BACKGROUND

Field of the Invention The present invention relates generally to methods and systems for heating subsurface formations. Certain embodiments relate to methods and systems for using temperature limited heaters with high power factors to heat subsurface formations such as hydrocarbon containing formations. Description of Related Art Hydrocarbons obtained from subterranean formations are often used as energy resources, as feedstocks, and as consumer products. Concerns over depletion of available hydrocarbon resources and concerns over declining overall quality of produced hydrocarbons have led to development of processes for more efficient recovery, processing and/or use of available hydrocarbon resources. In situ processes may be used to remove hydrocarbon materials from subterranean formations. Chemical and/or physical properties of hydrocarbon material in a subterranean formation may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formation. The chemical and physical changes may include in situ reactions that produce removable fluids, composition changes, solubility changes, density changes, phase changes, and/or viscosity changes of the hydrocarbon material in the formation. A fluid may be, but is not limited to, a gas, a liquid, an emulsion, a slurry, and/or a stream of solid particles that has flow characteristics similar to liquid flow. Heaters may be placed in wellbores to heat a formation during an in situ process. Examples of in situ processes utilizing downhole heaters are illustrated in U.S. Patent Nos. 2,634,961 to Ljungstrom; 2,732,195 to Ljungstrom; 2,780,450 to Ljungstrom; 2,789,805 to Ljungstrom; 2,923,535 to Ljungstrom; and 4,886,118 to Van Meurs et al. A heat source may be used to heat a subterranean formation. Electric heaters may be used to heat the subterranean formation by radiation and/or conduction. An electric heater may resistively heat an element. U.S. Patent No. 2,548,360 to Germain describes an electric heating element placed in a viscous oil in a wellbore. The heater element heats and thins the oil to allow the oil to be pumped from the wellbore. U.S. Patent No. 4,716,960 to Eastlund et al. describes electrically heating tubing of a petroleum well by passing a relatively low voltage current through the tubing to prevent formation of solids. U.S. Patent No. 5,065,818 to Van Egmond describes an electric heating element that is cemented into a well borehole without a casing surrounding the heating element. U.S. Patent No. 4,570,715 to Van Meurs et al. describes an electric heating element. The heating element has an electrically conductive core, a surrounding layer of insulating material, and a surrounding metallic sheath. The conductive core may have a relatively low resistance at high temperatures. The insulating material may have electrical resistance, compressive strength, and heat conductivity properties that are relatively high at high temperatures. The insulating layer may inhibit arcing from the core to the metallic sheath. The metallic sheath may have tensile strength and creep resistance properties that are relatively high at high temperatures. U.S. Patent No. 5,060,287 to Van Egmond describes an electrical heating element having a copper-nickel alloy core. Some heaters may break down or fail due to hot spots in the formation. The power supplied to the entire heater may need to be reduced if a temperature along any point of the heater exceeds, or is about to exceed, a maximum operating temperature of the heater to avoid failure of the heater and/or overheating of the formation at or near hot spots in the formation. Some heaters may not provide uniform heat along a length of the heater until the heater reaches a certain temperature limit. Some heaters may not heat a subsurface formation efficiently. Thus, it is advantageous to have a heater that provides uniform heat along a length of the heater; heats the subsurface formation efficiently; provides automatic temperature adjustment when a portion of the heater approaches a selected temperature; and/or has substantially linear magnetic properties and a high power factor below the selected temperature. SUMMARY OF THE INVENTION The invention provides a heater, comprising: a ferromagnetic member; an electrical conductor electrically coupled to the ferromagnetic member, wherein the electrical conductor is configured to provide heat output below the Curie temperature of the ferromagnetic member, and the electrical conductor is configured to conduct a majority of the electrical current of the heater at 25 0C; and wherein the heater automatically provides a reduced amount of heat approximately at and above the Curie temperature of the ferromagnetic member. The invention also provides in combination with the above invention: (a) the ferromagnetic member and the electrical conductor are electrically coupled such that a power factor of the heater remains above 0.85, above 0.9, or above 0.95 during use of the heater; (b) the heater has a turndown ratio of at least 1.1 , at least 2, at least 3, or at least 4; (c) the ferromagnetic member is electrically coupled to the electrical conductor such that a magnetic field produced by the ferromagnetic member confines a majority of the flow of the electrical current to the electrical conductor at temperatures below the Curie temperature of the ferromagnetic member; and (d) the electrical conductor provides a majority of heat output of the heater at temperatures up to the temperature at or near the Curie temperature of the ferromagnetic member. The invention also provides in combination with one or more of the above inventions: (a) the heater comprises in addition a second electrical conductor electrically coupled to the ferromagnetic member; and (b) the second electrical conductor comprises an electrical conductor with a higher electrical conductivity than the ferromagnetic member and the electrical conductor, and/or the second electrical conductor provides mechanical strength to support the ferromagnetic member at or near the Curie temperature of the ferromagnetic member. The invention also provides in combination with one or more of the above inventions: (a) the electrical conductor and the ferromagnetic member are concentric; and (b) the electrical conductor at least partially surrounds the ferromagnetic member. The invention also provides in combination with one or more of the above inventions: (a) the electrical conductor provides mechanical strength to support the ferromagnetic member at or near the Curie temperature of the ferromagnetic member; and (b) the electrical conductor is corrosion resistant material. The invention also provides in combination with one or more of the above inventions: (a) the heater exhibits an increase in operating temperature of at most 1.5 0C above or near a selected operating temperature when a thermal load proximate the heater decreases by 1 watt per meter; and (b) the heater provides a reduced amount of heat approximately at and above the Curie temperature of the ferromagnetic member, the reduced amount of heat being at most 10% of the heat output at 50 0C below the Curie temperature. The invention also provides in combination with one or more of the above inventions that the heater section provides, when electrical current is applied to the heater section: (a) a first heat output when the heater section is above 100 0C, above 200 °C, above 400 0C, or above 500 0C, or above 600 0C and below the selected temperature, and (b) a second heat output lower than the first heat output when the heater section is at and above the selected temperature. The invention also provides in combination with one or more of the above inventions: (a) the heater is used in a system configured to provide heat to a subsurface formation; and (b) the heater is used in a method for heating a subsurface formation, the method comprising: (1) applying electrical current to the heater to provide the heat output; and (2) allowing heat to transfer from the heater to a part of the subsurface formation. BRIEF DESCRIPTION OF THE DRAWINGS Advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings in which:

FIG. 1 depicts an illustration of stages of heating hydrocarbons in the formation. FIG. 2 shows a schematic view of an embodiment of a portion of an in situ conversion system for treating hydrocarbons in the formation. FIGS. 3, 4, and 5 depict cross-sectional representations of an embodiment of a temperature limited heater with an outer conductor having a ferromagnetic section and a non-ferromagnetic section. FIGS. 6, 7, 8, and 9 depict cross-sectional representations of an embodiment of a temperature limited heater with an outer conductor having a ferromagnetic section and a non-ferromagnetic section placed inside a sheath. FIGS. 10, 11, and 12 depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. FIGS. 13, 14, and 15 depict cross-sectional representations of an embodiment of a temperature limited heater with an outer conductor. FIGS. 16A and 16B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic inner conductor. FIGS. 17A and 17B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic inner conductor and a non-ferromagnetic core. FIGS. 18A and 18B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. FIGS. 19A and 19B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor that is clad with a corrosion resistant alloy. FIGS. 2OA and 2OB depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. FIG. 21 depicts a cross-sectional representation of an embodiment of a composite conductor with a support member. FIG. 22 depicts a cross-sectional representation of an embodiment of a composite conductor with a support member separating the conductors. FIG. 23 depicts a cross-sectional representation of an embodiment of a composite conductor surrounding a support member. FIG. 24 depicts a cross-sectional representation of an embodiment of a composite conductor surrounding a conduit support member. FIG. 25 depicts a cross-sectional representation of an embodiment of a conductor-in-conduit heater. FIG. 26 A and FIG. 26B depict an embodiment of an insulated conductor heater. FIG. 27 A and FIG. 27B depict an embodiment of an insulated conductor heater with a jacket located outside an outer conductor. e FIG. 28 depicts an embodiment of an insulated conductor located inside a conduit. FIG. 29 depicts an embodiment of a temperature limited heater in which the support member provides a majority of the heat output below the Curie temperature of the ferromagnetic conductor. FIGS. 30 and 31 depict embodiments of temperature limited heaters in which the jacket provides a majority of the heat output below the Curie temperature of the ferromagnetic conductor. FIG. 32 depicts experimentally measured resistance versus temperature at several currents for a temperature limited heater with a copper core, a carbon steel ferromagnetic conductor, and a stainless steel 347H stainless steel support member. FIG. 33 depicts experimentally measured resistance versus temperature at several currents for a temperature limited heater with a copper core, a cobalt-carbon steel ferromagnetic conductor, and a stainless steel 347H stainless steel support member. FIG. 34 depicts experimentally measured power factor versus temperature at two AC currents for a temperature limited heater with a copper core, a carbon steel ferromagnetic conductor, and a 347H stainless steel support member. FIG. 35 depicts experimentally measured turndown ratio versus maximum power delivered for a temperature limited heater with a copper core, a carbon steel ferromagnetic conductor, and a 347H stainless steel support member. FIG. 36 depicts temperature versus time for a temperature limited heater. FIG. 37 depicts temperature versus log time data for a 2.5 cm solid 410 stainless steel rod and a 2.5 cm solid 304 stainless steel rod. FIG. 38 displays temperature of the center conductor of a conductor-in-conduit heater as a function of formation depth for a temperature limited heater with a turndown ratio of 2: 1. FIG. 39 displays heater heat flux through a formation for a turndown ratio of 2:1 along with the oil shale richness profile. FIG. 40 displays heater temperature as a function of formation depth for a turndown ratio of 3 : 1. FIG. 41 displays heater heat flux through a formation for a turndown ratio of 3 : 1 along with the oil shale richness profile. FIG. 42 displays heater temperature as a function of formation depth for a turndown ratio of 4: 1. FIG. 43 depicts heater temperature versus depth for heaters used in a simulation for heating oil shale. FIG. 44 depicts heater heat flux versus time for heaters used in a simulation for heating oil shale. FIG. 45 depicts cumulative heat input versus time in a simulation for heating oil shale. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. DETAILED DESCRIPTION The above problems may be addressed using systems, methods, and heaters described herein. For example, the a heater includes a ferromagnetic member and an electrical conductor electrically coupled to the ferromagnetic member. The electrical conductor is configured to provide heat output below the Curie temperature of the ferromagnetic member. The electrical conductor is also configured to conduct a majority of the electrical current of the heater at 25 0C. The heater automatically provides a reduced amount of heat approximately at and above the Curie temperature of the ferromagnetic member. Certain embodiments of the inventions described herein in more detail relate to systems and methods for treating hydrocarbons in the formations. Such formations may be treated to yield hydrocarbon products, hydrogen, and other products. Terms used herein are defined as follows. "Hydrocarbons" are generally defined as molecules formed primarily by carbon and hydrogen atoms. Hydrocarbons may also include other elements such as, but not limited to, halogens, metallic elements, nitrogen, oxygen, and/or sulfur. Hydrocarbons may be, but are not limited to, kerogen, bitumen, pyrobitumen, oils, natural mineral waxes, and asphaltites. Hydrocarbons may be located in or adjacent to mineral matrices in the earth. Matrices may include, but are not limited to, sedimentary rock, sands, silicilytes, carbonates, diatomites, and other porous media. "Hydrocarbon fluids" are fluids that include hydrocarbons. Hydrocarbon fluids may include, entrain, or be entrained in non-hydrocarbon fluids (for example, hydrogen, nitrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, water, and ammonia). A "formation" includes one or more hydrocarbon containing layers, one or more non-hydrocarbon layers, an overburden, and/or an underburden. The overburden and/or underburden may include rock, shale, mudstone, or wet/tight carbonate. In some embodiments of in situ conversion processes, the overburden and/or the underburden may include a hydrocarbon containing layer or hydrocarbon containing layers that are relatively impermeable and are not subjected to temperatures during in situ conversion processing that results in significant characteristic changes of the hydrocarbon containing