Apparatus and method for generating an air flow Field of the Invention The present invention concerns an apparatus and method for generating an air flow. Background of the Invention Devices for generating an air flow have various applications, one of which is providing suction for use in a vacuum cleaner. It is desirable for a vacuum cleaner to have high suction levels to enable the cleaner to pick up debris from a surface (e.g. a floor) to be cleaned. Suction power may be measured in “Air Watts” to enable comparison between the suction performances of different vacuum cleaners and is typically calculated as a quantity that is proportional both to the air flow rate and the suction pressure. The performance, as measured in Air Watts, may vary according to the exact flow rate and/or vacuum pressure being applied. High suction (high pressure) is useful for tasks that involve using a crevice tool where the aerodynamic restriction is great and where the absolute flow rate may be low. During less restrictive tasks, such as cleaning a large area of floor, it may be desirable to have high flow, whilst not necessarily requiring high pressure. It would be desirable therefore to providing an apparatus which is able both to generate an air flow having high suction and to generate an air flow having high flow rate, whilst also delivering acceptable levels of suction power (as measured in air watts) at acceptable levels of efficiency (e.g. efficiency of conversion of electric power into suction power as measured in air watts). Other applications, for example hair dryers, may also benefit from the provision of improved air flow devices. Summary of the Invention The present invention provides an apparatus for generating an air flow. The apparatus comprises a duct, an electric motor and an impeller. The motor is arranged to drive the impeller to cause a flow of air through the duct. The apparatus is so arranged that there is a first air flow path that passes via the impeller. For example, it may be that the impeller directly creates a flow of air through the duct along the first flow path. The apparatus is also so arranged, or configurable to be so arranged, that there is a second air flow path separated at least in part from the first air flow path by a part (in the form of structure) of the apparatus. The first and second airflow paths are preferably in fluid communication with a common inlet of the apparatus, for example an inlet at one end of the duct. The second air flow path is preferably shaped such that it is spaced apart from the impeller, or at least such that air flowing along the second airflow path does not flow via the impeller. The apparatus is so arranged (or configurable to be so arranged) that air flowing via the first air flow path induces, at least in part, flow of air via the second air flow path. In embodiments, such an arrangement facilitates extra air flow which in turn enables higher flow rates whilst maintaining useful suction pressure. Such embodiments may be of particular advantage in relation to vacuum cleaning applications, for example where the common air inlet is connected to or provided by a vacuum cleaner head or cleaning tool. It may be that the air flow created by the impeller in certain embodiments is effectively amplified or multiplied by the use of the structure which forms the second airflow path. The apparatus may be configurable to close, or otherwise restrict the flow of air via, the second air flow path. Such an ability may be useful in embodiments in which high pressure flow is required without high flow rates – as might for example be the case when employing a single narrow cleaning tool on a vacuum cleaner. The apparatus may be arranged to have a mode of operation in which the second path is closed off or otherwise restricted. Such a mode of operation would be additional to a mode of operation in which the second path is open (i.e. not closed or at least less restricted). Thus there may be a first mode of operation in which the second path is open and a second mode of operation in which the second path is closed / restricted. Having two (or more) modes of operation in which the impact of the second flow path can be increased / decreased may in certain embodiments allow the same apparatus to be used to both generate an air flow having high suction (for example in the second mode of operation when the second airflow path is closed/less open) and to generate an air flow having high flow rate, whilst also delivering acceptable levels of suction power (for example in the first mode of operation when the second airflow path is more open). There may be more than two modes of operation, for example many modes, corresponding to how open / closed the second path is. An embodiment of the invention relates to an apparatus which is used to generate a low pressure high flow airflow via the first and second air flow paths and also to generate a high pressure low flow airflow via the first airflow path only, with the second air flow path being closed or restricted. In some embodiments it may be desirable to allow for a user / operator of the apparatus to be able to manually switch between such modes of operation. In some embodiments it may be desirable to allow, alternatively or additionally, for