Examples of thick film heaters are described in GB- A-2290210 and they generally comprise a substantially flat insulating substrate, typically in the form of a ceramic disc or metal plate with an insulating layer applied thereto. The insulating layer is typically of a glass, glass ceramic or ceramic (collectively referred to as ceramic hereafter). A resistive heating track is applied to, for example printed onto, the insulating substrate and in use this track carries a current which causes the track to heat. A protective overglaze is usually provided over the track, with apertures provided in suitable positions, for example to allow access to the track for the purposes of making electrical connections thereto.

The thick film heater may form a part of the wall of a liquid heating chamber, and in particular may form the base, or a part of the base, of a liquid heating vessel where it is usually arranged such that the resistive heating track is lowermost in the position of use of the vessel.

It is usual to provide an electric heater with an overheat protector, which deenergises the heater in the event that the heater overheats, such as may occur when the heater is energised in the absence of liquid in a liquid heating vessel ("dry switch-on") or if the vessel boils dry. An example of such a control is described in W095/34187.

In addition to an overheat protector, it is common, particularly in liquid heating vessels, to provide a temperature regulating control, which, for example, deenergises the heater when the liquid in the vessel boils or maintains the liquid in the vessel at a desired temperature.

In known controls, the operative components are mounted in accurately determined positions in a moulded plastics housing which is mounted against the heater so that in the final assembly of the apparatus the temperature-sensitive components, and possibly also current carrying components of the control engage the appropriate locations on the heater. For example, both the heater and the control may be commonly mounted to the body of a liquid heating vessel. The operating temperatures which must be endured by the plastics moulding are potentially very high, particularly in a fault condition. This limits the choice of materials available, and those suitable for operation at the necessary temperatures are expensive. Furthermore, the moulding requires careful design to maintain the operating tolerances of the various control components under the operating temperatures discussed above. Thus the design of such mouldings is highly skilled requiring such complex techniques as mould flow analysis, in order to achieve the required degree of accuracy while allowing for the thermal expansion of the plastics moulding. Such design processes are expensive.

The present invention seeks to improve upon known heater arrangements by providing a heater assembly which does not suffer from the above drawbacks and which is simpler to design and manufacture.

Thus, from a first aspect the present invention provides a heater assembly comprising: -an electric heater having a resistive heating track provided on an insulating substrate; and -a support for an operative member of a control

for the heater bonded to the heater.

Thus according to the invention, a support member for an operative member of the control is bonded to the heater and the operative member of the control can therefore be mounted to the support rather than to a plastics control housing. This greatly simplifies the considerations involved in the design of the control, and effectively does away with the need for a moulded housing for mounting the various operative components of the control by using the heater itself to provide support for the components.

Thus in broad terms, the invention provides an integrated heater and control assembly comprising: an electric heater having a resistive heating track provided on an insulating substrate; a thermally sensitive control for the heater, the components of the control being mounted to and supported directly by the heater.

The operative control member may, for example be an actuator, for example a bimetallic actuator or a movable member of the control, for example a trip lever or the like as will be described further below.

Advantageously, the support member and/or the control member may be made of metal, in which case these components may be manufactured easily and accurately as metal pressings. Metal components can operate continuously at higher temperatures than plastics materials and are less expensive to produce. When using metallic supports, the components can be positioned on an insulated portion of the substrate, which will then act not only to insulate the heating track from the substrate material but also the control components. Thus the heater provides both the physical support and electrical insulation previously provided by the control housing.

The support member may be bonded to the heater by soldering, brazing or welding, with epoxy resin or by

any other suitable method. The material used will to some extent depend on the temperature the heater will experience in use, and the melting point of the material being bonded to the heater. The preferred method of bonding, for reasons primarily of cost, simplicity and speed is soldering, provided the expected operating temperature of the heater is not too high. Solders are typically Pb, Sn and Zn alloys which melt at between 170° and 350°C, so provided heater temperatures will not approach the melting point of the solder (even in a fault condition) solder may be used. The solder may be printed on to the heater in the desired positions after the insulation and heating track have been printed and fired, and the component (s) positioned on the solder by suitable means, and the heater then passed through an oven to melt the solder. After cooling, the component is positioned firmly in place.

The use of a solder or braze as a bonding means in this context is though to be novel and advantageous, so from a further aspect, the invention provides a method of manufacturing a thick film electric heater comprising depositing solder or braze material in desired positions onto the heater, positioning the components to be mounted to the heater on said deposited material and heating and then solidifying the material to bond the components in position. When soldering is used as a bonding technique, it has been found advantageous to apply and fire a layer of a metal loaded ink, most preferably a silver loaded ink, to the area of the heater intended to receive the solder. This is particularly so when applying the solder to an insulating, e. g. a ceramic layer of the heater, as it provides a key for the solder which would otherwise not adhere successfully to the layer. This layer may be applied at the same time as contact pads or other silver components are being printed on the heater.

The material of the heater itself may be used as a

bonding medium. Accordingly, the support member may be bonded to the heater with ceramic. Thus in one method of manufacture, the support member can be positioned on a ceramic layer (eg an overglaze layer) prior to firing the latter and the firing of the ceramic forming the bond between the support member and the heater may take place at the same time as the layer itself is fired.

The portions of the support member that are bonded to the heater may be perforated, expanded or otherwise treated to increase the surface area presented to the bonding medium.

The support for the control member may be a portion of the member itself, eg one end of a leaf spring.

Preferably, however, the support is provided with a mounting location for the control member. That member can be mounted directly or indirectly to the support in any suitable manner, for example through a push on connector, but preferably it is pivotally mounted thereto, through a pivot provided on the support. The pivot may be in the form of at least one notch for engagement with a knife edge provided on the operative or movable member.

The support member may also have a reaction location for spring means arranged to bias the control member in a desired direction or position. The spring means may, for example, be arranged solely to bias the control member towards the heater, but preferably it constitutes a bistable, overcentre mechanism for biasing the control member either towards or away from the heater. To this end the resilient member may be a C- spring or an omega-spring mounted between the support member and the control member. The control member may, therefore be a bistable trip lever operating to open a set of contacts in the control in response to operation of a thermally sensitive actuator.

Thus, the control member is preferably arranged to co-operate with a thermally-sensitive actuator to open a

set of switch contacts in the electrical supply to the heater. The control member may carry the thermally- sensitive actuator, but preferably the thermally- sensitive actuator is separately mounted to the heater, and the control member is biased into engagement with the thermally-sensitive actuator, at least when the switch contacts are closed. The thermally-sensitive actuator is preferably a bimetallic actuator, most preferably a snap-acting bimetallic actuator for example of the type disclosed in GB1542252.

In one embodiment, the thermally-sensitive actuator may be arranged as an overheat protector in thermal contact with the heater such that it operates in the event that the heater overheats to cause the switch contacts to open and interrupt the supply of current to the heater.

In another embodiment, the thermally-sensitive actuator may be arranged to operate in the event that the liquid in the vessel reaches a predetermined temperature, in particular the boiling temperature of the liquid in the vessel. Such a thermally-sensitive actuator may be arranged proximate an outlet of a duct which directs vapour from the boiling liquid onto the actuator causing it to operate when the liquid boils.