layers of the overburden and/or the underburden. For example, the underburden may contain shale or mudstone, but the underburden is not allowed to heat to pyrolysis temperatures during the in situ conversion process. In some cases, the overburden and/or the underburden maybe somewhat permeable. "Formation fluids" and "produced fluids" refer to fluids removed from the formation and may include pyrolyzation fluid, synthesis gas, mobilized hydrocarbon, and water (steam). Formation fluids may include hydrocarbon fluids as well as non-hydrocarbon fluids. "Thermally conductive fluid" includes fluid that has a higher thermal conductivity than air at 101 kPa and a temperature in a heater. A "heater" is any system for generating heat in a well or a near wellbore region. Heaters may be, but are not limited to, electric heaters, circulated heat transfer fluid or steam, burners, combustors that react with material in or produced from the formation, and/or combinations thereof. "Temperature limited heater" generally refers to a heater that regulates heat output (for example, reduces heat output) above a specified temperature without the use of external controls such as temperature controllers, power regulators, rectifiers, or other devices. Temperature limited heaters may be AC (alternating current) or modulated (for example, "chopped") DC (direct current) powered electrical resistance heaters. "Curie temperature" is the temperature above which a ferromagnetic material loses all of its ferromagnetic properties. In addition to losing all of its ferromagnetic properties above the Curie temperature, the ferromagnetic material begins to lose its ferromagnetic properties when an increasing electrical current is passed through the ferromagnetic material. "Modulated direct current (DC)" refers to any time-varying current that allows for skin effect electricity flow in a ferromagnetic conductor. "Turndown ratio" for the temperature limited heater is the ratio of the highest AC or modulated DC resistance below the Curie temperature to the lowest AC or modulated DC resistance above the Curie temperature. The term "wellbore" refers to a hole in a formation made by drilling or insertion of a conduit into the formation. As used herein, the terms "well" and "opening," when referring to an opening in the formation may be used interchangeably with the term "wellbore." "Insulated conductor" refers to any elongated material that is able to conduct electricity and that is covered, in whole or in part, by an electrically insulating material. The term "self-controls" refers to controlling an output of a heater without external control of any type. In the context of reduced heat output heating systems, apparatus, and methods, the term "automatically" means such systems, apparatus, and methods function in a certain way without the use of external control (for example, external controllers such as a controller with a temperature sensor and a feedback loop, PID controller, or predictive controller). Hydrocarbons in formations may be treated in various ways to produce many different products. In certain embodiments, such formations are treated in stages. FIG. 1 illustrates several stages of heating a portion of the formation that contains hydrocarbons. FIG. 1 also depicts an example of yield ("Y") in barrels of oil equivalent per ton (y axis) of formation versus temperature ("T") of the heated formation in degrees Celsius (x axis). Desorption of methane and vaporization of water occurs during stage 1 heating. Heating the formation through stage 1 may be performed as quickly as possible. When the formation is initially heated, hydrocarbons in the formation desorb adsorbed methane. The desorbed methane may be produced from the formation. If the formation is heated further, water in the formation is vaporized. Water typically is vaporized in the formation between 160 0C and 285 0C at pressures of 600 kPa absolute to 7000 kPa absolute. In some embodiments, the vaporized water produces wettability changes in the formation and/or increased formation pressure. The wettability changes and/or increased pressure may affect pyrolysis reactions or other reactions in the formation. In certain embodiments, the vaporized water is produced from the formation. In other embodiments, the vaporized water is used for steam extraction and/or distillation in the formation or outside the formation. Removing the water from the formation and increasing the pore volume in the formation increases the storage space for hydrocarbons in the pore volume. In certain embodiments, after stage 1 heating, the portion of the formation is heated further, such that the temperature in the portion of the formation reaches (at least) an initial pyrolyzation temperature (such as a temperature at the lower end of the temperature range shown as stage 2). Hydrocarbons in the formation may be pyrolyzed throughout stage 2. A pyrolysis temperature range varies depending on the types of hydrocarbons in the formation. The pyrolysis temperature range may include temperatures between 250 0C and 900 °C. The pyrolysis temperature range for producing desired products may extend through only a portion of the total pyrolysis temperature range. In some embodiments, the pyrolysis temperature range for producing desired products may include temperatures between 250 °C and 400 °C, temperatures between 250 °C and 350 °C, or temperatures between 325 0C and 400 0C. If the temperature of hydrocarbons in the formation is slowly raised through the temperature range from 250 0C to 400 0C, production of pyrolysis products maybe substantially complete when the temperature approaches 400 °C. Heating the formation with a plurality of heaters may establish superposition of heat that slowly raises the temperature of hydrocarbons in the formation through the pyrolysis temperature range. In some in situ conversion embodiments, a portion of the formation is heated to the desired temperature instead of slowly heating the temperature through the pyrolysis temperature range. In some embodiments, the desired temperature is 300 0C. In some embodiments, the desired temperature is 325 0C. In some embodiments, the desired temperature is 350 °C. Other temperatures may be selected as the desired temperature. Superposition of heat from heaters allows the desired temperature to be relatively quickly and efficiently established in the formation. Energy input into the formation from the heaters may be adjusted to maintain the temperature in the formation at the desired temperature. The heated portion of the formation is maintained substantially at the desired temperature until pyrolysis declines such that production of desired formation fluids from the formation becomes uneconomical. Parts of the formation that are subjected to pyrolysis may include regions brought into the pyrolysis temperature range by heat transfer from only one heater. In certain embodiments, formation fluids including pyrolyzation fluids are produced from the formation. As the temperature of the formation increases, the amount of condensable hydrocarbons in the produced formation fluid may decrease. At very high temperatures, the formation may produce mostly methane and/or hydrogen. If the formation is heated throughout an entire pyrolysis range, the formation may produce only small amounts of hydrogen towards an upper limit of the pyrolysis range. After most of the available hydrogen is depleted, a minimal amount of fluid production will occur from the formation. After pyrolysis of hydrocarbons, a large amount of carbon and some hydrogen may still be present in the heated portion of the formation. A portion of carbon remaining in the heated portion of the formation may be produced from the formation in the form of synthesis gas. Synthesis gas generation may take place during stage 3 heating depicted in FIG. 1. Stage 3 may include heating the heated portion of the formation to a temperature sufficient to allow synthesis gas generation. Synthesis gas may be produced in a temperature range from 400 °C to 1200 °C, 500 0C to 1100 0C, or 550 0C to 1000 0C. The temperature of the heated portion of the formation when the synthesis gas generating fluid is introduced to the formation determines the composition of synthesis gas produced in the formation. Generated synthesis gas may be removed from the formation through one or more production wells. FIG. 2 depicts a schematic view of an embodiment of a portion of the in situ conversion system for treating the formation that contains hydrocarbons. Heaters 100 are placed in at least a portion of the formation. Heaters 100 provide heat to at least a portion of the formation to heat hydrocarbons in the formation. Energy may be supplied to heaters 100 through supply lines 102. Supply lines 102 may be structurally different depending on the type of heater or heaters used to heat the formation. Supply lines 102 for heaters may transmit electricity for electric heaters, may transport fuel for combustors, or may transport heat exchange fluid that is circulated in the formation. Production wells 104 are used to remove formation fluid from the formation. Formation fluid produced from production wells 104 maybe transported through collection piping 106 to treatment facilities 108. Formation fluids may also be produced from heaters 100. For example, fluid maybe produced from heaters 100 to control pressure in the formation adjacent to the heaters. Fluid produced from heaters 100 may be transported through tubing or piping to collection piping 106 or the produced fluid may be transported through tubing or piping directly to treatment facilities 108. Treatment facilities 108 may include separation units, reaction units, upgrading units, sulfur removal from gas units, fuel cells, turbines, storage vessels, and/or other systems and units for processing produced formation fluids. The in situ conversion system for treating hydrocarbons may include barrier wells 110. Barrier wells are used to form a barrier around a treatment area. The barrier inhibits fluid flow into and/or out of the treatment area. Barrier wells include, but are not limited to, dewatering wells, vacuum wells, capture wells, injection wells, grout wells, freeze wells, or combinations thereof. In some embodiments, barrier wells 110 are dewatering wells. Dewatering wells may remove liquid water and/or inhibit liquid water from entering a portion of the formation to be heated, or to the formation being heated. In the embodiment depicted in FIG. 2, the dewatering wells are shown extending only along one side of heaters 100, but dewatering wells typically encircle all heaters 100 used, or to be used, to heat the formation. As shown in FIG. 2, in addition to heaters 100, one or more production wells 104 are placed in the formation. Formation fluids may be produced through production well 104. In some embodiments, production well 104 includes a heater. The heater in the production well may heat one or more portions of the formation at or near the production well and allow for vapor phase removal of formation fluids. The need for high temperature pumping of liquids from the production well may be reduced or eliminated. Avoiding or limiting high temperature pumping of liquids may significantly decrease production costs. Providing heating at or through the production well may: (1) inhibit condensation and/or refluxing of production fluid when such production fluid is moving in the production well proximate the overburden, (2) increase heat input into the formation, and/or (3) increase formation permeability at or proximate the production well. In some in situ conversion process embodiments, an amount of heat supplied to the formation from a production well per meter of the production well is less than the amount of heat applied to the formation from a heater that heats the formation per meter of the heater. Some embodiments of heaters include switches (for example, fuses and/or thermostats) that turn off power to a heater or portions of a heater when a certain condition is reached in the heater. In certain embodiments, a temperature limited heater is used to provide heat to hydrocarbons in the formation. Temperature limited heaters may be in configurations and/or may include materials that provide automatic temperature limiting properties for the heater at certain temperatures. In certain embodiments, ferromagnetic materials are used in temperature limited heaters. Ferromagnetic material may self-limit temperature at or near the Curie temperature of the material to provide a reduced amount of heat at or near the Curie temperature when an alternating current is applied to the material. In certain embodiments, ferromagnetic materials are coupled with other materials (for example, highly conductive materials, high strength materials, corrosion resistant materials, or combinations thereof) to provide various electrical and/or mechanical properties. Some parts of the temperature limited heater may have a lower resistance (caused by different geometries and/or by using different ferromagnetic and/or non-ferromagnetic materials) than other parts of the temperature limited heater. Having parts of the temperature limited heater with various materials and/or dimensions allows for tailoring the desired heat output from each part of the heater. Using ferromagnetic materials in temperature limited heaters is typically less expensive and more reliable than using switches or other control devices in temperature limited heaters. Temperature limited heaters may be more reliable than other heaters. Temperature limited heaters may be less apt to break down or fail due to hot spots in the formation. In some embodiments, temperature limited heaters allow for substantially uniform heating of the formation. In some embodiments, temperature limited heaters are able to heat the formation more efficiently by operating at a higher average heat output along the entire length of the heater. The temperature limited heater operates at the higher average heat output along the entire length of the heater because power to the heater does not have to be reduced to the entire heater, as is the case with typical constant wattage heaters, if a temperature along any point of the heater exceeds, or is about to exceed, a maximum operating temperature of the heater. Heat output from portions of a temperature limited heater approaching a Curie temperature of the heater automatically reduces without controlled adjustment of alternating current applied to the heater. The heat output automatically reduces due to changes in electrical properties (for example, electrical resistance) of portions of the temperature limited heater. Thus, more power is supplied by the temperature limited heater during a greater portion of a heating process. In an embodiment, the system including temperature limited heaters initially provides a first heat output and then provides a reduced amount of heat, near, at, or above the Curie temperature of an electrically resistive portion of the heater when the temperature limited heater is energized by an alternating