automatic control of the switching to / from such a mode of operation. The apparatus may include a control unit arranged to control whether the apparatus operates in one or both of the first mode and the second mode. For example, the control unit may be configured to receive an input and to generate an output, in dependence on the input, determining the mode of operation of the apparatus. In the case where there are more than two modes of operation, the control unit may be configured to control the amount by which the second path is opened, the amount being dependent at least in part on the input. The control unit may receive multiple inputs. An input to the control unit may relate to the suction pressure and/or a measured pressure difference. An input to the control unit may relate to a measured flow rate. An input to the control unit may relate to electric power, current, voltage associated with / delivered to the motor. An input to the control unit may relate to rotational speed of the impeller. An input to the control unit may be a user input, for example from a switch, dial, button, key-pad, or the like. The structure between the first and second flow paths may be movable, for example to change the flow of air via the second airflow path. The structure may be movable to/from a first position corresponding to the first mode of operation mentioned above (i.e. second path open) and a second position corresponding to the second mode of operation (e.g. second path closed). The structure may be arranged to be moved in this manner in a direction along (e.g. parallel to) the axis of rotation of the impeller and / or along the direction of the duct. The structure may be ring-like and may have a central axis. The structure may be arranged to be manually moved. There may additionally or alternatively be a mechanism provided for moving the structure, for example under control of the control unit mentioned above. As mentioned above, in use the apparatus may be configured/operated such that air flowing via the first air flow path induces, at least in part, flow of air via the second air flow path. The manner in which the second airflow is induced may include the first airflow entraining the second airflow. It may be that the structure between the first and second flow paths assists in inducing the second airflow, for example by being shaped in a way that causes, or assists in causing, the air flowing via the first flow path to induce, at least in part, the flow of air via the second flow path. The structure may be so shaped and arranged to make use of the Venturi effect. The electric motor may be mounted in the duct. The apparatus may be so arranged that the first air flow path passes between the motor and the duct wall. The flow of air through the duct along the first flow path may surround the motor. The first flow path may be annular at least in part. The impeller may be mounted in the duct. The impeller may be a centrifugal impeller, which may for example be able to generate higher pressure differences than, say, a fan that only generates axial airflow. The centrifugal impeller may be positioned close to the motor in a way that would not be possible with an axial-flow fan, and therefore enables embodiments where efficient use of space is important – for example in portable / hand-held devices. An efficient use of space may also enable shorter ducts, which can improve efficiency by reducing energy losses. In an embodiment, the air that flows through the duct, along at least one (and preferably both) of the first and second airflow paths, surrounds and flows around (but preferably not through) the motor. The motor is preferably mounted downstream of the impeller. In this way the body of the motor may in part form or define the shape of the airflow downstream of the impeller. The motor may drive the impeller directly. The motor may be a digital motor. There may be gears and / or drive belts between the rotational output of the motor and the impeller. There may be embodiments in which there are more than one impeller and therefore optionally also more than one motor. The motor may have a casing or housing which defines at least in part its exterior surface. In use, there may be at least some airflow that is directly adjacent to (e.g. in contact with) an exterior surface of the motor. There may be an additional flow path, for example a third flow path (separate from the first and second flow paths at least in part). Such a third flow path may pass via the motor. Alternatively, there may only be the first and second flow paths for the flow of air via the duct. The duct may comprise a first inlet (for example an annular inlet) via which air is arranged to supply to the first air flow path. The duct may comprise a second inlet (for example an annular inlet) via which air is arranged to supply to the second air flow path. The second inlet may surround the first inlet. The first and second inlets may be supplied air from a common source (e.g. the common inlet mentioned above) upstream of the first and second inlets. The thickness of an annular inlet (i.e. the difference between the inner diameter and the outer diameter of the annular inlet) may be less than 2cm, possibly less than 1cm. The duct may comprise an outlet via which air is