However, in such an arrangement the inlet of the duct must be located towards the top of the vessel in order to receive the vapour from the boiling liquid, and this complicates the overall assembly of the vessel. Thus, in a preferred arrangement the heater assembly is provided with a sump as described in WO 97/04694, which can be positioned within the footprint of the heater and does not require any external vapour connection. In one arrangement, therefore, the thermally-sensitive actuator is in thermal contact with a sump, such that the actuator operates when the liquid in the vessel reaches a predetermined temperature. Advantageously, the sump is bonded to the heater by one of the techniques

described above. methods are novel and inventive in this context, independently of the other aspects of the invention, so from a yet further aspect, the invention provides a heater having a sump bonded thereto by one of the aforementioned methods.

In a preferred embodiment, the heater assembly is provided with two thermally-sensitive actuators, one actuator arranged as an overheat protector in thermal contact with the heater such that it operates in the event that the heater reaches a temperature indicative of a fault condition, and the other actuator arranged to operate in the event that the liquid in the vessel reaches a predetermined temperature, in particular the boiling temperature of the liquid in the vessel. Both actuators may co-operate with a single movable control member, to open the switch contacts when either actuator operates. However, a respective control member is preferably provided for each actuator, which control members may be associated with respective sets of switch contacts. In this case, each control member may be mounted from the same support member, or from respective separate supports.

The (or each) control member may be arranged to open the switch contacts indirectly, for example by acting on a resilient member mounting at least one switch contact. Preferably, however, the control member carries at least one switch contact and more preferably at least one co-operating switch contact is provided directly on the heater surface, for example bonded thereto. Most preferably, the heater contact is bonded in a suitable manner to a terminal portion of the heater track, thereby avoiding the need for a separate mounting and electrical connection to the heater contact. In one embodiment, two switch contacts are provided on the heater surface, each contact terminating a separate conductive path on the heater and the movable member carries a bridging member provided with two

complementary switch contacts, which are arranged to contact the switch contacts on the heater so that the bridging member electrically connects the separate conductive paths. According to this preferred arrangement the conductive paths to be connected are both provided on the heater surface and it is not therefore necessary for additional wires or leaf spring conductors to be used to provide a conductive connection to one of the switch contacts. This avoids the need for a secondary fixing operation to the insulating layer, thereby helping to avoid damage to that layer. It also greatly simplifies the electrical arrangement of the heater assembly and is itself considered to be a novel and inventive arrangement, which is of more general application than in the particular arrangements described above.

Thus, from a further aspect, the invention provides a heater assembly comprising: -an electric heater having a resistive heating track provided on an insulating substrate; -two switch contacts provided on the heater surface, each contact terminating a separate conductive path on the heater; and -an electrically conductive bridge having two complementary switch contacts, the bridge being mounted for movement towards and away from the heater, so as selectively to bridge the switch contacts on the heater and electrically connect the respective separate conductive paths or to open the electrical circuit between the paths.

From a yet further aspect, the invention provides a heater assembly comprising: -an electric heater having a resistive heating track provided on an insulating substrate, said resistive track having a permanent electrical connection to a power supply at its respective ends; -an interruption in said heating track, with

switch contacts provided at either side of the interruption; and -an electrically conductive bridge selectively movable into and out of engagement with said switch contacts for switching said track on or off.

The bridge may be provided by a bridging member mounted on a movable carrier, most preferably a pivotally mounted carrier. In the preferred embodiment, the movable carrier is the movable control member mentioned above which is mounted to a support bonded to the heater. The bridging member is most preferably mounted so as to be able to equalise the contact pressures between the respective sets of contacts.

Accordingly, the bridging member is preferably mounted so as to be able to rock with respect to its carrier.

The carrier may be made of a conductive, for example a metallic, material. In this case, when the bridging member connects the conductive paths, current may be conducted through the bridging member into the carrier allowing it to be provided with an electrical terminal, for example a spade terminal, for supplying current to an indicator arrangement, such as a neon bulb or light emitting diode (LED), when the bridging member connects the conductive paths. Of course, the terminal on the carrier could be used to supply current to any desired part of the liquid heating vessel, when the bridging member connects the conductive paths. With such an arrangement, the need for additional electrical connectors to indicators and the like is obviated.

The heater switch contacts may be in the form of a pad of metallic or metal-loaded, for example silver, material applied to the insulating substrate.

Preferably, however, the switch contacts are discrete contact members, e. g silver contacts, bonded onto terminal portions of the track.

The conductive paths referred to above may be applied to, for example printed on, the insulating

substrate of the heater in the form of a conductive track. The conductive paths may be of a resistive heating material or a low resistance material such as silver acting only as a current conductor. In a preferred arrangement, a low resistance conductive path leads from a mains current supply location to its switch contact and the other conductive path leads from its switch contact to, or forms at least part of the resistive heating track proper, and the lifting of the bridging member acts to break the power supply to the heating track.

In a particularly preferred arrangement, at least one terminal member of a mains electrical connector is bonded to the heater. Thus, a conductive track may be provided on the heater leading from one terminal of the mains connector to a switch contact or to the resistive heating track, for example, without the need for wires, strips or other conductive members running from the mains connector to the heater and without the need then to connect those conductors to the heater. This represents a significant saving in terms of material costs. Furthermore, the heater provides a mechanical support for the terminals of the mains connector, thereby obviating the need for a plastics moulding which has traditionally provided the mechanical support for the mains connector terminals.

It will be appreciated, therefore, that this arrangement is in itself novel and inventive and thus from a further aspect the invention provides a heater assembly comprising: -an electric heater having a resistive heating track provided on an insulating substrate; and -terminal means of an external power supply connector bonded to the heater.

If the substrate of the heater is sufficiently electrically insulated from the liquid in the vessel, only two mains terminals, i. e line and neutral, are

required. However, if an earth terminal is required, this may also be bonded to the heater.

Preferably, conductive portions are also provided on the insulating substrate to connect the terminals of the mains connector, the switch contacts and the resistive heating track.

The mains terminal (s) may be bonded to the heater by any of the methods described earlier. The bonding material is preferably solder or a conductive epoxy.

This is particularly so where any of the terminals is of brass, which would not be capable of withstanding the temperatures which would be experienced during firing of a ceramic bonding layer.

Where the heater has a metallic substrate, the earth terminal is preferably bonded directly to that substrate. The substrate is typically of stainless or other steel, and the desired mounting locations for the earth terminal can be printed with a silver loaded ink prior to printing the insulating layers which is then apertured in the desired area. The rest of the heater can then be produced, in the normal manner, by printing and firing insulating, conductive and resistive layers.

A bonding layer of e. g. silver loaded ink can then be applied to areas of the heater to receive components, e. g. contact members, terminals and supports and then fired. Solder is then printed onto those areas, the various components positioned and the solder melted to bond the components in position. This is extremely efficient as all the relevant components are bonded to the heater in a single operation.

The portions of the terminals and indeed other components that are bonded to the heater may be perforated, expanded or otherwise treated to increase the surface area presented to the bonding medium.

The mains terminals may be in the form of pins for a standard IEE appliance connection and may be angled or shaped so as to allow connection of a complementary

connector in a direction parallel to the plane of the heater. Preferably, however, the terminals are those of a"cordless"connector, i. e. a connector that mates with a complementary connector provided on a base as the liquid heating vessel is located on the base. The mains connector terminals may be bonded to the heater anywhere within the heater footprint and the location may be determined by reference to the location at which the mains connector terminals are required to exit the vessel housing.

In a particularly preferred embodiment, the terminals are those of a cordless connector of a type as described for example in WO 95/08204, that mates with a complementary base connector irrespective of the relative angular orientation of the vessel and base about a vertical axis.