current or a modulated direct current. The temperature limited heater may be energized by alternating current or modulated direct current supplied at the wellhead. The wellhead may include a power source and other components (for example, modulation components, transformers, and/or capacitors) used in supplying power to the temperature limited heater. The temperature limited heater may be one of many heaters used to heat a portion of the formation. In certain embodiments, the temperature limited heater includes a conductor that operates as a sMn effect or proximity effect heater when alternating current or modulated direct current is applied to the conductor. The skin effect limits the depth of current penetration into the interior of the conductor. For ferromagnetic materials, the skin effect is dominated by the magnetic permeability of the conductor. The relative magnetic permeability of ferromagnetic materials is typically between 10 and 1000 (for example, the relative magnetic permeability of ferromagnetic materials is typically at least 10 and maybe at least 50, 100, 500, 1000 or greater). As the temperature of the ferromagnetic material is raised above the Curie temperature and/or as the applied electrical current is increased, the magnetic permeability of the ferromagnetic material decreases substantially and the skin depth expands rapidly (for example, the skin depth expands as the inverse square root of the magnetic permeability). The reduction in magnetic permeability results in a decrease in the AC or modulated DC resistance of the conductor near, at, or above the Curie temperature and/or as the applied electrical current is increased. When the temperature limited heater is powered by a substantially constant current source, portions of the heater that approach, reach, or are above the Curie temperature may have reduced heat dissipation. Sections of the temperature limited heater that are not at or near the Curie temperature may be dominated by skin effect heating that allows the heater to have high heat dissipation due to a higher resistive load. Curie temperature heaters have been used in soldering equipment, heaters for medical applications, and heating elements for ovens. Some of these uses are disclosed in U.S. Patent Nos. 5,579,575 to Lamome et al.; 5,065,501 to Henschen et al.; and 5,512,732 to Yagnik et al. U.S. Patent No. 4,849,611 to Whitney et al. describes a plurality of discrete, spaced-apart heating units including a reactive component, a resistive heating component, and a temperature responsive component. An advantage of using the temperature limited heater to heat hydrocarbons in the formation is that the conductor is chosen to have a Curie temperature in a desired range of temperature operation. Operation within the desired operating temperature range allows substantial heat injection into the formation while maintaining the temperature of the temperature limited heater, and other equipment, below design limit temperatures. Design limit temperatures are temperatures at which properties such as corrosion, creep, and/or deformation are adversely affected. The temperature limiting properties of the temperature limited heater inhibits overheating or burnout of the heater adjacent to low thermal conductivity "hot spots" in the formation. In some embodiments, the temperature limited heater is able to lower or control heat output and/or withstand heat at temperatures above 25 0C, 37 0C, 100 0C, 250 0C, 500 0C, 700 0C, 800 0C, 900 0C, or higher up to 1131 0C, depending on the materials used in the heater. The temperature limited heater allows for more heat injection into the formation than constant wattage heaters because the energy input into the temperature limited heater does not have to be limited to accommodate low thermal conductivity regions adjacent to the heater. For example, in Green River oil shale there is a difference of at least a factor of 3 in the thermal conductivity of the lowest richness oil shale layers and the highest richness oil shale layers. When heating such a formation, substantially more heat is transferred to the formation with the temperature limited heater than with the conventional heater that is limited by the temperature at low thermal conductivity layers. The heat output along the entire length of the conventional heater needs to accommodate the low thermal conductivity layers so that the heater does not overheat at the low thermal conductivity layers and burn out. The heat output adjacent to the low thermal conductivity layers that are at high temperature will reduce for the temperature limited heater, but the remaining portions of the temperature limited heater that are not at high temperature will still provide high heat output. Because heaters for heating hydrocarbon formations typically have long lengths (for example, at least 10 m, 100 m, 300 m, at least 500 m, 1 km or more up to 10 km), the majority of the length of the temperature limited heater may be operating below the Curie temperature while only a few portions are at or near the Curie temperature of the temperature limited heater. The use of temperature limited heaters allows for efficient transfer of heat to the formation. Efficient transfer of heat allows for reduction in time needed to heat the formation to a desired temperature. For example, in Green River oil shale, pyrolysis typically requires 9.5 years to 10 years of heating when using a 12 m heater well spacing with conventional constant wattage heaters. For the same heater spacing, temperature limited heaters may allow a larger average heat output while maintaining heater equipment temperatures below equipment design limit temperatures. Pyrolysis in the formation may occur at an earlier time with the larger average heat output provided by temperature limited heaters than the lower average heat output provided by constant wattage heaters. For example, in Green River oil shale, pyrolysis may occur in 5 years using temperature limited heaters with a 12 m heater well spacing. Temperature limited heaters counteract hot spots due to inaccurate well spacing or drilling where heater wells come too close together. In certain embodiments, temperature limited heaters allow for increased power output over time for heater wells that have been spaced too far apart, or limit power output for heater wells that are spaced too close together. Temperature limited heaters also supply more power in regions adjacent the overburden and underburden to compensate for temperature losses in the regions. Temperature limited heaters may be advantageously used in many types of formations. For example, in tar sands formations or relatively permeable formations containing heavy hydrocarbons, temperature limited heaters may be used to provide a controllable low temperature output for reducing the viscosity of fluids, mobilizing fluids, and/or enhancing the radial flow of fluids at or near the wellbore or in the formation. Temperature limited heaters may be used to inhibit excess coke formation due to overheating of the near wellbore region of the formation. The use of temperature limited heaters, in some embodiments, eliminates or reduces the need for expensive temperature control circuitry. For example, the use of temperature limited heaters eliminates or reduces the need to perform temperature logging and/or the need to use fixed thermocouples on the heaters to monitor potential overheating at hot spots. In some embodiments, temperature limited heaters are more economical to manufacture or make than standard heaters. Typical ferromagnetic materials include iron, carbon steel, or ferritic stainless steel. Such materials are inexpensive as compared to nickel-based heating alloys (such as nichrome, Kanthal™(Bulten-Kanthal AB, Sweden), and/or LOHM™(Driver-Harris Company, Harrison, NJ)) typically used in insulated conductor (mineral insulated cable) heaters. In one embodiment of the temperature limited heater, the temperature limited heater is manufactured in continuous lengths as an insulated conductor heater to lower costs and improve reliability. The ferromagnetic alloy or ferromagnetic alloys used in the temperature limited heater determine the Curie temperature of the heater. Curie temperature data for various metals is listed in "American Institute of Physics Handbook," Second Edition, McGraw-Hill, pages 5-170 through 5-176. Ferromagnetic conductors may include one or more of the ferromagnetic elements (iron, cobalt, and nickel) and/or alloys of these elements. In some embodiments, ferromagnetic conductors include iron-chromium (Fe-Cr) alloys that contain tungsten (W) (for example, HCM12A and SAVE12 (Sumitomo Metals Co., Japan) and/or iron alloys that contain chromium (for example, Fe-Cr alloys, Fe-Cr-W alloys, Fe-Cr-V (vanadium) alloys, Fe-Cr-Nb (Niobium) alloys). Of the three main ferromagnetic elements, iron has a Curie temperature of approximately 770 °C; cobalt (Co) has a Curie temperature of approximately 1131 0C; and nickel has a Curie temperature of approximately 358 0C. An iron-cobalt alloy has a Curie temperature higher than the Curie temperature of iron. For example, iron-cobalt alloy with 2% by weight cobalt has a Curie temperature of approximately 800 0C; iron-cobalt alloy with 12% by weight cobalt has a Curie temperature of approximately 900 °C; and iron-cobalt alloy with 20% by weight cobalt has a Curie temperature of approximately 950 °C. Iron-nickel alloy has a Curie temperature lower than the Curie temperature of iron. For example, iron-nickel alloy with 20% by weight nickel has a Curie temperature of approximately 720 0C, and iron- nickel alloy with 60% by weight nickel has a Curie temperature of approximately 560 °C. Some non-ferromagnetic elements used as alloys raise the Curie temperature of iron. For example, an iron- vanadium alloy with 5.9% by weight vanadium has a Curie temperature of approximately 815 0C. Other non- ferromagnetic elements (for example, carbon, aluminum, copper, silicon, and/or chromium) maybe alloyed with iron or other ferromagnetic materials to lower the Curie temperature. Non-ferromagnetic materials that raise the Curie temperature may be combined with non-ferromagnetic materials that lower the Curie temperature and alloyed with iron or other ferromagnetic materials to produce a material with a desired Curie temperature and other desired physical and/or chemical properties. In some embodiments, the Curie temperature material is a ferrite such as NiFe2O^ In other embodiments, the Curie temperature material is a binary compound such as FeNi3 or FeaAl. Magnetic properties generally decay as the Curie temperature is approached. The "Handbook of Electrical Heating for Industry" by C. James Erickson (IEEE Press, 1995) shows a typical curve for 1% carbon steel (steel with 1% carbon by weight). The loss of magnetic permeability starts at temperatures above 650 0C and tends to be complete when temperatures exceed 730 °C Thus, the self-limiting temperature may be somewhat below the actual Curie temperature of the ferromagnetic conductor. The skin depth for current flow in 1% carbon steel is 0.132 cm (centimeters) at room temperature and increases to 0.445 cm at 720 0C. From 720 0C to 730 0C, the skin depth sharply increases to over 2.5 cm. Thus, a temperature limited heater embodiment using 1% carbon steel self-limits between 650 0C and 730 0C. Skin depth generally defines an effective penetration depth of alternating current or modulated direct current into the conductive material. In general, current density decreases exponentially with distance from an outer surface to the center along the radius of the conductor. The depth at which the current density is approximately lie of the surface current density is called the skin depth. For a solid cylindrical rod with a diameter much greater than the penetration depth, or for hollow cylinders with a wall thickness exceeding the penetration depth, the skin depth, δ, is: (1) δ = 1981.5* (ρ/(/x*f))1/2; in which: δ = skin depth in inches; p — resistivity at operating temperature (ohm-cm); μ = relative magnetic permeability; and f = frequency (Hz). EQN. 1 is obtained from "Handbook of Electrical Heating for Industry" by C. James Erickson (IEEE Press, 1995). For most metals, resistivity (p) increases with temperature. The relative magnetic permeability generally varies with temperature and with current. Additional equations may be used to assess the variance of magnetic permeability and/or skin depth on both temperature and/or current. The dependence of μ on current arises from the dependence of μ on the magnetic field. Materials used in the temperature limited heater may be selected to provide a desired turndown ratio. Turndown ratios of at least 1.1:1, 2:1, 3:1, 4:1, 5:1, 10:1, 30:1, or 50:1 maybe selected for temperature limited heaters. Larger turndown ratios may also be used. The selected turndown ratio depends on a number of factors including, but not limited to, the type of formation in which the temperature limited heater is located and/or a temperature limit of materials used in the wellbore. In some embodiments, the turndown ratio is increased by coupling additional copper or another good electrical conductor to the ferromagnetic material (for example, adding copper to lower the resistance above the Curie temperature). The temperature limited heater may provide a minimum heat output (power output) below the Curie temperature of the heater. In certain embodiments, the minimum heat output is at least 400 W/m (Watts per meter), 600 W/m, 700 W/m, 800 W/m, or higher up to 2000 W/m. The temperature limited heater reduces the amount of heat output by a section of the heater when the temperature of the section of the heater approaches or is above the Curie temperature. The reduced amount of heat may be substantially less than the heat output below the Curie temperature. In some embodiments, the reduced amount of heat is at most 400 W/m, 200 W/m, 100 W/m or may approach 0 W/m. In some embodiments, the temperature limited heater may operate substantially independently of the thermal load on the heater in a certain operating temperature range. "Thermal load" is the rate that heat is transferred from a heating system to its surroundings. It is to be understood that the thermal load may vary with temperature of the surroundings and/or the thermal conductivity of the surroundings. In an embodiment, the temperature limited heater operates at or above the Curie temperature of the temperature limited heater such that the operating temperature of the heater increases at most by 1.5 0C, 1 °C, or 0.5 °C for a decrease in thermal load of 1 W/m proximate to a portion of the heater. The AC or modulated DC resistance and/or the heat output of the temperature limited heater may decrease sharply above the Curie temperature due to the Curie effect. In certain embodiments, the value of the electrical resistance or heat output above or near the Curie temperature is at most one-half of the value of electrical resistance or heat output at a certain point below the Curie temperature. In some embodiments, the heat output