exhausted. The outlet may be a common outlet which is in fluid communication with the first air flow path and the second airflow path. The outlet may be an annular outlet. The outlet may be positioned at an end of the duct. The first flow path may taper in the direction of the air flow. The second flow path may taper in the direction of the air flow. The shape of, for example the tapering of, the passageways which define the air flow paths may assist in the inducing of airflow along the second airflow path. As mentioned above, the structure between the first and second flow paths may be shaped to assist in inducing the second airflow. It will be appreciated that the shape of the structure may therefore determine the shape of at least part of the passageway(s) along which the first airflow and/or the second airflows pass. The first flow path when viewed in cross-section may have a sigmoid profile (i.e. generally S-shaped). The second flow path when viewed in cross-section may have a sigmoid profile (i.e. generally S-shaped). The cross-section referred to here may be a longitudinal section that is coplanar with longitudinal axis of the duct/impeller/motor/structure and/or air-flow. The structure when viewed in cross-section may have a sigmoid profile. It is preferred that the structure has an aerodynamically efficient shape. The structure may be curved in a manner that entrains airflow, for example with the use of the Coandă effect at least in part. Embodiments of the present invention have particular application in relation to portable electric-powered apparatus, for example devices configured for domestic use by a single person at a time. The apparatus may be a hand-held apparatus. The apparatus may be a vacuum cleaner. In such a case, the motor and impeller may be located downstream of a dust/dirt separator. It may also be the case that the first air flow path is downstream of such a dust/dirt separator (for example, with the common inlet mentioned above being between the dust/dirt separator and the impeller) The apparatus may be a hair dryer. The apparatus may be a fan and/or air conditioning apparatus and/or air heater. The apparatus may be configured to be capable of delivering at least 100 Air Watts of airflow power. The apparatus may be configured to be capable of delivering an air flow at a rate of at least 30 litres / sec, for example at a power of at least 20 Air Watt and preferably at a power of at least 30 Air Watts. The apparatus of the invention may be provided as a kit of parts. Such a kit may comprise one or more of the aforementioned duct, motor, impeller, structure between the air flow paths, control unit, and any other distinct parts mentioned herein. The invention also provides a method of generating a high pressure low flow airflow and a low pressure high flow airflow with the same apparatus, which is preferably in the form of a portable apparatus. The method includes a step of generating a low pressure high flow airflow via at least two passageways such that air flowing via a first passageway assists in causing flow, or increases the flow, via a second passageway, and also includes a step of generating a high pressure low flow airflow via that least the first passageway but such that air flow via the second passageway is restricted (including completely prevented). It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a view of motor-impeller apparatus for generating airflow according to a first embodiment of the invention; Figure 2 shows a graph comparing the performance of the apparatus of Figure 1 with a comparable motor and impeller; Figure 3 shows a second embodiment, as a modification of the apparatus of Figure 1 in which the air amplification device is provided in the same length of duct; Figure 4 shows a third embodiment, based closely on the apparatus of Figure 3 but including a slidable ring which can be moved between two positions; Figure 5 shows the air inlet of the motor-impeller parts of the apparatus, when viewed head-on in a fourth embodiment of the invention where the apparatus is incorporated into a vacuum cleaner; Figure 6 shows a cross-section about the plane represented by the line A-A in Figure 5, corresponding to an open mode where air amplification is provided; Figure 7 shows a cross-section about the plane represented by the line A-A in Figure 5, corresponding to a closed mode in which air amplification is disabled; Figure 8 shows an external view of the vacuum cleaner of the fourth embodiment; Figure 9 shows a graph of the performance of the fourth embodiment; and Figure 10 shows a schematic view of the air amplification device incorporated into a hairdryer in a fifth embodiment of the invention. Detailed Description Figure 1 shows an apparatus for generating an air flow according to a first embodiment. The apparatus 10 comprises an electric motor 12 connected to an impeller 14, which are both located within a duct 16. The motor 12 is arranged to drive the impeller 14 to cause a flow of air through the duct 16 along a first air flow path 18, which passes via the impeller and through a tapered annular passageway 20 that is shaped to induce further airflow along a second air flow path 22, by means of the Venturi effect. The second air flow path 22 includes a portion along