Thus in the preferred embodiment, the terminals comprise a central terminal pin with two concentric terminal rings arranged therearound. The line and neutral electrical connections are to the central pin and inner ring, and an earth connection is made to the outer ring. The concentric rings may be provided with cut-outs to allow the conductive tracks to run from the pin and inner ring to the rest of the heating track without danger of shorting.

If necessary, the earth terminal member may be bonded to the substrate by welding, more particularly laser welding to give a stronger bond.

It will be appreciated that the terminals, for example the outer earth ring may be used to mount other components of the control, with suitable electrical insulation therebetween if necessary and so perform a dual function.

The resistive heating track may be arranged in any suitable manner. Advantageously, the track may be arranged as a series of discrete lengths of track material joined by bridges of a material of higher

conductivity, for example silver, or a silver-loaded material. The conductive bridges may be arranged to circumvent obstacles on the heater, for example the portions of the support member or the mains connector that are bonded to the heater. This is particularly important when such obstacles are metallic as sufficient clearance must be given between the obstacles and the track to prevent any risk of an undesired electrical connection between them. In this way, the heated area of the heater can be maximised.

The insulating substrate may take any suitable form. Thus, for example, the insulating substrate may comprise a metallic plate, for example of stainless steel or mild steel provided with an insulating layer of ceramic (as herein defined) on which is provided the resistive heating track. The liquid-facing side of the metallic plate may also be provided with a layer of ceramic or other material to electrically insulate it from the liquid and/or to protect it from the corrosive effects of the liquid.

Preferably, the insulating substrate is substantially flat, as this allows easy manufacture of the substrate and easy application of the resistive heating track. In addition, where the insulating substrate forms the base or part of the base of a liquid heating vessel, a flat base is desirable from an aesthetic point of view. In this case, attachment means is required to attach the heater to the vessel.

Preferably, the attachment means are provided on a flange bonded to and depending from a peripheral region of the heater. The flange is bonded to the heater after the various insulating layers, resistive heating tracks and so on have been applied to substrate by conventional means. The flange may be bonded to the heater by any of the methods discussed above, preferably soldering. Most preferably, this is done at the same time as all other components are being bonded to the heater. The

provision of a flange around the periphery of the heater adds considerable strength to the heater which means that the metal plate of the heater can be made of a thinner, and thus less expensive, material.

This arrangement in itself is novel and so from a further aspect the present invention provides a thick film electric heater for liquid heating apparatus comprising a generally planar body with a flange bonded to and depending from a peripheral region of the planar body.

Most preferably the flange is welded, more preferably laser welded to the planar body. Thus when the planar body comprises a metal, e. g. stainless steel, plate the flange can be welded directly thereto. The flange material, in such cases, should be such as to permit welding.

In liquid heating applications, a liquid-tight seal needs to be provided between the wall of a liquid receiving chamber and the heater. To this end, resilient sealing means may be clamped between the peripheral portion of the heater and a face of the chamber wall by clamping means associated with the flange.

Advantageously, the flange is L-shaped in section and one limb of the L-shape is bonded to the heater.

Preferably, an edge of the wall of the liquid heating chamber or vessel is received between the one limb of the L-shaped flange portion and the clamping means, so that the compressive forces acting on the sealing member are exerted substantially by the flange and substantial pressure is not applied to the bond between the flange and the heater by the resilient sealing member.

The flange may be made from a strip of material formed into a cylinder with its end joined. The flange may be formed by forming a series of cuts in one edge of the strip, which after forming can be bent over to form a number of tabs which together form the flange.

The clamping means may be, for example, a bayonet ring engaging a complementary formation on the flange.

Preferably, however, the clamping means comprises at least one retaining projection provided at a distal region of the flange and a locking ring dimensioned substantially to circumscribe the flange and for engaging the vessel body, the locking ring being urged towards the edge of the vessel by the retaining projection.

This arrangement is in itself novel and inventive and of application to any kind of heater which forms part of the wall of a liquid heating chamber. Thus from a yet further aspect, the present invention provides an electric liquid heating apparatus comprising: -a liquid heating chamber; -a planar heater; -a flange depending from a peripheral region of the heater, the flange having at least one clamping projection at a distal region thereof; -a locking ring dimensioned substantially to circumscribe the flange; and -a resilient sealing member arranged between the locking ring and the periphery of the heater; and wherein, an edge of the wall of the liquid heating chamber is received between the sealing member and the locking ring.

Thus according to this arrangement the sealing member is confined between the periphery of the heater, the depending flange and the edge of the wall of the liquid heating chamber, e. g. the liquid heating vessel, such that the sealing member deforms to provide a reliable seal. The locking ring and the clamping projection provide a quick and simple means for urging the edge of the liquid heating vessel towards the periphery of the heater.

The locking ring, sealing member and flange will generally follow the contour of the periphery of the

heater and may therefore take any shape, in particular circular, square or elliptical. The locking ring is preferably in the form of a split ring such that it can expand to fit over the clamping projection during assembly. Spacer rings may be provided between the locking ring and the vessel edge or between the sealing member and the vessel edge in order to accommodate different thicknesses of vessel material, for example glass, plastics or stainless steel.

The clamping projection is preferably formed integrally with the flange by deformation thereof.

Preferably, the clamping projection is angled radially outwardly towards the heater so as to facilitate the positioning of the locking ring thereon. There may be a single retaining projection extending about the entire distal region of the flange. Preferably however, the flange has a plurality of discrete projections spaced around its outer surface.

The edge of the liquid heating chamber may be arranged to be downwardly stepped in order that the upper surface of the heater lies flush with an inner surface of the chamber in the position of use.

In either event, the invention further provides a liquid heating apparatus comprising a liquid receiving chamber, and a planar thick film heater mounted in an opening in a wall of the chamber, the heater lying flush with the surrounding chamber wall.

In the European market, liquid heating vessels, for example kettles, are usually arranged to the liquid they contain, usually water, to the boil and then to turn themselves off automatically. It is often the case that the user will turn on the liquid heating vessel and leave it unattended, returning after the vessel has turned itself off. The user, who requires boiling water, will then turn the vessel back on and allow the pre-heated liquid to boil before dispensing it. Such an arrangement is not altogether satisfactory in terms of

energy usage and user time.

According to a further aspect of the present invention there is provided an electric heater for liquid heating apparatus comprising a primary heating means arranged to be energised on initial energisation of the heater for heating the liquid in the apparatus to a predetermined temperature and a secondary heating means energised after that temperature has been detected, said secondary heating means having a power output generally equal to the rate of heat loss from the liquid heating apparatus when the liquid therein is at the predetermined temperature.

Thus according to this aspect of the invention, the liquid in the heating apparatus can be brought to the boil, or to another predetermined temperature, by primary heating means. Once the predetermined temperature is reached, the liquid is maintained at the predetermined temperature purely by the secondary heating means, rather than using any form of thermostatic control. The heat required to maintain the liquid at the desired temperature can be determined empirically, and typically may be 25-30W. In this way, once the apparatus is initially activated by a user and the liquid has reached the predetermined temperature, the liquid will remain at this temperature.

The heater may be a thick film heater, with the respective heating means being in the form of resistive heating tracks. The resistive heating tracks making up the primary and secondary heating means are preferably of different materials, more particularly having different resistivities.

Detection of the predetermined temperature may be achieved by a mechanical thermally-sensitive actuator, which selectively switches the respective heater tracks.

The secondary heating means may be completely separately energisable from the primary heating means.