above or near the Curie temperature is at most 40%, 30%, 20%, 10%, or less (down to 1%) of the heat output at a certain point below the Curie temperature (for example, 30 °C below the Curie temperature, 40 °C below the Curie temperature, 50 0C below the Curie temperature, or 100 °C below the Curie temperature). In certain embodiments, the electrical resistance above or near the Curie temperature decreases to 80%, 70%, 60%, 50%, or less (down to 1%) of the electrical resistance at a certain point below the Curie temperature (for example, 30 0C below the Curie temperature, 40 °C below the Curie temperature, 50 0C below the Curie temperature, or 100 °C below the Curie temperature). In some embodiments, AC frequency is adjusted to change the skin depth of the ferromagnetic material. For example, the skin depth of 1% carbon steel at room temperature is 0.132 cm at 60 Hz, 0.0762 cm at 180 Hz, and 0.046 cm at 440 Hz. Since heater diameter is typically larger than twice the skin depth, using a higher frequency (and thus a heater with a smaller diameter) reduces heater costs. For a fixed geometry, the higher frequency results in a higher turndown ratio. The turndown ratio at a higher frequency is calculated by multiplying the turndown ratio at a lower frequency by the square root of the higher frequency divided by the lower frequency. In some embodiments, a frequency between 100 Hz and 1000 Hz, between 140 Hz and 200 Hz, or between 400 Hz and 600 Hz is used (for example, 180 Hz, 540 Hz, or 720 Hz). In some embodiments, high frequencies maybe used. The frequencies may be greater than 1000 Hz. To maintain a substantially constant skin depth until the Curie temperature of the temperature limited heater is reached, the heater may be operated at a lower frequency when the heater is cold and operated at a higher frequency when the heater is hot. Line frequency heating is generally favorable, however, because there is less need for expensive components such as power supplies, transformers, or current modulators that alter frequency. Line frequency is the frequency of a general supply of current. Line frequency is typically 60 Hz, but may be 50 Hz or another frequency depending on the source for the supply of the current. Higher frequencies may be produced using commercially available equipment such as solid state variable frequency power supplies. Transformers that convert three-phase power to single-phase power with three times the frequency are commercially available. For example, high voltage three-phase power at 60 Hz may be transformed to single-phase power at 180 Hz and at a lower voltage. Such transformers are less expensive and more energy efficient than solid state variable frequency power supplies. In certain embodiments, transformers that convert three-phase power to single-phase power are used to increase the frequency of power supplied to the temperature limited heater. In certain embodiments, modulated DC (for example, chopped DC, waveform modulated DC, or cycled DC) may be used for providing electrical power to the temperature limited heater. A DC modulator or DC chopper may be coupled to a DC power supply to provide an output of modulated direct current. In some embodiments, the DC power supply may include means for modulating DC. One example of a DC modulator is a DC-to-DC converter system. DC-to-DC converter systems are generally known in the art. DC is typically modulated or chopped into a desired waveform. Waveforms for DC modulation include, but are not limited to, square-wave, sinusoidal, deformed sinusoidal, deformed square- wave, triangular, and other regular or irregular waveforms. The modulated DC waveform generally defines the frequency of the modulated DC. Thus, the modulated DC waveform may be selected to provide a desired modulated DC frequency. The shape and/or the rate of modulation (such as the rate of chopping) of the modulated DC waveform may be varied to vary the modulated DC frequency. DC may be modulated at frequencies that are higher than generally available AC frequencies. For example, modulated DC may be provided at frequencies of at least 1000 Hz. Increasing the frequency of supplied current to higher values advantageously increases the turndown ratio of the temperature limited heater. In certain embodiments, the modulated DC waveform is adjusted or altered to vary the modulated DC frequency. The DC modulator may be able to adjust or alter the modulated DC waveform at any time during use of the temperature limited heater and at high currents or voltages. Thus, modulated DC provided to the temperature limited heater is not limited to a single frequency or even a small set of frequency values. Waveform selection using the DC modulator typically allows for a wide range of modulated DC frequencies and for discrete control of the modulated DC frequency. Thus, the modulated DC frequency is more easily set at a distinct value whereas AC frequency is generally limited to incremental values of the line frequency. Discrete control of the modulated DC frequency allows for more selective control over the turndown ratio of the temperature limited heater. Being able to selectively control the turndown ratio of the temperature limited heater allows for a broader range of materials to be used in designing and constructing the temperature limited heater. In some embodiments, the modulated DC frequency or the AC frequency is adjusted to compensate for changes in properties (for example, subsurface conditions such as temperature or pressure) of the temperature limited heater during use. The modulated DC frequency or the AC frequency provided to the temperature limited heater is varied based on assessed downhole condition conditions. For example, as the temperature of the temperature limited heater in the wellbore increases, it may be advantageous to increase the frequency of the current provided to the heater, thus increasing the turndown ratio of the heater. In an embodiment, the downhole temperature of the temperature limited heater in the wellbore is assessed. In certain embodiments, the modulated DC frequency, or the AC frequency, is varied to adjust the turndown ratio of the temperature limited heater. The turndown ratio may be adjusted to compensate for hot spots occurring along a length of the temperature limited heater. For example, the turndown ratio is increased because the temperature limited heater is getting too hot in certain locations. In some embodiments, the modulated DC frequency, or the AC frequency, are varied to adjust a turndown ratio without assessing a subsurface condition. Temperature limited heaters may generate an inductive load. The inductive load is due to some applied electrical current being used by the ferromagnetic material to generate a magnetic field in addition to generating a resistive heat output. As downhole temperature changes in the temperature limited heater, the inductive load of the heater changes due to changes in the magnetic properties of ferromagnetic materials in the heater with temperature. The inductive load of the temperature limited heater may cause a phase shift between the current and the voltage applied to the heater. A reduction in actual power applied to the temperature limited heater may be caused by a time lag in the current waveform (for example, the current has a phase shift relative to the voltage due to an inductive load) and/or by distortions in the current waveform (for example, distortions in the current waveform caused by introduced harmonics due to a non-linear load). Thus, it may take more current to apply a selected amount of power due to phase shifting or waveform distortion. The ratio of actual power applied and the apparent power that would have been transmitted if the same current were in phase and undistorted is the power factor. The power factor is always less than or equal to 1. The power factor is 1 when there is no phase shift or distortion in the waveform. Actual power applied to a heater due to a phase shift is described by EQN. 2: (2) P = I x V x cos(0); in which P is the actual power applied to the temperature limited heater; I is the applied current; V is the applied voltage; and θ is the phase angle difference between voltage and current. If there is no distortion in the waveform, then cos(0) is equal to the power factor. At higher frequencies (for example, modulated DC frequencies at least 1000 Hz, 1500 Hz, or 2000 Hz), the problem with phase shifting and/or distortion is more pronounced. In some embodiments, electrical voltage and/or electrical current is adjusted to change the skin depth of the ferromagnetic material. Increasing the voltage and/or decreasing the current may decrease the skin depth of the ferromagnetic material. A smaller skin depth allows the temperature limited heater to have a smaller diameter, thereby reducing equipment costs. In certain embodiments, the applied current is at least 1 amp, 10 amps, 70 amps, 100 amps, 200 amps, 500 amps, or greater up to 2000 amps. In some embodiments, alternating current is supplied at voltages above 200 volts, above 480 volts, above 650 volts, above 1000 volts, above 1500 volts, or higher up to 10000 volts. In an embodiment, the temperature limited heater includes an inner conductor inside an outer conductor. The inner conductor and the outer conductor are radially disposed about a central axis. The inner and outer conductors may be separated by an insulation layer. In certain embodiments, the inner and outer conductors are coupled at the bottom of the temperature limited heater. Electrical current may flow into the temperature limited heater through the inner conductor and return through the outer conductor. One or both conductors may include ferromagnetic material. The insulation layer may comprise an electrically insulating ceramic with high thermal conductivity, such as magnesium oxide, aluminum oxide, silicon dioxide, beryllium oxide, boron nitride, silicon nitride, or combinations thereof. The insulating layer may be a compacted powder (for example, compacted ceramic powder). Compaction may improve thermal conductivity and provide better insulation resistance. For lower temperature applications, polymer insulation made from, for example, fluoropolymers, polyimides, polyamides, and/or polyethylenes, may be used. In some embodiments, the polymer insulation is made of perfluoroalkoxy (PFA) or polyetheretherketone (PEEK™ (Victrex Ltd, England)). The insulating layer may be chosen to be substantially infrared transparent to aid heat transfer from the inner conductor to the outer conductor. In an embodiment, the insulating layer is transparent quartz sand. The insulation layer may be air or a non-reactive gas such as helium, nitrogen, or sulfur hexafluoride. If the insulation layer is air or a non-reactive gas, there may be insulating spacers designed to inhibit electrical contact between the inner conductor and the outer conductor. The insulating spacers may be made of, for example, high purity aluminum oxide or another thermally conducting, electrically insulating material such as silicon nitride. The insulating spacers may be a fibrous ceramic material such as Nextel™ 312 (3M Corporation, St. Paul, Minnesota), mica tape, or glass fiber. Ceramic material maybe made of alumina, alumina- silicate, alumina-borosilicate, silicon nitride, boron nitride, or other materials. In certain embodiments, the outer conductor is chosen for corrosion and/or creep resistance. In one embodiment, austentitic (non-ferromagnetic) stainless steels such as 304H, 347H, 347HH, 316H, 310H, 347HP, NF709 (Nippon Steel Corp., Japan) stainless steels, or combinations thereof may be used in the outer conductor. The outer conductor may also include a clad conductor. For example, a corrosion resistant alloy such as 800H or 347H stainless steel may be clad for corrosion protection over a ferromagnetic carbon steel tubular. If high temperature strength is not required, the outer conductor may be constructed from the ferromagnetic metal with good corrosion resistance such as one of the ferritic stainless steels. PB one embodiment, a ferritic alloy of 82.3% by weight iron with 17.7% by weight chromium (Curie temperature of 678 °C) provides desired corrosion resistance. The Metals Handbook, vol. 8, page 291 (American Society of Materials (ASM)) includes a graph of Curie temperature of iron-chromium alloys versus the amount of chromium in the alloys. In some temperature limited heater embodiments, a separate support rod or tubular (made from 347H stainless steel) is coupled to the temperature limited heater made from an iron-chromium alloy to provide strength and/or creep resistance. The support material and/or the ferromagnetic material may be selected to provide a 100,000 hour creep-rupture strength of at least 20.7 MPa at 650 °C. In some embodiments, the 100,000 hour creep-rupture strength is at least 13.8 MPa at 650 °C or at least 6.9 MPa at 650 °C. For example, 347H steel has a favorable creep-rupture strength at or above 650°C. In some embodiments, the 100,000 hour creep-rupture strength ranges from 6.9 MPa to 41.3 MPa or more for longer heaters and/or higher earth or fluid stresses. In temperature limited heaters embodiments with the inner ferromagnetic conductor and the outer ferromagnetic conductor, the skin effect current path occurs on the outside of the inner conductor and on the inside of the outer conductor. Thus, the outside of the outer conductor may be clad with the corrosion resistant alloy, such as stainless steel, without affecting the skin effect current path on the inside of the outer conductor. A ferromagnetic conductor with a thickness at least the skin depth at the Curie temperature allows a substantial decrease in AC resistance of the ferromagnetic material as the skin depth increases sharply near the Curie temperature. In certain embodiments when the ferromagnetic conductor is not clad with a highly conducting material such as copper, the thickness of the conductor may be 1.5 times the skin depth near the Curie temperature, 3 times the skin depth near the Curie temperature, or even 10 or more times the skin depth near the Curie temperature. If the ferromagnetic conductor is clad with copper, thickness of the ferromagnetic conductor may be substantially the same as the skin depth near the Curie temperature. In some embodiments, the ferromagnetic conductor clad with copper has a thickness of at least three-fourths of the skin depth near the Curie temperature. In certain embodiments, the temperature limited heater includes a composite conductor with a ferromagnetic tubular and a non-ferromagnetic, high electrical conductivity core. The non-ferromagnetic, high electrical conductivity core reduces a required diameter of the conductor. For example, the conductor may be composite 1.19 cm diameter conductor with a core of 0.575 cm diameter copper clad with a 0.298 cm thickness of ferritic stainless steel or carbon steel surrounding the core. A composite conductor allows the electrical resistance of the temperature limited heater to decrease more steeply near the Curie temperature. As