the axis around which the tapered annular passageway 20 extends. The structure 24 that forms part of the tapered annular passageway 20 thus separates this portion of the second air flow path 22 from the first air flow path 18. The structure that forms the tapered annular passageway 20 may be considered as forming an air amplification device. The portion of the duct that forms the second air flow path immediately upstream of the region where the first air flow path and the second airflow path merge, tapers in the direction of air flow. Air is sucked from a common inlet 26 that feeds air both to the first air flow path 18 and to the second air flow path 22. The inlet 26 may for example be connected to a debris separator part of a vacuum cleaner. The air flowing from the first air flow path and from the second air flow path is exhausted via a common outlet 28. The portion of the duct immediately downstream of the region at which the first air flow path and the second airflow path merge, flares outwardly in the direction of air flow. The arrangement of the apparatus may be thought of as an air flow apparatus having an air amplifier device, which acts as a type of fluid diode or one way valve, preventing the flow from simply recirculating. Figure 2 is a graph which compares the performance of the apparatus of Figure 1 with a comparable motor and impeller simply placed in-line with a single duct (i.e. no feedback and no air amplifier). The x-axis is a measure of air flow rate (volume per second) and the y-axis is a measure of air watts (proportional to suction pressure). The solid line 40 is a line of best fit through the triangular data points which are measurements of the performance of the apparatus of Figure 1. The broken (dashed) line 50 is a line of best fit through the data points being measurements of the comparable apparatus (with no feedback and no air amplifier). It will be seen that while at low flow rates, the suction pressure available with the comparable apparatus (with no feedback and no air amplifier) is better than the apparatus of Figure 1, the suction pressure available at high flow rates is markedly better using the apparatus of Figure 1. The apparatus of Figure 1 could readily be adopted to function as part of a vacuum cleaner apparatus by means of a topologically equivalent apparatus in which a high velocity debris separator (e.g. a cyclone separator) and dust filter are provided immediately upstream of the impeller (in the first air flow path) and a low velocity agglomerator is provided immediately upstream of the air amplification device (in the second air flow path). In such a set-up, the motor exit flow is used to generate additional suction that is effectively routed back to the cleaner head. Tests have shown that such an apparatus can provide an additional 5 litres per second of total air flow as a result of the use of such an air amplifier device. A configuration like this, in which the air amplification device is connected as a separate entity downstream of the motor with its own inlet tube connected back to the cleaner head, may suit a robotic style vacuum floor cleaner in which the cleaner head is permanently fixed to the main body of the vacuum cleaner. Figure 3 shows a second embodiment, as a modification of the apparatus of Figure 1. Reference numbers having the same last two digits are used to identify similar parts as between Figures 1 and 3. In this apparatus, the electric motor 112, the impeller 114, and the structure 124 that forms part of the tapered passageway 120 of the air amplification device are all provided in the same length of duct 116, there being no separate duct (of the same diameter) that provides the feedback airflow. There is still a first air flow path 118, which relates to the flow of air directly via the centrifugal impeller 114 and which induces the further airflow along the second air flow path 122. The tapered annular passageway 120 which assists in providing the air amplification function is arranged in parallel with the motor-impeller. In this case, the first air flow path 118 includes a portion immediately downstream of the impeller, wherein air passes via an annular constriction 130 before merging with the second flow path 122. The second flow path bypasses the impeller via the tapering annular passageway 120, the outer diameter of which being defined by the inner surface of the duct 116. The outlet of the tapering annular passageway 120 (the point at, or at least the point very near to, which the first and second air flows merge) has an inner diameter that is the same as or slightly less than the outer diameter of the annular constriction 130 of the first air flow path. The motor 112 is provided downstream of the impeller 114 which it drives. The merged first and second airflows are annular in cross-section and flow around the outside of the exterior of the motor 112. An arrangement such as that shown in Figure 3 provides a more compact air amplification device, which might be better suited to a stick-based vacuum cleaner having the air amplifier device and its associated ducting contained within the main body. Figure 4 shows a third embodiment, based closely on the apparatus of Figure 3. Reference numbers having the same last two digits are used to