Alternatively, the secondary heating means may be

arranged to be permanently energised when the heater is activated, for example in parallel or series with the primary heating means.

In a preferred arrangement, the primary heating means forms a part of the secondary heating means, which comprises an additional portion of resistive heating track of high resistance relative to the main heating element and in series with the main heating element.

When current is directed through the main heating element and the additional portion, the current level is reduced relative to that of the main heating element alone and the power output is consequently reduced. An advantage of this series arrangement is that a single overheat protector will protect the heater in both boiling and simmer modes. Advantageously, the additional portion is short circuited during operation of the primary heating means.

Preferably the heat input of the secondary heater is slightly greater than that required to keep the liquid in the vessel at boiling point, so that vapour bubbles will continue to be produced. This will provide a visual indication to a user that liquid is still at boiling temperature. Preferably the heating effect of the secondary heating means is substantially localised so that a localised stream of bubbles will be seen rising from the heater. This is easily achieved with a material of high resistivity, as discussed above. To view the bubbles, the vessel is preferably made from a transparent material such as glass or polycarbonate, or at least has a transparent portion.

This is in itself a novel and innovative arrangement, so from a further aspect, the invention provides a liquid heating apparatus comprising a liquid heating vessel and an electric heater provided in a base of the vessel, the heater having heating means operable in use to maintain liquid in the vessel substantially at boiling after it has boiled, said heating means being

localised to produce a rising stream of vapour bubbles in the liquid, the vessel being transparent or having a transparent portion to allow such bubbles to be seen by a user.

A manual override may be provided to permanently deenergise the secondary heating means if traditional operation, i. e. total switching off on reaching the predetermined temperature, is required.

In liquid heating vessels, in particular kettles, it is traditional for an on-off knob to be provided on or adjacent the handle of the vessel. The knob may be part of a self contained switch unit connected electrically to the main control of the vessel, or be mechanically connected to the main control. The provision of an on-off knob places constraints on the design of the control, since the control must be capable of generating sufficient force to operate the knob.

This has an effect on the strength of the materials used, and so on cost.

From another aspect, the invention seeks to provide a liquid heating apparatus which does not require the provision of a knob, so from a yet further aspect of the invention, there is provided a heating appliance, and in particular but not exclusively a liquid heating appliance comprising a heater, wherein the heater of the appliance is energised in response to an actuating movement of the appliance. Thus, in accordance with this aspect of the invention, the heater of the a liquid heating vessel can be simply switched on by moving the vessel by its handle rather than using a manually operated switch or button. This will avoid the need for the knob, electrical or mechanical connections between a knob or button and the heater control thereby producing a significant cost saving. In addition, the operating forces in the control can be reduced, allowing less strong materials to be used, reducing costs further.

The actuating movement may be a simple displacement

of the vessel, for example a linear or rotary displacement, and the actuating movement may be sensed in a number of ways. For example the movement could be sensed electronically or electrically, for example with a magnet and coil arrangement, and the heater energised in response to a sensing signal. Preferably, however, a mechanical sensing mechanism is used. In a preferred arrangement, a mechanism such as a pivotally mounted weight is used to sense an acceleration of the vessel, and a mechanical linkage provided between the mechanism and means for closing a set of switch contacts in the electrical supply to the heater.

In the preferred embodiment, a pivotally mounted weight is coupled to a cam which operates a movable member of the control of the heater to close the switch contacts. In the particular arrangements described earlier, the cam may act upon the trip lever arm so as to move it to a position in which the switch contacts on the adjacent track portions are bridged or closed to energise the heater. By effectively tripping a lever arm, the weight may be freed so as not to interfere with the subsequent operation of the control.

The pivotally mounted weight may be arranged so as to rotate about its mounting by virtue of centrifugal force if the vessel is rotated about its axis.

Preferably, however, the weight is arranged so as to act as an inertial mass responding to a linear or rotational acceleration of the vessel.

Preferably the actuating movement is a rotational one, which lends itself particularly well to cordless appliances having 360° connectors as discussed above.

In such appliances, the rotation is effected after the appliance has been placed on its base. With a rotational movement, a substantial acceleration can be achieved by rotating the vessel first in one direction and then in another, maximum acceleration being achieved when the direction of motion changes.

Preferably, means are provided to switch off the heater as the vessel is placed on its base so that a deliberate actuating movement is required to switch on the heater and so that the vessel cannot accidentally be energised when there is no liquid in the vessel merely by placing it on the base. This is also a novel an innovative arrangement in broad terms, having application both to corded and cordless appliances, so from a yet further aspect, the invention provides a liquid heating apparatus comprising a heater and a control for the heater, the control comprising switch means for controlling the supply of energy to the heater, the control further having means for opening said switch means as the apparatus is being physically connected to its power supply.

In the case of a corded appliance, the socket attached to the power lead of the appliance may be provided with means which cooperate with the control as the socket is being introduced into the power connector inlet of the appliance. In the case of a cordless vessel as described above, such means may be provided on the base connector part. The base part projection may be provided by a projecting rim of the connector, as shown for example in WO 95/08204. Once the appliance part projection moves past the projection on the base part, it will be able to move once the control operates.

In the preferred embodiment, a movable projection may be provided within the appliance connector part which is depressed by a corresponding projection on the base or power lead connector part as the two parts are interengaged, to cause opening of the switch. The cooperating projections may also be arranged to switch off the heater by opening the switch contacts when the connector parts are disengaged.

Preferably the projection in the appliance connector part is pivotally mounted and cooperates with a trip lever arm of the control unit.

Thus in the arrangement described above, the appliance projection part may act on the movable contact support to trip open the switch contacts. The projection preferably engages with extends through the earth terminal of the connector, and the base connector projection is formed by a horizontally projecting rim formed on the forward end of the connector. This base connector part will also then act to deflect the appliance part projection when the appliance is removed from its base to switch off the appliance when it is lifted from the base.

This is believed to be a novel and advantageous arrangement, so from a yet further aspect the invention provides a cordless electrical appliance comprising a base connector part having a horizontally projecting rim provided adjacent the free end of the base connector part, and an appliance connector part having a member extending into the connector and coupled to a switch of a control of the appliance, the appliance member being deflected by the connector projection upon relative movement of the connector parts towards or away from each other so as to switch off the appliance.

Thus in the preferred embodiment of the invention, it will be seen that two distinct movements of the vessel are used to switch the vessel on and off: a rotary motion to switch it on and a vertical motion to switch it off.

It will be appreciated that the various inventions described above and their preferred features may be used alone or in combination. The invention extends to a heater assembly comprising a heater according to any aspect of the invention, a liquid heating vessel comprising a heater assembly according to any aspect of the invention and a method of manufacturing a heater, heater assembly or liquid heating vessel according to any aspect of the invention.

Some embodiments of the invention will now be

described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a heater assembly according to an embodiment of the present invention;.

Figure 2 is a view similar to that of Figure 1 with only those components bonded to the heater substrate shown; Figure 3 is a perspective view of the heater assembly of Figure 1 from without the heater substrate; Figure 4 is a detailed view of the mounting of the overheat actuator of Figure 1; Figure 5 is a detailed view of the mounting arrangement for the boiling actuator of Figure 1; Figure 6 is a view from below of the track layout of the electric heater of Figure 1; Figure 7 is a detailed view of the attachment means of Figure 1.