the skin depth increases near the Curie temperature to include the copper core, the electrical resistance decreases very sharply. The composite conductor may increase the conductivity of the temperature limited heater and/or allow the heater to operate at lower voltages. In an embodiment, the composite conductor exhibits a relatively flat resistance versus temperature profile at temperatures below a region near the Curie temperature of the ferromagnetic conductor of the composite conductor, m some embodiments, the temperature limited heater exhibits a relatively flat resistance versus temperature profile between 100 0C and 750 0C or between 300 0C and 600 0C. The relatively flat resistance versus temperature profile may also be exhibited in other temperature ranges by adjusting, for example, materials and/or the configuration of materials in the temperature limited heater. In certain embodiments, the relative thickness of each material in the composite conductor is selected to produce a desired resistivity versus temperature profile for the temperature limited heater. FIGS. 3-31 depict various embodiments of temperature limited heaters. One or more features of an embodiment of the temperature limited heater depicted in any of these figures may be combined with one or more features of other embodiments of temperature limited heaters depicted in these figures. In certain embodiments described herein, temperature limited heaters are dimensioned to operate at a frequency of 60 Hz AC. It is to be understood that dimensions of the temperature limited heater may be adjusted from those described herein in order for the temperature limited heater to operate in a similar manner at other AC frequencies or with modulated DC. FIG. 3 depicts a cross-sectional representation of an embodiment of the temperature limited heater with an outer conductor having a ferromagnetic section and a non-ferromagnetic section. FIGS. 4 and 5 depict transverse cross-sectional views of the embodiment shown in FIG. 3. In one embodiment, ferromagnetic section 140 is used to provide heat to hydrocarbon layers in the formation. Non-ferromagnetic section 142 is used in the overburden of the formation. Non-ferromagnetic section 142 provides little or no heat to the overburden, thus inhibiting heat losses in the overburden and improving heater efficiency. Ferromagnetic section 140 includes a ferromagnetic material such as 409 stainless steel or 410 stainless steel. Ferromagnetic section 140 has a thickness of 0.3 cm. Non- ferromagnetic section 142 is copper with a thickness of 0.3 cm. Inner conductor 144 is copper. Inner conductor 144 has a diameter of 0.9 cm. Electrical insulator 146 is silicon nitride, boron nitride, magnesium oxide powder, or another suitable insulator material. Electrical insulator 146 has a thickness of 0.1 cm to 0.3 cm. FIG. 6 depicts a cross-sectional representation of an embodiment of a temperature limited heater with an outer conductor having a ferromagnetic section and a non-ferromagnetic section placed inside a sheath. FIGS. 7, 8, and 9 depict transverse cross-sectional views of the embodiment shown in FIG. 6. Ferromagnetic section 140 is 410 stainless steel with a thickness of 0.6 cm. Non-ferromagnetic section 142 is copper with a thickness of 0.6 cm. Inner conductor 144 is copper with a diameter of 0.9 cm. Outer conductor 148 includes ferromagnetic material. Outer conductor 148 provides some heat in the overburden section of the heater. Providing some heat in the overburden inhibits condensation or refluxing of fluids in the overburden. Outer conductor 148 is 409, 410, or 446 stainless steel with an outer diameter of 3.0 cm and a thickness of 0.6 cm. Electrical insulator 146 is magnesium oxide powder with a thickness of 0.3 cm. In some embodiments, electrical insulator 146 is silicon nitride, boron nitride, or hexagonal type boron nitride. Conductive section 150 may couple inner conductor 144 with ferromagnetic section 140 and/or outer conductor 148. FIG. 10 depicts a cross-sectional representation of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. The heater is placed in a corrosion resistant jacket. A conductive layer is placed between the outer conductor and the jacket. FIGS. 11 and 12 depict transverse cross-sectional views of the embodiment shown in FIG. 10. Outer conductor 148 is a 3A" Schedule 80 446 stainless steel pipe. In an embodiment, conductive layer 152 is placed between outer conductor 148 and jacket 154. Conductive layer 152 is a copper layer. Outer conductor 148 is clad with conductive layer 152. In certain embodiments, conductive layer 152 includes one or more segments (for example, conductive layer 152 includes one or more copper tube segments). Jacket 154 is a WA" Schedule 80 347H stainless steel pipe or a WA" Schedule 160347H stainless steel pipe. In an embodiment, inner conductor 144 is 4/0 MGT- 1000 furnace cable with stranded nickel-coated copper wire with layers of mica tape and glass fiber insulation. 4/0 MGT-1000 furnace cable is UL type 5107 (available from Allied Wire and Cable (Phoenixville, Pennsylvania)). Conductive section 150 couples inner conductor 144 and jacket 154. In an embodiment, conductive section 150 is copper. FIG. 13 depicts a cross-sectional representation of an embodiment of a temperature limited heater with an outer conductor. The outer conductor includes a ferromagnetic section and a non-ferromagnetic section. The heater is placed in a corrosion resistant jacket. A conductive layer is placed between the outer conductor and the jacke.t. FIGS. 14 and 15 depict transverse cross-sectional views of the embodiment shown in FIG. 13. Ferromagnetic section 140 is 409, 410, or 446 stainless steel with a thickness of 0.9 cm. Non-ferromagnetic section 142 is copper with a thickness of 0.9 cm. Ferromagnetic section 140 and non-ferromagnetic section 142 are placed in jacket 154. Jacket 154 is 304 stainless steel with a thickness of 0.1 cm. Conductive layer 152 is a copper layer. Electrical insulator 146 is silicon nitride, boron nitride, or magnesium oxide with a thickness of 0.1 to 0.3 cm. Inner conductor 144 is copper with a diameter of 1.0 cm. In an embodiment, ferromagnetic section 140 is 446 stainless steel with a thickness of 0.9 cm. Jacket 154 is 410 stainless steel with a thickness of 0.6 cm. 410 stainless steel has a higher Curie temperature than 446 stainless steel. Such a temperature limited heater may "contain" current such that the current does not easily flow from the heater to the surrounding formation and/or to any surrounding water (for example, brine, groundwater, or formation water). In this embodiment, a majority of the current flows through ferromagnetic section 140 until the Curie temperature of the ferromagnetic section is reached. After the Curie temperature of ferromagnetic section 140 is reached, a majority of the current flows through conductive layer 152. The ferromagnetic properties of jacket 154 (410 stainless steel) inhibit the current from flowing outside the jacket and "contain" the current. Jacket 154 may also have a thickness that provides strength to the temperature limited heater. FIG. 16A and FIG. 16B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic inner conductor. Inner conductor 144 is a 1" Schedule XXS 446 stainless steel pipe. In some embodiments, inner conductor 144 includes 409 stainless steel, 410 stainless steel, Invar 36, alloy 42-6, alloy 52, or other ferromagnetic materials. Inner conductor 144 has a diameter of 2.5 cm. Electrical insulator 146 is silicon nitride, boron nitride, magnesium oxide, polymers, Nextel ceramic fiber, mica, or glass fibers. Outer conductor 148 is copper or any other non-ferromagnetic material such as aluminum. Outer conductor 148 is coupled to jacket 154. Jacket 154 is 304H, 316H, or 347H stainless steel. In this embodiment, a majority of the heat is produced in inner conductor 144. FIG. 17A and FIG. 17B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic inner conductor and a non-ferromagnetic core. Inner conductor 144 includes 446 stainless steel, 409 stainless steel, 410 stainless steel or other ferromagnetic materials. Core 168 is tightly bonded inside inner conductor 144. Core 168 is a rod of copper or other non-ferromagnetic material. Core 168 is inserted as a tight fit inside inner conductor 144 before a drawing operation. In some embodiments, core 168 and inner conductor 144 are coextrusion bonded. Outer conductor 148 is 347H stainless steel. A drawing or rolling operation to compact electrical insulator 146 may ensure good electrical contact between inner conductor 144 and core 168. In this embodiment, heat is produced primarily in inner conductor 144 until the Curie temperature is approached. Resistance then decreases sharply as alternating current penetrates core 168. FIG. 18A and FIG. 18B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. Inner conductor 144 is nickel-clad copper. Electrical insulator 146 is silicon nitride, boron nitride, or magnesium oxide. Outer conductor 148 is a 1" Schedule XXS carbon steel pipe. In this embodiment, heat is produced primarily in outer conductor 148, resulting in a small temperature differential across electrical insulator 146. FIG. 19A and FIG. 19B depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor that is clad with a corrosion resistant alloy. Inner conductor 144 is copper. Outer conductor 148 is a 1" Schedule XXS 446 stainless steel pipe. Outer conductor 148 is coupled to jacket 154. Jacket 154 is made of corrosion resistant material (for example, 347H stainless steel). Jacket 154 provides protection from corrosive fluids in the wellbore (for example, sulfidizing and carburizing gases). Heat is produced primarily in outer conductor 148, resulting in a small temperature differential across electrical insulator 146. FIG. 2OA and FIG. 2OB depict cross-sectional representations of an embodiment of a temperature limited heater with a ferromagnetic outer conductor. The outer conductor is clad with a conductive layer and a corrosion resistant alloy. Inner conductor 144 is copper. Electrical insulator 146 is silicon nitride, boron nitride, or magnesium oxide. Outer conductor 148 is a 1" Schedule 80 446 stainless steel pipe. Outer conductor 148 is coupled to jacket 154. Jacket 154 is made from corrosion resistant material. In an embodiment, conductive layer 152 is placed between outer conductor 148 and jacket 154. Conductive layer 152 is a copper layer. Heat is produced primarily in outer conductor 148, resulting in a small temperature differential across electrical insulator 146. Conductive layer 152 allows a sharp decrease in the resistance of outer conductor 148 as the outer conductor approaches the Curie temperature. Jacket 154 provides protection from corrosive fluids in the wellbore. In some embodiments, the conductor (for example, an inner conductor, an outer conductor, or a ferromagnetic conductor) is the composite conductor that includes two or more different materials. In certain embodiments, the composite conductor includes two or more ferromagnetic materials. In some embodiments, the composite ferromagnetic conductor includes two or more radially disposed materials. In certain embodiments, the composite conductor includes a ferromagnetic conductor and a non-ferromagnetic conductor. In some embodiments, the composite conductor includes the ferromagnetic conductor placed over a non-ferromagnetic core. Two or more materials may be used to obtain a relatively flat electrical resistivity versus temperature profile in a temperature region below the Curie temperature and/or a sharp decrease (a high turndown ratio) in the electrical resistivity at or near the Curie temperature. In some cases, two or more materials are used to provide more than one Curie temperature for the temperature limited heater. The composite electrical conductor may be used as the conductor in any electrical heater embodiment described herein. For example, the composite conductor may be used as the conductor in a conductor-in-conduit heater or an insulated conductor heater. In certain embodiments, the composite conductor may be coupled to a support member such as a support conductor. The support member may be used to provide support to the composite conductor so that the composite conductor is not relied upon for strength at or near the Curie temperature. The support member may be useful for heaters of lengths of at least 100 m. The support member may be a non- ferromagnetic member that has good high temperature creep strength. Examples of materials that are used for a support member include, but are not limited to, Haynes® 625 alloy and Haynes® HRl 20® alloy (Haynes International, Kokomo, IN), NF709, Incoloy® 800H alloy and 347H alloy (Allegheny Ludlum Corp., Pittsburgh, PA). In some embodiments, materials in a composite conductor are directly coupled (for example, brazed, metallurgically bonded, or swaged) to each other and/or the support member. Using a support member may decouple the ferromagnetic member from having to provide support for the temperature limited heater, especially at or near the Curie temperature. Thus, the temperature limited heater may be designed with more flexibility in the selection of ferromagnetic materials. FIG. 21 depicts a cross-sectional representation of an embodiment of the composite conductor with the support member. Core 168 is surrounded by ferromagnetic conductor 166 and support member 172. In some embodiments, core 168, ferromagnetic conductor 166, and support member 172 are directly coupled (for example, brazed together or metallurgically bonded together). In one embodiment, core 168 is copper, ferromagnetic conductor 166 is 446 stainless steel, and support member 172 is 347H alloy. In certain embodiments, support member 172 is a Schedule 80 pipe. Support member 172 surrounds the composite conductor having ferromagnetic conductor 166 and core 168. Ferromagnetic conductor 166 and core 168 are joined to form the composite conductor by, for example, a coextrusion process. For example, the composite conductor is a 1.9 cm outside diameter 446 stainless steel ferromagnetic conductor surrounding a 0.95 cm diameter copper core. This composite conductor inside a 1.9 cm Schedule 80 support member produces a turndown ratio of 1.7. In certain embodiments, the diameter of core 168 is adjusted relative to a constant outside diameter of ferromagnetic conductor 166 to adjust the turndown ratio of the temperature limited heater. For example, the diameter of core 168 maybe increased to 1.14 cm while maintaining the outside diameter of ferromagnetic conductor 166 at 1.9 cm to increase the turndown ratio of the heater to 2.2. In some embodiments, conductors (for example, core 168 and ferromagnetic conductor 166) in the composite conductor are separated by support member 172. FIG. 22 depicts a cross-sectional representation of an embodiment of the composite conductor with support member 172 separating the conductors. In one embodiment, core 168 is copper with a diameter of 0.95 cm, support member 172 is 347H alloy with an outside diameter of 1.9 cm, and