identify similar parts as between Figures 3 and 4. The apparatus 210 comprises a motor 212 for driving an impeller 214 to generated an airflow 218 in a duct 216. In this apparatus, there is provided a slidable ring 232 which can be moved between a first position shown in broken line in Figure 4 in which the operation of the apparatus is much as described in Figure 3 and a second position shown in solid line in Figure 4 in which the first air flow path 218 is open but the second air flow path is closed off. In this manner, the apparatus may have a first mode of operation corresponding to the first position of the ring which provides high flow rates not possible without the air amplification effect, and also a second mode of operation corresponding to the second position of the ring which provides high pressure suction at low flow rates not possible with the air amplification effect enabled. In this embodiment, the position of the ring is moved manually, so that the user can select which mode of operation is preferred. A sliding control knob (not shown) is provided which is attached to the ring and protrudes via an elongate slot (with appropriate sealing plates to reduce the risk of loss of suction) to allow for such movement of the ring 232 along the axis of the duct 216 between its two operational positions. Figures 5 to 8 show a fourth embodiment used in a handheld vacuum cleaner 310. The motor 312, impeller 314 and air amplification device of the fourth embodiment are functionally similar to corresponding parts of the apparatus of the third embodiment. Those parts are however illustrated by means of less schematic drawings in this fourth embodiment. Reference numbers having the same last two digits are used to identify similar parts as between the third and fourth embodiments. The air inlet of the motor- impeller parts of the apparatus, when viewed head-on, is shown in Figure 5, in which part of the impeller 314 can be seen. Figures 6 and 7 each show a cross-section about the plane represented by the line A-A in Figure 5. Figure 6 corresponds to an open mode where air amplification is provided whereas Figure 7 corresponds to a closed mode in which air amplification is disabled. The air amplifier forms part of the motor and impeller unit. Similarly to the third embodiment, the electric motor 312, the impeller 314, and the structure 324 that forms part of the tapered passageway 320 of the air amplification device are all provided in the same length of duct 316. The duct flares outwardly (increasing diameter) from its inlet to a maximum diameter at a point immediately downstream of the impeller. The diameter of the duct is constant from this point to the outlet. Figure 8 shows schematically a hand-held vacuum cleaner in which the motor- impeller parts of the apparatus 310 are used. In this embodiment, the motor is similar to the electric motor used in Dyson’s V6™ cordless stick vacuum. As shown in Figures 6 and 7, the main parts of the motor 312 include a central shaft 334 which is connected to the rotor, around which the stator is provided. Figure 8 shows the main body 346 of the vacuum cleaner which houses the motor-impeller apparatus 312 and is directly attached to a rigid handle 338 designed to be handheld by an upright user. The inner side of the handle carries an on/off switch in the form of a trigger (not shown). The handle extends down to a battery pack 336, which is a rounded cuboid and acts to weight the main body 346 such that it is easy to hold. The battery provides power to the electric motor 312. The main body 346 supports a separation system which functions to remove dust and debris from the air drawn into the vacuum cleaner by the motor-impeller system. The separation system consists of a primary cyclonic separator (not shown separately), a secondary separator (in the form of a cyclone pack 344), a bin 342 and pre and post filters (not shown separately). The bin 342 is substantially cylindrical in shape. Coaxial with the outer wall of the bin is a second cylindrical wall of a smaller radius, such that the walls define an annular chamber. The lower portion of this chamber acts as a bin for dust and debris, whilst the upper portion forms the primary cyclonic separator. When in use, dirty air is drawn in through the cleaner-head or crevice tool 348 and enters the bin 342 of the vacuum cleaner through an inlet nozzle. The air then passes through the first stage of the separation system, which is the primary separator. This separator acts to remove the largest pieces of dust and debris from the air. The air exits the bin through a perforated shroud and then passes through a plurality of conduits into the secondary separator as provided by the cyclone pack 344. The cyclone pack generates multiple identical cyclones which act to remove the finer dust particles from the air, which pass into a fine-dust-collector which sits within the main body. The air is then passed through a pre- filter before passing through the motor 312. The pre-filter removes a large percentage of the remaining fine dust still entrained in the air. Once the air has been pulled through the motor, it is passed through a post filter before being released as clean air back into the atmosphere. The post filter acts to remove any particulates