Figure 8 shows a heater in accordance with a second embodiment of the invention; Figure 9 shows a partially constructed assembly of the invention; Figure 10 shows the assembly of Figure 9 in a later stage of assembly; and Figure 11 shows the finished assembly.

Referring to Figures 1 to 6, a heater assembly comprises an electric heater 1 which takes the form of a circular stainless steel or mild steel plate provided with an insulating layer 22 onto which is printed a resistive heating track 23. The track 23 is provided with a protective overglaze (now shown), similarly apertured to allow access where necessary to the underlying track or insulating substrate. The upper surface of the heater 1 may be treated with a suitable surface coating or to electrically insulate the heater from the liquid, for example water, in the vessel to which the heater will be mounted and/or to protect the upper surface of the heater 1 from the corrosive effects

of the liquid. The assembly comprises a control comprising an overheat protection function and a boiling sensing function.

As shown most clearly in Figure 2, the lower surface of the heater 1 has mounted thereto an overheat protector support member 3a; a boiling control support member 3b; a mains connector 5 in the form of a central line pin 7 located in the centre of the circular heater 1, an inner neutral ring 9 and an outer earth ring 11 both mounted concentrically about the live pin 7; a sump 13; four switch contacts 15a-15d; a neon terminal 17 and a mounting flange 19.

In this description, corresponding components of the boiling control and the overheat protection control are given the same reference numeral, with the subscript "a"given to those components of the overheat protector and the subscript"b"to those of the boiling control.

The locations on the surface of the heater 1 occupied by the bonded components of the heater assembly are indicated with the reference numeral of the component itself followed by a prime, for example"15"'.

The respective support members 3a, 3b are of press- cut and folded metal for example mild steel. Each support member 3a, 3b is mounted to the heater 1 by three feet 21, which are bonded to the insulating layer 22 of the heater 1. As can be seen from Figure 6, the locations 21'for the feet 21 on the surface of the heater 1 are left free from resistive heating track 23 and a clearance is provided around these locations 21' to ensure that there is no chance of electrical shorting between the resistive heating track 23 and the support members 3a, 3b. The support members 3a, 3b are also electrically isolated from the metal plate of the heater 1 by the insulating layer 22 of the heater 1.

The support members 3a, 3b each have two pivot locations in the form of respective notches 25a, 25b which receive the knife edges 27a, 27b of respective

trip levers 29a, 29b.'Opposite the respective notches 25a, 25b are provided respective reaction locations 31a, 31b, also in the form of notches, for receiving one edge of a an omega spring 33a, 33b, for biasing the respective trip levers 29a, 29b into one of two stable positions, i. e. an on position and an off position.

Each support member 3a, 3b is provided with a neon terminal 35a, 35b in the form of a spade connector formed integrally with the support member 3a, 3b. At the opposite end of the respective support member 3a, 3b to the notches 25a, 25b is defined a hole 37a, 37b which receives one end of a pivot rod 39a, 39b of a cam member 41a, 41b of an inertial switching mechanism as will be described further below.

The mains connector 5 is a so called 360°C connector which will allow engagement with a corresponding connector in a base unit (in the vessel irrespective of the relative rotational orientation.

The particular terminal configuration is that of the Applicant's well known P72 connector, described in WO 95/08204.

The line pin 7 of the mains connector 5 is formed as a cylindrical stainless steel deep drawing 43 into the end of which is laser welded a silver pin 45 to ensure reliable electrical connection. The line pin 7 has an integrally formed skirt 47 by means of which it is bonded to the surface of the heater 1. The skirt may be expanded or perforated to increase the surface area which it presents to the bonding medium and thereby increase the reliability of the bond. The line pin 7 is bonded to the surface of the heater 1 using solder as will be described further below. As is shown in Figure 6, the location 7'for receiving the line pin 7 is provided with a conductive track 49 which connects the line pin to the location 15c'for the switch contact 15c.

The neutral ring 9 of the mains connector 5 is

preferably formed from a strip of silver edged stainless steel which is formed into a ring and joined at its ends. In this way, the edge of the neutral ring 9 which is inserted into a complementary socket in a mains connector is silver to ensure a good electrical connection therewith. The unsilvered edge of the neutral ring 9 is provided with an integral inner skirt 51 portion by means of which the ring 9 is bonded to the surface of the heater 1. The skirt portion 51 of the neutral ring 9 is bonded to a mounting location 51 on the surface of the heater 1 to form an electrically conductive connection between a conductive track 53 provided on the insulating surface of the heater 1 and the neutral ring 9. The conductive track 53 runs to and provides the mounting locations 15a'and 17'for the contact 15a and neon tab 17 respectively. The skirt portion 51 of the neutral ring 9 may be expanded or perforated to ensure a reliable bond. A cutaway 55 is provided in the neutral ring 9 by removing some of the skirt portion and the wall of the ring 9. This cutaway 55 is aligned with, and thereby avoids, the conductive track 49 which runs from the line pin 7 to the switch contact 15c so that no electrical connection is made between the neutral ring 9 and the line pin 7.

The earth ring 11 is of brass, and constructed in a similar manner to the neutral ring 9, but is bonded directly to the surface of the metal plate of the heater 1 so as to form an earth connection therewith. The insulating layer is suitably apertured to accommodate this connection. The earth ring 11 is provided with three cutaways, two 57a, 57b of which allow the passage of the conductive tracks 49 and 53 underneath the earth ring 11 without electrical contact. A further wider cutaway 57c is provided to accommodate conductive bridges 59 which provide clearance for the sump 13 on the surface of the heater 1.

As will be described further below, a window 61 is

provided in the wall of the earth ring 11 for receiving a projection 63 of a lever arm 65. The wall of the earth ring 11 is also provided with two holes 57 which engage and retain corresponding projections 69 on a plastics moulding 71 which mounts the lever arm 65.

The sump 13 is formed as a stainless steel deep drawing and is bonded to the insulating layer of the heater 1 by means of a skirt portion 73. As shown in Figure 6, the region 13'of the heater 1 on which the sump is located is circumvented by conductive bridges 75 to prevent any risk of electrical contact between the heating track 23 and the sump 13.

The neon terminal 17 is a press-cut folded metal component. The neon terminal 17 is bonded with silver- loaded ceramic to a conductive pad 17'provided on the insulating layer of the heater 1 which pad is connected to the conductive pad 15b'of the neutral switch contact 15b. Thus the neon terminal 17 is electrically isolated from the metal plate of the heater 1, but is connected permanently electrically connected to the neutral ring 9 of the mains connector 5 via the conductive path 53.

The mounting flange 19, which has a substantially L-shaped cross-section, is bonded by means of one limb 77 to a peripheral region 19'of the insulating layer of the heater 1. As will be described in further detail below, the mounting flange 19 is provided with a plurality of locking projections 79 which take the form of triangular teeth bent radially outwardly from the plane of the mounting flange 19. The flange may be made in the same manner as the earth and neutral rings 11,9, by forming a strip into a cylindrical shape and joining its ends. The mounting limb 77 may be formed as a plurality of bent over tabs.

The switch contacts 15 are conductively bonded to respective conductive pads 15'provided on the surface of the heater 1. The conductive pads 15b'and 15c'are electrically connected to the conductive tracks 49 and

53 from the live and neutral terminals 7 and 9 of the mains connector 5. The conductive pads 15a'and 15d' are electrically connected by conductive bridges 47 to the resistive heating track 23.