ferromagnetic conductor 166 is 446 stainless steel with an outside diameter of 2.7 cm. Such a conductor produces a turndown ratio of at least 3. The support member depicted in FIG. 22 has a higher creep strength relative to other support members depicted in FIGS. 21, 23, and 24. In certain embodiments, support member 172 is located inside the composite conductor. FIG. 23 depicts a cross-sectional representation of an embodiment of the composite conductor surrounding support member 172. Support member 172 is made of 347H alloy. Inner conductor 144 is copper. Ferromagnetic conductor 166 is 446 stainless steel. In one embodiment, support member 172 is 1.25 cm diameter 347H alloy, inner conductor 144 is 1.9 cm outside diameter copper, and ferromagnetic conductor 166 is 2.7 cm outside diameter 446 stainless steel. Such a conductor produces a turndown ratio larger than 3, and the turndown ratio is higher than the turndown ratio for the embodiments depicted in FIGS. 21, 22, and 24 for the same outside diameter. In some embodiments, the thickness of inner conductor 144, which is copper, is reduced to reduce the turndown ratio. For example, the diameter of support member 172 is increased to 1.6 cm while maintaining the outside diameter of inner conductor 144 at 1,9 cm to reduce the thickness of the conduit. This reduction in thickness of inner conductor 144 results in a decreased turndown ratio relative to the thicker inner conductor embodiment The turndown ratio, however, remains at least 3. In one embodiment, support member 172 is a conduit (or pipe) inside inner conductor 144 and ferromagnetic conductor 166. FIG. 24 depicts a cross-sectional representation of an embodiment of the composite conductor surrounding support member 172. In one embodiment, support member 172 is 347H alloy with a 0.63 cm diameter hole in its center. In some embodiments, support member 172 is a preformed conduit. In certain embodiments, support member 172 is formed by having a dissolvable material (for example, copper dissolvable by nitric acid) located inside the support member during formation of the composite conductor. The dissolvable material is dissolved to form the hole after the conductor is assembled. In an embodiment, support member 172 is 347H alloy with an inside diameter of 0.63 cm and an outside diameter of 1.6 cm, inner conductor 144 is copper with an outside diameter of 1.8 cm, and ferromagnetic conductor 166 is 446 stainless steel with an outside diameter of 2.7 cm. In certain embodiments, the composite electrical conductor is used as the conductor in the conductor-in- conduit heater. For example, the composite electrical conductor maybe used as conductor 174 in FIG. 25. FIG. 25 depicts a cross-sectional representation of an embodiment of the conductor-in-conduit heater. Conductor 174 is disposed in conduit 176. Conductor 174 is a rod or conduit of electrically conductive material. Low resistance sections 178 is present at both ends of conductor 174 to generate less heating in these sections. Low resistance section 178 is formed by having a greater cross-sectional area of conductor 174 in that section, or the sections are made of material having less resistance. In certain embodiments, low resistance section 178 includes a low resistance conductor coupled to conductor 174. Conduit 176 is made of an electrically conductive material. Conduit 176 is disposed in opening 180 in hydrocarbon layer 182. Opening 180 has a diameter able to accommodate conduit 176. Conductor 174 may be centered in conduit 176 by centralizers 184. Centralizers 184 electrically isolate conductor 174 from conduit 176. Centralizers 184 inhibit movement and properly locate conductor 174 in conduit 176. Centralizers 184 are made of ceramic material or a combination of ceramic and metallic materials. Centralizers 184 inhibit deformation of conductor 174 in conduit 176. Centralizers 184 are touching or spaced at intervals between approximately 0.1 m (meters) and approximately 3 m or more along conductor 174. A second low resistance section 178 of conductor 174 may couple conductor 174 to wellhead 112, as depicted in FIG. 25. Electrical current may be applied to conductor 174 from power cable 186 through low resistance section 178 of conductor 174. Electrical current passes from conductor 174 through sliding connector 188 to conduit 176. Conduit 176 may be electrically insulated from overburden casing 190 and from wellhead 112 to return electrical current to power cable 186. Heat may be generated in conductor 174 and conduit 176. The generated heat may radiate in conduit 176 and opening 180 to heat at least a portion of hydrocarbon layer 182. Overburden casing 190 maybe disposed in overburden 192. Overburden casing 190 is, in some embodiments, surrounded by materials (for example, reinforcing material and/or cement) that inhibit heating of overburden 192. Low resistance section 178 of conductor 174 may be placed in overburden casing 190. Low resistance section 178 of conductor 174 is made of, for example, carbon steel. Low resistance section 178 of conductor 174 may be centralized in overburden casing 190 using centralizers 184. Centralizers 184 are spaced at intervals of approximately 6 m to approximately 12 m or, for example, approximately 9 m along low resistance section 178 of conductor 174. In a heater embodiment, low resistance section 178 of conductor 174 is coupled to conductor 174 by one or more welds. In other heater embodiments, low resistance sections are threaded, threaded and welded, or otherwise coupled to the conductor. Low resistance section 178 generates little and/or no heat in overburden casing 190. Packing 194 may be placed between overburden casing 190 and opening 180. Packing 194 may be used as a cap at the junction of overburden 192 and hydrocarbon layer 182 to allow filling of materials in the annulus between overburden casing 190 and opening 180. In some embodiments, packing 194 inhibits fluid from flowing from opening 180 to surface 196. In certain embodiments, the composite electrical conductor may be used as a conductor in an insulated conductor heater. FIG. 26A and FIG. 26B depict an embodiment of the insulated conductor heater. Insulated conductor 200 includes core 168 and inner conductor 144. Core 168 and inner conductor 144 are a composite electrical conductor. Core 168 and inner conductor 144 are located within insulator 146. Core 168, inner conductor 144, and insulator 146 are located inside outer conductor 148. Insulator 146 is silicon nitride, boron nitride, • magnesium oxide, or another suitable electrical insulator. Outer conductor 148 is copper, steel, or any other electrical conductor. In some embodiments, jacket 154 is located outside outer conductor 148, as shown in FIG. 27A and FIG. 27B. In some embodiments, jacket 154 is 304 stainless steel and outer conductor 148 is copper. Jacket 154 provides corrosion resistance for the insulated conductor heater. In some embodiments, jacket 154 and outer conductor 148 are preformed strips that are drawn over insulator 146 to form insulated conductor 200. In certain embodiments, insulated conductor 200 is located in a conduit that provides protection (for example, corrosion and degradation protection) for the insulated conductor. In FIG. 28, insulated conductor 200 is located inside conduit 176 with gap 202 separating the insulated conductor from the conduit. For a temperature limited heater in which the ferromagnetic conductor provides a majority of the resistive heat output below the Curie temperature, a majority of the current flows through a material (the ferromagnetic material) that has highly non-linear functions of magnetic field (H) versus magnetic induction (B). These non-linear functions may cause strong inductive effects and distortion leading to a loss of power factor in the temperature limited heater at temperatures below the Curie temperature. These effects may render the temperature limited heater difficult to control and may result in additional current flow through surface and/or overburden power supply conductors. Expensive and/or difficult to implement control systems such as variable capacitors or modulated power supplies may be used to attempt to compensate for these effects and control temperature limited heaters where the majority of the resistive heat output is provided by current flow through the ferromagnetic material. In certain temperature limited heater embodiments, the ferromagnetic conductor confines a majority of the flow of electrical current to an outer electrical conductor (for example, a sheath, a jacket, a support member, a corrosion resistant member, or other electrically resistive member) coupled to the ferromagnetic conductor at temperatures below or near the Curie temperature of the ferromagnetic conductor. In some embodiments, the ferromagnetic conductor confines a majority of the flow of electrical current to another electrical conductor (for example, an inner conductor or an intermediate conductor (an electrical conductor between layers). The ferromagnetic conductor is located in the cross section of the temperature limited heater such that the magnetic properties of the ferromagnetic conductor at or below the Curie temperature of the ferromagnetic conductor confine the majority of the flow of electrical current to the outer electrical conductor. The majority of the flow of electrical current is confined to the outer electrical conductor due to the skin effect of the ferromagnetic conductor. Thus, the majority of the current is flowing through material having substantially linear resistive properties (for example, the outer electrical conductor) throughout most of the operating range of the heater. The ferromagnetic properties of the ferromagnetic conductor disappear above the Curie temperature, thus significantly reducing or eliminating inductive effects and/or distortion. The ferromagnetic conductor and the outer electrical conductor are located in the cross section of the temperature limited heater so that the skin effect of the ferromagnetic material limits the penetration depth of electrical current in the outer electrical conductor and the ferromagnetic conductor at temperatures below the Curie temperature of the ferromagnetic conductor. Thus, the outer electrical conductor provides a majority of the electrically resistive heat output of the temperature limited heater at temperatures up to a temperature at or near the Curie temperature of the ferromagnetic conductor. Because the majority of the current flows through the outer electrical conductor below the Curie temperature, the temperature limited heater has a resistance versus temperature profile that at least partially reflects the resistance versus temperature profile of the material in the outer electrical conductor. Thus, the resistance versus temperature profile of the temperature limited heater is substantially linear below the Curie temperature of the ferromagnetic conductor if the material in the outer electrical conductor has a linear resistance versus temperature profile. In certain embodiments, the material in the outer electrical conductor is selected so that the temperature limited heater has a desired resistance versus temperature profile below the Curie temperature of the ferromagnetic conductor. As the temperature of the temperature limited heater approaches or exceeds the Curie temperature of the ferromagnetic conductor, the reduction in the ferromagnetic properties of the ferromagnetic conductor allows electrical current to flow through a greater portion of the electrically conducting cross section of the temperature limited heater. Thus, the electrical resistance of the temperature limited heater is reduced and the temperature limited heater automatically provides reduced heat output at or near the Curie temperature of the ferromagnetic conductor. In certain embodiments, a highly electrically conductive member (for example, an inner conductor, a core, or another conductive member of, for example, copper or aluminum) is coupled to the ferromagnetic conductor and the outer electrical conductor to reduce the electrical resistance of the temperature limited heater at or above the Curie temperature of the ferromagnetic conductor. The ferromagnetic conductor that confines the majority of the flow of electrical current to the outer electrical conductor at temperatures below the Curie temperature may have a relatively small cross section compared to the ferromagnetic conductor in temperature limited heaters that use the ferromagnetic conductor to provide the majority of resistive heat output up to or near the Curie temperature. A temperature limited heater that uses the outer conductor to provide a majority of the resistive heat output below the Curie temperature has low magnetic inductance at temperatures below the Curie temperature because less current is flowing through the ferromagnetic conductor as compared to temperature limited heater where the majority of the resistive heat output below the Curie temperature is provided by the ferromagnetic material. Magnetic field (H) at radius (r) is proportional to the current (I) flowing through the ferromagnetic conductor and the core divided by the radius (r) of the ferromagnetic conductor: (3) H oc I/r. Since only a portion of the current flows through the ferromagnetic conductor for a temperature limited heater that uses the outer conductor to provide a majority of the resistive heat output below the Curie temperature, the magnetic field of the temperature limited heater may be significantly less than the magnetic field of the temperature limited heater where the majority of the current flows through the ferromagnetic material. At lower magnetic fields, relative magnetic permeability (μ) may be greater. The skin depth (δ) of the ferromagnetic conductor is inversely proportional to the square root of the relative magnetic permeability (μ.): (4) δ oc (l//0'/!. Increasing the relative magnetic permeability decreases the skin depth of the ferromagnetic conductor. However, because only a portion of the current flows through the ferromagnetic conductor for temperatures below the Curie temperature, the radius (or thickness) of the ferromagnetic conductor may be decreased for ferromagnetic materials with large relative magnetic permeabilities to compensate for the decreased skin depth while still allowing the skin effect to limit the penetration depth of the electrical current to the outer electrical conductor at temperatures below the Curie temperature of the ferromagnetic conductor. The radius (thickness) of the ferromagnetic conductor may be between 0.3 mm and 8 mm, between 0.3 mm and 2 mm, or between 2 mm and 4 mm depending on the relative magnetic permeability of the ferromagnetic conductor). Increasing the relative magnetic permeability of the ferromagnetic conductor provides a higher turndown ratio and a sharper decrease in electrical resistance for the temperature limited heater at or near the Curie temperature of the ferromagnetic conductor. Ferromagnetic materials (such as iron, iron-cobalt alloys, or low impurity carbon