that may have been introduced into the air by the motor. With reference to Figures 6 and 7, in a similar manner to the third embodiment, the electric motor 312 is mounted in the duct 316 and drives a centrifugal impeller 314 which is mounted in the duct immediately upstream of the motor. In both the open mode and the closed mode of operation the flow of air is entrained in the duct around the motor (such that air flows in the region between the motor and the duct wall and along a first path 318 via the impeller), the open mode having additional air flow (along the second path 322) being induced as a result of the slidable ring 332 opening up a second passageway and moving an air amplification structure into an operational position. The use of a centrifugal impeller 314 which causes an airflow with both axial (along the axis of rotation of the impeller) and radial components (flowing radially out away from the central axis) has advantages in that higher pressures are achievable, and that the impeller is relatively compact. The compact nature of the overall design also enables greater efficiency as energy losses as a result of long ducting or the like are reduced. The geometry of the apparatus also enables more uniform entrainment of air as the annular inlet to the first air flow path and the annular inlet to the second airflow path are both axially symmetric with the axis of rotation of both the impeller 314 and the drive shaft 334 of the motor 312. A slidable ring 332 moves automatically between the open position shown in Figure 6 and the closed position shown in Figure 7 in which the second air flow path 322 is closed off by the ring. In this embodiment the ring 332 has an aerodynamic profile being in the general shape of a sigmoid curve when viewed in cross-section (although not necessarily in the exact shape of a mathematical sigmoid curve it will be appreciated). This shape of the ring (when in the open position shown in Figure 6) defines an inner tapering curved (in cross-section) passageway for the first airflow path 318 for flow directly generated by the impeller 314, and an outer tapering curved (in cross-section) passageway for the second airflow path 322, for airflow induced by the air passing via the first airflow path 318. The two airflows are separated (in this region) by the ring 332. The outlet of the annular constriction of the first passageway has a width 352 of about 5mm (distance between inner and outer diameters). The outlet of the apparatus 310 has a slot width 354 (distance between inner and outer diameters) of about 10mm. The aerodynamic shape of the ring 332 and the tapered passageways has been found to create good results in terms of the air flow amplification effects observed at high flow rates. With the air amplifier function enabled (open mode, shown in Figure 6) higher flow rates are made possible. With the air amplifier function disabled (closed mode, shown in Figure 7), the apparatus is able to match the peak suction power that would normally be associated with the motor in its conventional application (as part of the V6 vacuum cleaner). In this fourth embodiment, the position of the ring 332 is moved automatically. Figure 7 shows schematically, a control unit 356, pressure sensor 358 and solenoid device 360 which are connected to the apparatus 310 in order to achieve this automatic control (those parts being omitted from Figure 6 for the avoidance of repetition). The ring 332 is moved by the solenoid 360 device, which is activated by the control unit 356 in dependence on a measurement of suction pressure from the pressure sensor 358 received at the control unit. When the control unit 356 detects that the suction pressure drops below a first threshold level (signified by a rise in absolute pressure detected by the pressure sensor 358), the control unit causes the solenoid to move the ring to its open position to ensure high flow rates. When the control unit 356 detects that the suction pressure rises above a second threshold level, e.g. the same as or higher than the first level (signified by a reduction in absolute pressure detected by the pressure sensor 358), the control unit cause the ring 332 to move to its closed position to ensure high suction pressure, at low flow rates. The graph shown in Figure 9, of Air-Watts (y-axis) against flow rate (x-axis), shows the point 362 at which switchover occurs, and the extension of the range of operation, in terms of the higher flow rates, made possible with the use of the embodiments. The dotted line 364 in the graph shows the air-watts and flow-rates possible if only the closed mode were available. It will be seen that the second mode of operation (illustrated by the solid line 366 in the graph extending to the right of the switchover point 362) extends the range of flow rates possible by a considerable margin. The same apparatus can be capable of delivering over 100 air watts of suction power at flow rates in the region of 10 to 20 litres per second (as might be required when using narrow / focussed / small-area suction tools) and also be capable of delivering flow rates in the region of 25 to 35 litres per second whilst still maintaining at least 20 air watts of suction power (as might be required when using wider / large-area suction