With reference to Figure 6, the insulating layer of the heater 1 is provided with a resistive heating track 23 in the form of a plurality of discrete curved sections of track interconnected by conductive bridges 81. The main heating track 23 extends between the switch contact mounting location 15a and the switch contact mounting location 15d. The conductive bridges 81 allow the resistive heating track to circumvent obstacles on the surface of the heater 1, such as the locations for the bonding of components, without decreasing the radius of curvature of the track to the extent that current crowding and consequent hot spots occur.

A serpentine portion 83 of the main heating track 23 is provided for the location of an overheat protector bimetallic actuator 85. The serpentine formation of this portion 83 ensures that there is only a relatively low potential difference across the actuator 85, reducing the likelihood of shorting across the actuator.

Included within this serpentine portion, but not forming part of the main heating track 23, is an additional heating track 87 connected between the line conductive track 43 and the switch contact 15d. The material of the additional heating track 87 has a much higher resistivity, than that of the main heating track 23 so that it has a much higher resistance than the track 23 for reasons which will be discussed further below.

As is described in WO 97/39603, the track 23 is provided with an area 89 or areas 89 which will selectively fuse in the event of serious overheating of the heater 1. These areas 89 are displaced from the ends of the track by the track portion 83. This means

that the voltage gradient between the adjacent track portions is reduced, leading to a reduction in the current which occurs on failure of the track, thereby preventing external power supply fuses blowing.

Having described in some detail the heater 1 and the various components bonded to it, it is useful now to describe a process by which this assembly is manufactured. Firstly, a plate eg of stainless steel, having a hole for registry with the sump 13, is prepared in any suitable way, for example by the grit blasting method disclosed in WO 97/47223.

Then a metal loaded, e. g. a silver or copper loaded ink is deposited, e. g. printed onto the metal substrate in those regions where a connection is to be made directly to the plate, e. g. in the mounting location 11' for the earth ring 11. This is then dried or fired.

One or more insulating layers is then deposited, e. g. printed, onto the plate to a desired thickness with apertures in appropriate locations on the metal loaded ink to allow access to the plate as necessary. The track 23 is then applied e. g. printed onto the insulating layer. This is done in two operations, due to the two different track materials used. The insulating layers and track may be fired separately or together as appropriate.

After the track 23 has been applied, the conductive paths 49,53,75,81 are printed as silver or alloy loaded inks and dried or fired.

Thereafter, a protective overglaze is printed over the tracks 23, with apertures in the appropriate locations to allow access to the conductive paths, the insulating substrate and the plate as necessary.

Thereafter, silver or other metal loaded ink may be applied to those areas 21', 19', 13'of the insulating layers or protective overglaze intended to receive components so as to form a keying layer for solder.

This layer is then dried and fired. Solder is then

applied to all those areas'for receiving components i. e. the supports 3a, 3b, the line pin 7, the neutral ring 9, the earth ring 11, the sump 13, the contacts 15a-15d, the neon tab 17 and the mounting flange 19.

This may be done by commercially available pick and place machinery. Once in position, the assembly can be heated e. g. to 300°C to melt the solder and bond the components to the heater. This process allows all the components to be attached in a single operation. Solder is preferred as it is relatively inexpensive and will melt at temperatures which will not adversely affect the brass earth ring. Also, the surface tension effect of the solder when molten will tend to centralise the components in their positions.

The assembly is then ready to mount the further components of the control as will be described below.

Referring back to Figure 1, a plastics lever arm 65 is pivotally mounted in the plastics moulding 71 which embraces the earth ring 11 and is retained thereon by projections which engage in the holes 67 formed in the wall of the earth ring 11. The pivotal axis 93 of the lever arm 65 is arranged such that as the projection 63 which extends through the opening 61 in the earth ring 11, is engaged by the projecting bead of a P72 base connector it is forced radially outwardly such that the lever arm 65 pivots away from the surface of the heater 1. The effect of this will be explained below.

Also mounted to the plastics moulding 71 is the overheat protection actuator 85. This actuator is of the domed snap-acting type described in GB 1542252. The actuator 85 is biased in use against the serpentine portion 83 of the heater track 23 by a spring member 97 which receives the tongue of the actuator 85 in a clip 99. The spring 97 lightly braces the actuator 85 into contact with the heater 1 and is clipped onto the plastics moulding 71. When the temperature of the actuator 85 reaches its predetermined operating

temperature, the actuator 85 reverses its curvature in a snap-action so that the periphery of the actuator 85 urges the trip lever 65 away from the heater 1. The actuator 85 is selected to operate at a temperature associated with a fault condition of the heater 1, such as the heater being energised without any liquid in the vessel.

The free end of the lever arm 65 engages each of the trip levers 29a, 29b. The omega springs 33a, 33b of the trip levers 29a, 29b are selected such that the weight of the lever arm 65 is not sufficient to cause the trip levers 29a, 29b to pivot about their respective pivotal axes against the bias of the omega springs 33a, 33b. Thus, in the position of use of the heater assembly, the lever arm 65 is supported by the trip levers 29a, 29b.

Each trip lever 29a, 29b carries a respective contact bridge 101a, 101b which is clipped onto a lug 103a, 103b on the trip levers 29a, 29b. Each contact bridge 101a, 101b is provided with two switch contacts 105a, 105b which in the"on"position of the trip lever 29a, 29b contact the complementary switch contacts 15a- 15d to allow current to flow via the respective contact bridge 101a, 101b between the pairs of switch contacts 15a-15b and 15c-15d, thereby allowing current to flow to the heating track 23.

As mentioned above, each respective trip lever 29a, 29b is formed as a metal pressing and has a coined knife edge 27a, 27b about which the trip lever 29a, 29b pivots. The omega springs 33a, 33b engage between the reaction locations 31a, 31b on the support member 3a, 3b and reaction locations on the trip levers 29a, 29b to bias the trip levers 29a, 29b either into an"on" position, in which the switch contacts 105a, 105b are urged against the switch contacts 15a-15b, or an"off" position, in which the switch contacts 105a, 105b are urged away from the switch contacts 15a-15b.

Between the heater 1 and the extremity of each trip lever 29a, 29b furthest from the lever arm 65 is provided a plastics cam member llla, lllb. As shown in Figure 5, each cam member llla, lllb pivotally mounted about a substantially horizontal axis between its respective support member 3a, 3b and the plastics moulding 71, by means of pivot spindles 113a, 113b. A bob weight 115a, 115b is formed integrally with and depends from each cam member llla, lllb. The inertia of the bob weight 115a, 115b causes the cam member llla, lllb to pivot about its respective axis when the heater assembly is rotated suddenly about its central axis or when the heater assembly undergoes a translational movement in a direction having a substantial component perpendicular to the pivot axis of the cam member llla, lllb. The cam surface of each cam member llla, lllb comprises a pair of wings 117 extending outwardly from the pivot axis, such that when the cam members llla, lllb rotate in either direction under the influence of the bobweight 115a, 115b one or other cam surface wing 117 will engage its respective trip lever 29a, 29b and urges it away from the heater surface against the action of the omega springs 33a, 33b into the"on"position. In this way, the cam members llla, lllb and bobweights 115a, 115b form an inertial switching mechanism.

Referring now to Figure 5, at the end of the boiling trip lever 29b proximate the cam member lllb, is mounted a generally V-shaped spring member 121. This spring 121 is clipped onto a plastics mounting block 123 mounted on the end of the boiling trip lever 29b. The spring 121 biases a boiling sensitive bimetallic actuator 125 against a thermally conductive plate e. g. of copper 127 which is in thermal contact with the sump 13. The actuator 125 is of the type as described in GB 1542252 and the tongue of the actuator 125 is retained in a clip 131 in the spring member 121. A similar arrangement of a bimetal, sump and conductor plate is

described in more detail in WO 97/04694.