steel) with high relative magnetic permeabilities (for example, at least 200, at least 1000, at least 1 x 104, or at least 1 x 105) and/or high Curie temperatures (for example, at least 600 0C, at least 700 °C, or at least 800 0C) tend to have less corrosion resistance and/or less mechanical strength at high temperatures. The outer electrical conductor may provide corrosion resistance and/or high mechanical strength at the high temperatures for the temperature limited heater. Confining the majority of the flow of electrical current to the outer electrical conductor below the Curie temperature of the ferromagnetic conductor reduces variations in the power factor. Because only a portion of the electrical current flows through the ferromagnetic conductor below the Curie temperature, the non-linear ferromagnetic properties of the ferromagnetic conductor have little or no effect on the power factor of the temperature limited heater, except at or near the Curie temperature. Even at or near the Curie temperature, the effect on the power factor is reduced compared to temperature limited heaters in which the ferromagnetic conductor provides a majority of the resistive heat output below the Curie temperature. Thus, there is less or no need for external compensation (for example, variable capacitors or waveform modification) to adjust for changes in the inductive load of the temperature limited heater to maintain a relatively high power factor. In certain embodiments, the temperature limited heater, which confines the majority of the flow of electrical current to the outer electrical conductor below the Curie temperature of the ferromagnetic conductor, maintains the power factor above 0.85, above 0.9, or above 0.95 during use of the heater. Any reduction in the power factor occurs only in sections of the temperature limited heater at a temperature near the Curie temperature. Most sections of the temperature limited heater are typically not at or near the Curie temperature during use and these sections have a high power factor that approaches 1.0. Thus, the power factor for the entire temperature limited heater is maintained above 0,85, above 0.9, or above 0.95 during use of the heater even if some sections of the heater have power factors below 0.85. The highly electrically conductive member, or inner conductor, increases the turndown ratio of the temperature limited heater. In certain embodiments, thickness of the highly electrically conductive member is increased to increase the turndown ratio of the temperature limited heater. In some embodiments, ,the outer diameter of the outer electrical conductor is reduced to increase the turndown ratio of the temperature limited heater. In certain embodiments, the turndown ratio of the temperature limited heater is between 2 and 10, between 3 and 8, or between 4 and 6 (for example, the turndown ratio is at least 2, at least 3, or at least 4). FIG. 29 depicts an embodiment of a temperature limited heater in which the support member provides a majority of the heat output below the Curie temperature of the ferromagnetic conductor. Core 168 is an inner conductor of the temperature limited heater. In certain embodiments, core 168 is a highly electrically conductive material such as copper or aluminum. Ferromagnetic conductor 166 is a thin layer of ferromagnetic material between support member 172 and core 168. In certain embodiments, ferromagnetic conductor 166 is iron or an iron alloy. In some embodiments, ferromagnetic conductor 166 includes ferromagnetic material with a high relative magnetic permeability. For example, ferromagnetic conductor 166 may be purified iron such as Armco ingot iron (Armco, Brazil). Iron with some impurities typically has a relative magnetic permeability on the order of 400. Purifying the iron by annealing the iron in hydrogen gas (H2) at 1450 °C increases the relative magnetic permeability of the iron to a value on the order of 1 * 105. Increasing the relative magnetic permeability of ferromagnetic conductor 166 allows the thickness of the ferromagnetic conductor to be reduced. For example, the thickness of unpurified iron may be approximately 4.5 mm while the thickness of the purified iron is approximately 0.76 mm. In certain embodiments, support member 172 provides support for ferromagnetic conductor 166 and the temperature limited heater. Support member 172 may be made of a material that provides good mechanical strength at temperatures near or above the Curie temperature of ferromagnetic conductor 166. In certain embodiments, support member 172 is a corrosion resistant member. Support member 172 may both provide support for ferromagnetic conductor 166 and be corrosion resistant. Support member 172 is made from a material that provides electrically resistive heat output at temperatures up to and/or above the Curie temperature of ferromagnetic conductor 166. In an embodiment, support member 172 is 347H stainless steel. In some embodiments, support member 172 is another electrically conductive, good mechanical strength, corrosion resistant material. For example, support member 172 may be 304H, 316H, 347HH, NF709, Incoloy® 800H alloy (Inco Alloys International, Huntington, West Virginia), Haynes® HRl 20® alloy, or Inconel® 617 alloy. In some embodiments, support member 172 includes different alloys in portions of the temperature limited heater. For example, a lower portion of support member 172 may be 347H stainless steel and an upper portion of the support member is NF709. In certain embodiments, different alloys are used in different portions of the support member to increase the mechanical strength of the support member while maintaining desired heating properties for the temperature limited heater. In the embodiment depicted in FIG. 29, ferromagnetic conductor 166, support member 172, and core 168 are dimensioned so that the skin depth of the ferromagnetic conductor limits the penetration depth of the majority of the flow of electrical current to the support member when the temperature is below the Curie temperature of the ferromagnetic conductor. Thus, support member 172 provides a majority of the electrically resistive heat output of the temperature limited heater at temperatures up to a temperature at or near the Curie temperature of ferromagnetic conductor 166. In certain embodiments, the temperature limited heater depicted in FIG. 29 is smaller (for example, an outside diameter of 3 cm, 2.9 cm, 2.5 cm, or less) than other temperature limited heaters that do not use support member 172 to provide the majority of electrically resistive heat output. The temperature limited heater depicted in FIG. 29 may be smaller because ferromagnetic conductor 166 is thin as compared to the size of the ferromagnetic conductor needed for a temperature limited heater where the majority of the resistive heat output is provided by the ferromagnetic conductor. In some embodiments, the support member and the corrosion resistant member are different members in the temperature limited heater. FIGS. 30 and 31 depict embodiments of temperature limited heaters in which the jacket provides a majority of the heat output below the Curie temperature of the ferromagnetic conductor. Jacket 154 is a corrosion resistant member. Jacket 154, ferromagnetic conductor 166, support member 172, and core 168 (in FIG. 30) or inner conductor 144 (in FIG. 31) are dimensioned so that the skin depth of the ferromagnetic conductor limits the penetration depth of the majority of the flow of electrical current to the thickness of the jacket. In certain embodiments, jacket 154 is a material that is corrosion resistant and provides electrically resistive heat output below the Curie temperature of ferromagnetic conductor 166. For example, jacket 154 is 825 stainless steel, 446 stainless steel, or 347H stainless steel. In some embodiments, jacket 154 has a small thickness (for example, on the order of 0.5 mm). In FIG. 30, core 168 is highly electrically conductive material such as copper or aluminum. Support member 172 is 347H stainless steel or another material with good mechanical strength at or near the Curie temperature of ferromagnetic conductor 166. In FIG. 31, support member 172 is the core of the temperature limited heater and is 347H stainless steel or another material with good mechanical strength at or near the Curie temperature of ferromagnetic conductor 166. Inner conductor 144 is highly electrically conductive material such as copper or aluminum. In some embodiments, the temperature limited heater is used to achieve lower temperature heating (for example, for heating fluids in a production well, heating a surface pipeline, or reducing the viscosity of fluids in a wellbore or near wellbore region). Varying the ferromagnetic materials of the temperature limited heater allows for lower temperature heating. In some embodiments, the ferromagnetic conductor is made of material with a lower Curie temperature than that of 446 stainless steel. For example, the ferromagnetic conductor may be an alloy of iron and nickel. The alloy may have between 30% by weight and 42% by weight nickel with the rest being iron. In one embodiment, the alloy is Invar 36. Invar 36 is 36% by weight nickel in iron and has a Curie temperature of 277 0C. In some embodiments, an alloy is a three component alloy with, for example, chromium, nickel, and iron. For example, an alloy may have 6% by weight chromium, 42% by weight nickel, and 52% by weight iron. The ferromagnetic conductor made of these types of alloys provides a heat output between 250 watts per meter and 350 watts per meter. A 2.5 cm diameter rod of Invar 36 has a turndown ratio of approximately 2 to 1 at the Curie ' temperature. Placing the Invar 36 alloy over a copper core may allow for a smaller rod diameter. A copper core may result in a high turndown ratio. For temperature limited heaters that include a copper core or copper cladding, the copper may be protected with a relatively diffusion-resistant layer such as nickel. In some embodiments, the composite inner conductor includes iron clad over nickel clad over a copper core. The relatively diffusion-resistant layer inhibits migration of copper into other layers of the heater including, for example, an insulation layer. In some embodiments, the relatively impermeable layer inhibits deposition of copper in a wellbore during installation of the heater into the wellbore. The temperature limited heater may be a single-phase heater or a three-phase heater. In a three-phase heater embodiment, the temperature limited heater has a delta or a wye configuration. Each of the three ferromagnetic conductors in the three-phase heater may be inside a separate sheath. A connection between conductors may be made at the bottom of the heater inside a splice section. The three conductors may remain insulated from the sheath inside the splice section. In some three-phase heater embodiments, three ferromagnetic conductors are separated by insulation inside a common outer metal sheath. The three conductors may be insulated from the sheath or the three conductors may be connected to the sheath at the bottom of the heater assembly. In another embodiment, a single outer sheath or three outer sheaths are ferromagnetic conductors and the inner conductors may be non-ferromagnetic (for example, aluminum, copper, or a highly conductive alloy). Alternatively, each of the three non-ferromagnetic conductors are inside a separate ferromagnetic sheath, and a connection between the conductors is made at the bottom of the heater inside a splice section. The three conductors may remain insulated from the sheath inside the splice section. In some embodiments, the three-phase heater includes three legs that are located in separate wellbores. The legs may be coupled in a common contacting section (for example, a central wellbore, a connecting wellbore, or an solution filled contacting section). In an embodiment, the temperature limited heater includes a hollow core or hollow inner conductor. Layers forming the heater may be perforated to allow fluids from the wellbore (for example, formation fluids or water) to enter the hollow core. Fluids in the hollow core may be transported (for example, pumped or gas lifted) to the surface through the hollow core. In some embodiments, the temperature limited heater with the hollow core or the hollow inner conductor is used as a heater/production well or a production well. Fluids such as steam may be injected into the formation through the hollow inner conductor.

EXAMPLES Non-restrictive examples of temperature limited heaters and properties of temperature limited heaters are set forth below. A 6 foot temperature limited heater element was placed in a 6 foot 347H stainless steel canister. The heater element was connected to the canister in a series configuration. The heater element and canister were placed in an oven. The oven was used to raise the temperature of the heater element and the canister. At varying temperatures, a series of electrical currents were passed through the heater element and returned through the canister. The resistance of the heater element and the power factor of the heater element were determined from measurements during passing of the electrical currents. FIG. 32 depicts experimentally measured resistance versus temperature at several currents for a temperature limited heater with a copper core, a carbon steel ferromagnetic conductor, and a 347H stainless steel support member. The ferromagnetic conductor was a low-carbon carbon steel with a Curie temperature of 770 0C. The ferromagnetic conductor was sandwiched between the copper core and the 347H support member. The copper core had a diameter of 0.5". The ferromagnetic conductor had an outside diameter of 0.765". The support member had an outside diameter of 1.05". The canister was a 3" Schedule 160 347H stainless steel canister. Data 204 depicts resistance versus temperature for 300A at 60 Hz AC applied current. Data 206 depicts resistance versus temperature for 400A at 60 Hz AC applied current. Data 208 depicts resistance versus temperature for 500A at 60 Hz AC applied current. Curve 210 depicts resistance versus temperature for 1OA DC applied current. The resistance versus temperature curves show that the AC resistance of the temperature limited heater linearly increased up to a temperature near the Curie temperature of the ferromagnetic conductor. Near the Curie temperature, the AC resistance decreased rapidly until the AC resistance equaled the DC resistance above the Curie temperature. The linear dependence of the AC resistance below the Curie temperature at least partially reflects the linear dependence of the AC resistance of 347H at these temperatures. Thus, the linear dependence of the AC resistance below the Curie temperature indicates that the majority of the current is flowing through the 347H support member at these temperatures. FIG. 33 depicts experimentally measured resistance versus temperature at several currents for a temperature limited heater with a copper core, a cobalt-carbon steel ferromagnetic conductor, and a 347H