tools). The apparatus 310 of the fourth embodiment requires little in the way of extra ducting (lengthwise) in order to achieve the integrated air amplifications and is therefore relative compact. The compact arrangement resulting from the combining of the air amplifier structure directly with the motor-impeller apparatus and ducting also means that the apparatus may be relatively weight efficient, which can be advantageous when the apparatus forms part of a portable or hand held device. The flow of air is evenly distributed around the motor 312 and forms a uniform entrained annular air stream which may be beneficial in many applications. The passageways are smoothly shaped and relatively short, enabling an aerodynamically efficient design. Figure 10 illustrates a hairdryer 410 according to a fifth embodiment of the invention, which utilises a motor 412, impeller 414 and air amplification device of a type similar to that described in relation to the third embodiment. Reference numbers having the same last two digits are used to identify similar parts as between the third and fifth embodiments. The motor 412 and impeller 414 are arranged in a duct that draws air from an inlet side (indicated by arrows 478), and exhausts air via a heating element 476 followed by an outlet (indicated by arrow 480), thus blowing heated air out of the hair dryer. The hairdryer 400 consists of a cylindrical main body 482 which houses the motor 412, impeller 414 and air amplification device. The hairdryer is wired to a plug via a power lead 470, which can be connected to the mains in order to provide power to the motor. A rigid cylindrical handle 438 is attached perpendicular to the main body, and is designed to be held easily in the hand of the user. The handle carries an on/off switch in addition to controls to adjust the heat and/or speed of the airflow. An air inlet is provided on the flat rear face of the main body 482, through which air can be drawn (arrows 478) into the hairdryer. This air passes via the motor 412 and air amplification device before passing through a heating element 476. This air then exits the main body via an outlet on the front face of the cylindrical main body. The direction of airflow through the main body is indicated by arrows 478 and 480 in Figure 10. A slidable ring (not shown in Figure 10) can be moved – as in the third embodiment – to cause the hair dryer to switch between a first mode of operation providing high flow rates, and a second mode of operation providing high pressure air flow. In this embodiment, the position of the ring is moved manually, so that the user can select which mode of operation is preferred. The removable attachment 474 on the hairdryer as shown in Figure 10 provides a constricted outlet which enables the hairdryer to produce a focussed narrow jet of air, requiring high pressure, but relatively low flow rates. This sort of attachment functions best if the hair dryer is in its second mode of operation (where the second air flow passageway(s) are closed off). A second removable attachment 472 for the hairdryer is shown in Figure 10, which is in the form of a diffuser attachment having a wide outlet which enables the hairdryer to produce a wide flow of air, requiring relatively high flow rates. This sort of attachment functions best if the hair dryer is in its first mode of operation (where the second air flow passageway(s) are open and the air amplification function is enabled). Thus, interchangeable attachments may demand different levels of pressure from the impeller 414. The impeller design determines the flow rate and head characteristics of the motor. Applications where a relatively low pressure drop exists suit a more axial flow design with the penalty being low total head, whereas radial designs suit applications where components have higher pressure drops but the penalty is lower flow. Mixed flow bridges the two but at a compromise of both flow and pressure. The air amplifier motor design of the present embodiment can switch between the high pressure mode (typically associated with radial flow design) and the high flow mode (typically associated with axial flow design). Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. The cyclone separator and bin shown in Figure 8 could instead be provided in series with and coaxially with the airflow, in a manner similar to Dyson’s V11 handheld vacuum cleaner. The motor, impeller and air-amplification device shown in the main body of the hair dryer in Figure 10 could instead be positioned within the body of the handle of the hairdryer. Other applications are envisaged for the motor, impeller and air amplification device described in relation to the illustrated embodiments. It may have application in relation to fans, air heaters, or other consumer goods that utilise air flows in their operation. The switching between modes of operation may be automated or otherwise controlled by reference to parameters other than a single absolute pressure measurement. For example, a combination of multiple pressure measurements before and after the motors could be measured to provide a measure of pressure drop. A measure from which airflow can be calculated could alternatively or additionally be used. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.