Heat in the sump 13 is transferred to the conductor plate 127 and thence into the actuator 125. The boiling actuator 125 is selected to actuate at a temperature of the conductor plate 127 associated with the boiling of the liquid in the vessel. The conductor plate 127 provides a thermal lag between the sump 13 and the actuator 125 to prevent nuisance tripping of the bimetal 125 and allow boiling to continue for a predetermined time. When the predetermined temperature is reached the bimetal reverses its curvature and urges the spring member 121 and thus the extremity of the trip lever 29b towards the surface of the heater 1. This causes the trip lever 29b to pivot about the knife edge 27b against the force of the omega spring 33b into the"off" position.

The operation of the heater assembly will now be described.

When the liquid heating vessel to which the heater assembly is mounted is initially placed on a base (not shown) provided with a cordless connector, particularly of the kind described in WO 95/08204, the bead of the base connector engages with the projection 63 of the lever arm 65 and causes the lever arm 65 to pivot away from the surface of the heater 1. In so pivoting, the lever arm 65 urges the extremities of the trip levers 29a and 29b away from the surface of the heater. Each trip lever 29a, 29b then pivots overcentre about its respective knife edge 27a, 27b against the force of the omega springs 33a, 33b into the"off"position in which the contact bridges 101a, 101b are lifted, thereby opening the contacts 105 and 15. Thus, both trip levers 29 are in the"off"position and no current is supplied to the resistive heating track 23.

A subsequent rotational movement of the liquid heating vessel on the base about the axis of the connector 5 will both cause bobweights 115a, 115b to

swing and the respective cam members llla, lllb to pivot about their axes. The pivoting of the cam members llla, lllb causes the cam surfaces of the cam wings 117 to urge each trip lever 29a, 29b overcentre into the"on" position, closing the contacts 105 and 15. As the trip levers 29a, 29b have been moved away from the cam members llla, lllb, the latter will not interfere with subsequent operation of the control.

In this position, the contact bridge 101a carried by trip lever 29a connects the contacts 15a and 15b such that current is supplied from the neutral ring 9 to one end of the main heating track 23. It will be noted that the main heating track 23 is connected at its other end to the live pin 7 via switch contact 15d, conductive track 43 and additional heating track 87. However, when trip lever 29b is in its"on"position, contact bridge 101b connects contacts 15c and 15d which forms a short circuit across the additional heating track 87, such that no current flows through the additional heating track 87. Thus, when both trip levers 29a and 29b are in the"on"position, current flows through the main heating track 23 to heat the liquid in the vessel, and no current flows through the additional heating track 87. Typically, the heating track 23 will generate 2.2 kW at 220V supply voltage.

As liquid in the vessel heats, the temperature of the sump lags behind the rest of the heater, until boiling occurs at which point its temperature rises rapidly. This rise in temperature is transmitted along the plate 127 into the actuator 125 which operates to urge the trip lever 29b overcentre into its"off" position, in which the contacts 105c and 105d are lifted from the heater thereby opening the switch contacts 105c-15c and 105d-15d. Thus, the additional heating track 87 is no longer short circuited and current flows from the live pin 7 via conductive track 49 and additional heating track 87 through main heating track

23. It is to be noted that switch contacts 15a-105a and 15b-105b are still closed such that one end of the main heating track 23 remains connected to the neutral ring 9. The resistance of the additional heating track 87 is such that the current flowing through the main heating track 23 when it is connected in series with the additional heating track 87 is very significantly less than the current flowing through the main heating track 23 when the main heating track 23 is directly connected to the live pin 7. Consequently, when the trip lever 29b is tripped to its"off"position and the trip lever 29a is in its"on"position, as just described, the main heating track 23 and the additional heating track 65 are energised with a relatively low current and therefore have a relatively low power output. The resistance of the additional heating track 87 is selected such that the power output of the main heating track 23 and the additional heating track 65 when running at this relatively low current is just sufficient to keep the liquid in the vessel at boiling temperature, i. e. the power output of the combined heating track is approximately equal to the rate of heat loss from the liquid heating vessel at the boiling temperature. The required power output may be determined empirically and will be, for example, about 30W.

This heating effect will be concentrated substantially entirely in the section 87. Typically the resistance of track 23 will be about 22Q to produce 2.2 kW at 220V, and that of section 87 about 1620Q to produce 30W. In this situation, only about. 5W will be generated in the track 23 and 29.5W in the section 87.

This localised heating effect can be used to produce a constant stream of vapour bubbles rising through the vessel, such that if the vessel is transparent or has a transparent portion, it can be seen that the liquid in the vessel is still at boiling temperature.

The combined heating track (main track 23 plus additional track 87) will continue to be energised until the trip lever 29a is urged into its"off"position.

This may occur in two ways. Firstly, if the liquid heating vessel is lifted from the base, the bead of the base connector will engage the projection 63 and cause the lever arm 65 to pivot and urge the trip lever 29a overcentre into its"off"position. Secondly, if the temperature of the heater 1 becomes sufficiently great, the overheat actuator 89 will operate, urging the lever arm 65 against the trip lever 29a and moving it overcentre into its"off"position.

Should the trip lever 29b also be in the on position, for example because the liquid in the vessel has not yet boiled, the pivoting of the lever arm 65 due to either of these occurrences will also urge the trip lever 29b into the off position, ensuring that no current at all flows through the main heating track 23.

Figure 7 shows a detailed view of the edge 151 of a liquid heating vessel 153 to which the heater assembly according to the invention is mounted. The edge 151 is an inner edge of the base of a plastics, glass or metal vessel defining a circular hole, 155 in which the mounting flange 19 of the heater is received. The edge 151 is stepped as shown in Figure 7 so that the heater 1 fits flush with the upper surface 157 of the base of the liquid heating vessel. A sealing member 159, in the form of a ring of rubber or suitable elastomer is located between the heater 1 and the stepped portion 161 of the edge 151. A locking circlip 163 is located over the retaining projections of the mounting flange 19 and is retained in engagement with the vessel edge 151 by the retaining projections whose taper assists location of the latter.

As can be seen, the sealing member 159 is compressed both horizontally and vertically between mounting flange 19 and the edge 161 to form a reliable

seal at that point. The axial compressive sealing forces are exerted mainly against the limb 77 on the mounting flange 19 and the retaining projections 79, such that very little stress is put on the edge of the heater 1 itself, thereby protecting the bond between the mounting flange 19 and the heater 1.

Spacers (not shown) may be provided between the circlip 163 and the edge 151 to accommodate different thicknesses of the material of the liquid heating vessel.

A second embodiment of the invention will now be discussed with reference to Figures 8 to 11. The main differences in this embodiment over that described above are that a single switching contact is provided on the element in each pool of the supply, rather than a pair of switching contacts as in the earlier embodiment, that the respective support members form part of the electrical circuit of the heater assembly, that the support members now act to mount both the dry switch on lever arm and the respective bobweights, and the boiling responsive bimetallic actuator is mounted to the heat conduction plate rather than to an end of the trip lever.