stainless steel support member. The cobalt-carbon steel ferromagnetic conductor was a carbon steel conductor with 6% cobalt by weight and a Curie temperature of 843 °C. The ferromagnetic conductor was sandwiched between the copper core and the 347H support member. The copper core had a diameter of 0.465". The ferromagnetic conductor had an outside diameter of 0.765". The support member had an outside diameter of 1.05". The canister was a 3" Schedule 160 347H stainless steel canister. Data 212 depicts resistance versus temperature for IOOA at 60 Hz AC applied current. Data 214 depicts resistance versus temperature for 400A at 60 Hz AC applied current. Curve 216 depicts resistance versus temperature for 1OA DC. The AC resistance of this temperature limited heater turned down at a higher temperature than the previous temperature limited heater. This was due to the added cobalt increasing the Curie temperature of the ferromagnetic conductor. The AC resistance was substantially the same as the AC resistance of a tube of 347H steel having the dimensions of the support member. This indicates that the majority of the current is flowing through the 347H support member at these temperatures. The resistance curves in FIG. 33 are generally the same shape as the resistance curves in FIG. 32. FIG. 34 depicts experimentally measured power factor versus temperature at two AC currents for the temperature limited heater with the copper core, the cobalt-carbon steel ferromagnetic conductor, and the 347H stainless steel support member. Curve 218 depicts power factor versus temperature for IOOA at 60 Hz AC applied current. Curve 220 depicts power factor versus temperature for 400A at 60 Hz AC applied current. The power factor was close to unity (1) except for the region around the Curie temperature. In the region around the Curie temperature, the non-linear magnetic properties and a larger portion of the current flowing through the ferromagnetic conductor produce inductive effects and distortion in the heater and lower the power factor. FIG. 34 shows that the minimum value of the power factor for this heater remained above 0.85 at all temperatures in the experiment. Because only portions of the temperature limited heater used to heat a subsurface formation may be at the Curie temperature at any given point in time and the power factor for these portions does not go below 0.85 during use, the power factor for the entire temperature limited heater would remain above 0.85 (for example, above 0.9 or above 0.95) during use. From the data in the experiments for the temperature limited heater with the copper core, the cobalt-carbon steel ferromagnetic conductor, and the 347H stainless steel support member, the turndown ratio was calculated as a function of the maximum power delivered by the temperature limited heater. The results of these calculations are depicted in FIG. 35. The curve in FIG. 35 shows that the turndown ratio remains above 2 for heater powers up to approximately 2000 W/m. This curve is used to determine the ability of a heater to effectively provide heat output in a sustainable manner. A temperature limited heater with the curve similar to the curve in FIG. 35 would be able to provide sufficient heat outputs while maintaining temperature limiting properties that inhibit the heater from overheating or malfunctioning. FIG. 36 depicts temperature (°C) versus time (hrs) for a temperature limited heater. The temperature limited heater was a 1.83 m long heater that included a copper rod with a diameter of 1.3 cm inside a 2.5 cm Schedule XXH 410 stainless steel pipe and a 0.325 cm copper sheath. The heater was placed in an oven for heating. Alternating current was applied to the heater when the heater was in the oven. The current was increased over two hours and reached a relatively constant value of 400 amps for the remainder of the time. Temperature of the stainless steel pipe was measured at three points at 0.46 m intervals along the length of the heater. Curve 240 depicts the temperature of the pipe at a point 0.46 m inside the oven and closest to the lead-in portion of the heater. Curve 242 depicts the temperature of the pipe at a point 0.46 m from the end of the pipe and furthest from the lead- in portion of the heater. Curve 244 depicts the temperature of the pipe at about a center point of the heater. The point at the center of the heater was further enclosed in a 0.3 m section of 2.5 cm thick Fiberfrax® (Unifrax Corp., Niagara Falls, NY) insulation. The insulation was used to create a low thermal conductivity section on the heater (a section where heat transfer to the surroundings is slowed or inhibited (a "hot spot")). The temperature of the heater increased with time as shown by curves 244, 242, and 240. Curves 244, 242, and 240 show that the temperature of the heater increased to about the same value for all three points along the length of the heater. The resulting temperatures were substantially independent of the added Fiberfrax® insulation. Thus, the operating temperatures of the temperature limited heater were substantially the same despite the differences in thermal load (due to the insulation) at each of the three points along the length of the heater. Thus, the temperature limited heater did not exceed the selected temperature limit in the presence of a low thermal conductivity section. FIG. 37 depicts temperature (0C) versus log time (hrs) data for a 2.5 cm solid 410 stainless steel rod and a 2.5 cm solid 304 stainless steel rod. At a constant applied AC electrical current, the temperature of each rod increased with time. Curve 246 shows data for a thermocouple placed on an outer surface of the 304 stainless steel rod and under a layer of insulation. Curve 248 shows data for a thermocouple placed on an outer surface of the 304 stainless steel rod without a layer of insulation. Curve 250 shows data for a thermocouple placed on an outer surface of the 410 stainless steel rod and under a layer of insulation. Curve 252 shows data for a thermocouple placed on an outer surface of the 410 stainless steel rod without a layer of insulation. A comparison of the curves shows that the temperature of the 304 stainless steel rod (curves 246 and 248) increased more rapidly than the temperature of the 410 stainless steel rod (curves 250 and 252). The temperature of the 304 stainless steel rod (curves 246 and 248) also reached a higher value than the temperature of the 410 stainless steel rod (curves 250 and 252). The temperature difference between the non-insulated section of the 410 stainless steel rod (curve 252) and the insulated section of the 410 stainless steel rod (curve 250) was less than the temperature difference between the non-insulated section of the 304 stainless steel rod (curve 248) and the insulated section of the 304 stainless steel rod (curve 246). The temperature of the 304 stainless steel rod was increasing at the termination of the experiment (curves 246 and 248) while the temperature of the 410 stainless steel rod had leveled out (curves 250 and 252). Thus, the 410 stainless steel rod (the temperature limited heater) provided better temperature control than the 304 stainless steel rod (the non-temperature limited heater) in the presence of varying thermal loads (due to the insulation). A numerical simulation (FLUENT available from Fluent USA, Lebanon, NH) was used to compare operation of temperature limited heaters with three turndown ratios. The simulation was done for heaters in an oil shale formation (Green River oil shale). Simulation conditions were: 61 m length conductor-in-conduit Curie heaters (center conductor (2.54 cm diameter), conduit outer diameter 7.3 cm) downhole heater test field richness profile for an oil shale formation 16.5 cm (6.5 inch) diameter wellbores at 9.14 m spacing between wellbores on triangular spacing - 200 hours power ramp-up time to 820 watts/m initial heat injection rate constant current operation after ramp up Curie temperature of 720.6 0C for heater formation will swell and touch the heater canisters for oil shale richnesses at least 0.14 L/kg (35 gals/ton) FIG. 38 displays temperature (0C) of a center conductor of a conductor-in-conduit heater as a function of formation depth (m) for a temperature limited heater with a turndown ratio of 2: 1. Curves 254-276 depict temperature profiles in the formation at various times ranging from 8 days after the start of heating to 675 days after the start of heating (254: 8 days, 256: 50 days, 258: 91 days, 260: 133 days, 262: 216 days, 264: 300 days, 266: 383 days, 268: 466 days, 270: 550 days, 272: 591 days, 274: 633 days, 276: 675 days). At a turndown ratio of 2:1, the Curie temperature of 720.6 0C was exceeded after 466 days in the richest oil shale layers. FIG. 39 shows the corresponding heater heat flux (W/m) through the formation for a turndown ratio of 2: 1 along with the oil shale richness (l/kg) profile (curve 278). Curves 280-312 show the heat flux profiles at various times from 8 days after the start of heating to 633 days after the start of heating (280: 8 days; 282: 50 days; 284: 91 days; 286: 133 days; 288: 175 days; 290: 216 days; 292: 258 days; 294: 300 days; 296: 341 days; 298: 383 days; 300: 425 days; 302: 466 days; 304: 508 days; 306: 550 days; 308: 591 days; 310: 633 days; 312: 675 days). At a turndown ratio of 2:1, the center conductor temperature exceeded the Curie temperature in the richest oil shale layers. FIG. 40 displays heater temperature (0C) as a function of formation depth (m) for a turndown ratio of 3: 1. Curves 314-336 show temperature profiles through the formation at various times ranging from 12 days after the start of heating to 703 days after the start of heating (314: 12 days; 316: 33 days; 318: 62 days; 320: 102 days; 322: 146 days; 324: 205 days; 326: 271 days; 328: 354 days; 330: 467 days; 332: 605 days; 334: 662 days; 336: 703 days). At a turndown ratio of 3:1, the Curie temperature was approached after 703 days. FIG. 41 shows the corresponding heater heat flux (W/m) through the formation for a turndown ratio of 3:1 along with the oil shale richness (l/kg) profile (curve 338). Curves 340-360 show the heat flux profiles at various times from 12 days after the start of heating to 605 days after the start of heating (340: 12 days, 342: 32 days, 344: 62 days, 346: 102 days, 348: 146 days, 350: 205 days, 352: 271 days, 354: 354 days, 356: 467 days, 358: 605 days, 360: 749 days). The center conductor temperature never exceeded the Curie temperature for the turndown ratio of 3 : 1. The center conductor temperature also showed a relatively flat temperature profile for the 3: 1 turndown ratio. FIG. 42 shows heater temperature (0C) as a function of formation depth (m) for a turndown ratio of 4: 1. Curves 362-382 show temperature profiles through the formation at various times ranging from 12 days after the start of heating to 467 days after the start of heating (362: 12 days; 364: 33 days; 366: 62 days; 368: 102 days, 370: 147 days; 372: 205 days; 374: 272 days; 376: 354 days; 378: 467 days; 380: 606 days, 382: 678 days). At a turndown ratio of 4: 1, the Curie temperature was not exceeded even after 678 days. The center conductor temperature never exceeded the Curie temperature for the turndown ratio of 4: 1. The center conductor showed a temperature profile for the 4: 1 turndown ratio that was somewhat flatter than the temperature profile for the 3:1 turndown ratio. These simulations show that the heater temperature stays at or below the Curie temperature for a longer time at higher turndown ratios. For this oil shale richness profile, a turndown ratio of at least 3 : 1 may be desirable. Simulations have been performed to compare the use of temperature limited heaters and non-temperature limited heaters in an oil shale formation. Simulation data was produced for conductor-in-conduit heaters placed in 16.5 cm (6.5 inch) diameter wellbores with 12.2 m (40 feet) spacing between heaters a formation simulator (for example, STARS from Computer Modelling Group, LTD., Houston, TX), and a near wellbore simulator (for example, ABAQUS from ABAQUS, Inc., Providence, RI). Standard conductor-in-conduit heaters included 304 stainless steel conductors and conduits. Temperature limited conductor-in-conduit heaters included a metal with a Curie temperature of 760 0C for conductors and conduits. Results from the simulations are depicted in FIGS. 43-45. FIG. 43 depicts heater temperature (0C) at the conductor of a conductor-in-conduit heater versus depth (m) of the heater in the formation for a simulation after 20,000 hours of operation. Heater power was set at 820 watts/meter until 760 0C was reached, and the power was reduced to inhibit overheating. Curve 384 depicts the conductor temperature for standard conductor-in-conduit heaters. Curve 384 shows that a large variance in conductor temperature and a significant number of hot spots developed along the length of the conductor. The temperature of the conductor had a minimum value of 490 0C. Curve 386 depicts conductor temperature for temperature limited conductor-in-conduit heaters. As shown in FIG. 43, temperature distribution along the length of the conductor was more controlled for the temperature limited heaters. In addition, the operating temperature of the conductor was 730 °C for the temperature limited heaters. Thus, more heat input would be provided to the formation for a similar heater power using temperature limited heaters. FIG. 44 depicts heater heat flux (W/m) versus time (yrs) for the heaters used in the simulation for heating oil shale. Curve 388 depicts heat flux for standard conductor-in-conduit heaters. Curve 390 depicts heat flux for temperature limited conductor-in-conduit heaters. As shown in FIG. 44, heat flux for the temperature limited heaters was maintained at a higher value for a longer period of time than heat flux for standard heaters. The higher heat flux may provide more uniform and faster heating of the formation. FIG. 45 depicts cumulative heat input (kJ/m)(kilojoules per meter) versus time (yrs) for the heaters used in the simulation for heating oil shale. Curve 392 depicts cumulative heat input for standard conductor-in-conduit heaters. Curve 394 depicts cumulative heat input for temperature limited conductor-in-conduit heaters. As shown in FIG. 45, cumulative heat input for the temperature limited heaters increased faster than cumulative heat input for standard heaters. The faster accumulation of heat in the formation using temperature limited heaters may decrease the time needed for retorting the formation. Onset of retorting of the oil shale formation may begin around an average cumulative heat input of 1.1 x 10s kJ/meter. This value of cumulative heat input is reached around 5 years for temperature limited heaters and between 9 and 10 years for standard heaters. Further modifications and alternative embodiments of various aspects of the invention maybe apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. In addition, it is to be understood that features described herein independently may, in certain embodiments, be combined.