Turning now to detail, it will be seen from Figure 8 that the heater 202 of this embodiment has a track 204 which is laid out in a very similar manner to that of the earlier embodiment. The main difference is that respective single contact mounting pads 206a, 206b are provided in each side of the electrical supply to the track 204. Mounting pads 208a, 208b are provided for receiving mounting legs 210a, 210b of respective trip lever support members 212a, 212b. A further mounting location 214 is provided for a neon tab 216.

It will be seen from Figure 9 that the support member 212b (which together with the other support member 212a is made of brass) connects the foremost two of the pads 208a which are connected to respective

portions of the heating track 204.-The effect of this will be described further below. Furthermore, the respective support members, due to their shape are, to some degree, flexible, which allows them to accommodate relative thermal expansion which occurs in use.

A line pin 220, neutral ring 222 and earth ring 224, a sump 218 and the support members 212a, 212b are all bonded to the heater as in the earlier embodiment.

In this embodiment, however, it is preferred to weld and more preferably to laser weld the earth ring 224 to the heater substrate. This gives a stronger bond and allows the ring 224 better to receive knocks during use without becoming detached from the heater 200. When the earth ring 224 is welded to the substrate, which is normally stainless steel, it may be necessary, for example, to use a copper ring, or a silver plated mild steel ring rather than a brass ring to facilitate the welding. Also, in order to facilitate positioning of the neutral ring 222 and strengthen the connector parts, an insulating washer e. g. of insulating paper (not shown) may be located over the line pin 220 for locating the neutral ring 222. This washer may have apertures to receive an injected epoxy which acts further to insulate the line and neutral terminals 220,222 and also adds strength to the connector.

Also, as in the earlier embodiment the respect supports 212a, 212b support respective trip levers 230a, 230b. In this embodiment, the respective trip levers 230a, 230b are of brass and are spring loaded by conventional C-springs 232a, 232b. The trip levers 230a, 230b mount contacts 226a, 226b at their ends, for contacting contacts 228a, 228b mounted on the heater at locations 206a, 206b.

A further difference in this embodiment over the earlier embodiment is that in this embodiment the heat conduction plate 233 which is attached to the sump mounts the boiling responsive snap acting bimetallic

actuator 234 which was previously mounted to an end of the corresponding trip lever. In this embodiment, therefore, the actuator 234 is retained on the heat conduction plate 233 by means of a beryllium copper spring clip 236. The spring clip 236 has a tongue 238 which engages in a slot 240 formed in the upper surface of the heat conduction plate 226. A moulded plastics plug 238 is mounted on an arm 240 of the trip arm 230b and is arranged in close proximity to the outer perimeter of the actuator 234 so as to translate its movement to the trip lever 230b.

It will also be seen, particularly from Figures 10 and 11 that the respective trip lever support members 230a, 230b now mount the bobweights 250a, 250b, and to this end are provided with journals 252a and 252b formed above their rearmost mounting legs 210a, 210b.

Actuating surfaces 254a, 254b of the respective bobweights act on respective surfaces 256a, 256b of the trip levers 230a, 230b. Also, the respective trip lever support members 212a, 212b each have an upstanding leg 260a, 260b at their forward end with journals 262a, 262b for receiving pivots 264 of the dry switch on lever arm 266. This lever arm 266, as in the earlier embodiment, mounts a snap-acting bimetallic actuator 268 which is mounted on a beryllium copper leaf spring 270 which acts, in use, to bias the actuator 268 resiliently against the serpentine portion of the heating track 202.

As in the earlier embodiment this lever arm 266 also has lugs 272 which extend under corresponding lugs 274 provided on the forward end of the respective trip lever arms 230a, 230b. Also, it is provided with a projection 276 which extends through an aperture 278 in the annular earth terminal 224 for engagement with a power connector when the appliances is placed on or lifted from a support.

Operation of this embodiment will now briefly be described.

As in the earlier embodiment, when the liquid heating vessel, to which the heater assembly is mounted is placed on a base unit with a cordless connector of the kind described in W095/08204, the bead of the base connector engages with the projection 276 of the lever arm 266 to cause it to pivot away from the heater surface thereby lifting the lugs 274a, 274b of the trip levers 230a, 230b which then trip open, thereby lifting contacts 280a away from the contacts on the track. Both trip levers are in the"off"position and no current is supplied to the resistor heating track 234.

As in the earlier embodiment, a rotational movement of the vessel will cause the control surfaces 254a, 254b of the respective bobweights to act on the surfaces 256a, 250b of the trip arms 230a, 230b, causing the trip levers to move over centre, and thereby to close the contacts 226a, 226b with those 228a, 228b on the heater 200.

As will be appreciated from Figure 8, in this condition, power is supplied to the heater track 202 from the line pin 220 via the contact pair 228b, 226b into the trip lever 230b, into the trip lever support 212b and into the track via the foremost mounting legs 210b thereof. The neutral ring 222 is connected to the track via the contact pair 228a, 226a to the trip arm 230a and thence through the support member 212a into the foremost mounting pad 208a.

It will be seen that the mounting legs 210b of the trip lever support 212b bridge two of the contact regions so that in this condition a length 290 of the track 202 between the respective support members is, as in the earlier case short circuited, but that current flows through the remainder of the track.

As in the earlier embodiment when liquid in the vessel boils the bimetallic actuator 234 will operate causing the trip lever 230b to trip, thereby opening the contact pair 228b, 226b. When this happens, power will

be conducted through the track through the line pin 220 to the innermost track 290 of the serpentine portion 292 through the support member 212b into the main track portion, around the track to the foremost mounting pad 208a and through the support member 210a and trip lever 232a to the contact pair 226a, 228a and thence to the neutral ring 222. As in the earlier embodiment, the track properties are chosen so that the heating effect is concentrated in the radially outer serpentine region 292 so as to produce a constant stream of vapour bubbles rising through the vessel.

As in the earlier embodiment, this situation will continue until such time as the vessel is lifted from its base, in which case the projection 276 on the lever arm 266 will be pivoted by the base power connector so as to lift the lever arm 266 and trip the trip lever 210a so as to lift the contact 226a from the track.

Also should the temperature of the heater becomes sufficiently high, the bimetallic actuator 268 will operate and urge the lever arm 266 away from the track so as again to pick up the trip arm 272 and open the contact pair 226a, 228a thereby disabling the heater.

Of course it will be appreciated that there are other differences between this embodiment and the earlier embodiment. For example in this embodiment the mounting flange 300 has a bayonet-type fitting. Also, it may be advantageous to laser weld the flange to the substrate for reasons of strength to this end, the flange 300 should be of a material, compatible for welding in this way, for example tin plated or silver plated mild steel.

It will be appreciated that various inventions described herein can be applied individually, or as shown in the preferred embodiment, integrated in the same apparatus. For example, the connector construction effecting the switching off of a heater when a cordless appliance is positioned on its base can be applied to

all manner of liquid heating vessels (not just those using thick film heaters), and can be applied to different connector constructions, for example that shown in WO 95/08204 and also to non-360° connector, such as the Applicant's P75 connector (shown in UK Design 2052182). Similarly, the invention relating to maintaining the liquid in a vessel at boiling can be applied to any type of heater, not just thick film heaters.

It will be understood by the skilled person that the preferred method of manufacturing a heater as described herein represents a significant advance, doing away with the need for a housing for the control, and using the heater itself as the support for the control, and other component of the assembly. The assembly may be preassembled and tested after manufacture before being shipped to appliance manufacturers for assembly into an appliance. This will significantly reduce scrap rates at that stage, which at present are quite high, since is not until this stage that the heater and control are normally brought together.