LIQUID HEATING VESSELS AND CONTROLS THEREFOR

Abstract: An electric liquid vessel has a high power setting and a low power setting, and control means permitting a user to select said high power setting. The control automatically reduces the power when liquid in the vessel boils. The control also allows a long or short boil to be selected.

Description: Liquid Heatinσ Vessels and Controls Therefor The present invention relates to liquid heating vessels and for controls for such vessels, and in particular, but not exclusively, to water heating vessels and controls for such vessels which may be used to boil water and then maintain the water at a predetermined temperature. Such vessels are particularly popular in the Far East where water supplies may not be entirely clean or may be highly chlorinated and where it is therefore desirable to boil the water for a prolonged period (typically over 1 minute) to sterilise or de-odorise the water. After boiling, the water is kept at a relatively high temperature, usually around 90°C. There is also the requirement that the water in the vessel may be re- boiled, for example if it has been standing for some time, or if fresh water is added to the vessel. The vessels are usually substantially sealed and have a lid mounted pump to dispense water from the vessel under air pressure without the need to open the lid of the vessel. Accordingly, such vessels are generally known as "airpots", and as used herein, the term is intended to refer broadly to liquid heating vessels having electrical heating means and means for pumping the heated liquid out of the vessel. Airpots traditionally have two independently controlled heating elements, one for boiling the water and rated, typically, at around 700 and the other for keeping the boiled water warm, and rated, typically, at about 70W. These elements are independently controlled and are usually formed as belts wrapped around and secured to a lower part of the vessel wall, or clamped onto the bottom of the vessel . Whilst an airpot of this type may be suitable for certain applications, with such a low powered boiling element, it can take some considerable time to boil water, which means that heated water cannot be produced quickly if needed. The present invention in a first aspect seeks to provide a liquid heating vessel which can heat water at two or more desired rates From a first aspect, therefore, the invention provides a water liquid heating vessel having electrical heating means having a high power setting and a low power setting, and control means permitting a user to select said high power setting. For the first time, therefore, the present invention allows a user to choose whether he or she wishes to boil water slowly or quickly. Thus while for normal operation the user may be content to have water heated at a lower power, for example 500-700W, if water is required more quickly, a higher power setting, for example 1600-2000W may be selected. Although the control means may be arranged so as to allow a user to select either a low power or high power setting, preferably the control means is arranged such that the heating means defaults to its lower power setting, and means are provided whereby a user may preferentially select the higher power setting. As mentioned above, airpots are designed such that water may be boiled for a prolonged period for sterilisation or de-odorising purposes. In existing airpot designs, operating at about 700W, steam generated during the period of prolonged boiling is conducted out of the vessel by a steam vent. The amount of steam generated is relatively small, due to the relatively low heating power. However, the high power levels discussed above, give rise to particular problems. Firstly, the airpot will be noisy, particularly whilst heating the water, but also during a prolonged boil. Secondly, with a vigorous boil continuing for over one minute, there will be a considerable waste of energy, since such high powers are not required to maintain the liquid in a boil. Clearly this is not desirable. Thirdly, and more importantly, however, large amounts of steam will be produced. This will not only potentially release large amounts of steam into the room, thereby causing problems with condensation and increased humidity, but the steam may not be able to escape sufficiently quickly from the vessel , - thereby building up pressure inside the vessel. This could be extremely dangerous in the event that the vessel lid is opened, or if liquid is dispensed from the vessel, as it will be expelled at high pressure, thereby potentially splashing a user. The present invention also seeks to mitigate these problems, and from a second aspect provides a liquid heating vessel having electrical heating means having a high power setting and a low power setting, and control means comprising means for detecting boiling of liquid within the vessel, and means responsive to the boiling detection means for switching the heating means from its high power setting to its low power setting once the liquid in the vessel has boiled so as to permit continue boiling of the liquid at the lower power setting for a predetermined period of time after boiling has been detected. The invention also provides in this regard a control for a liquid heating vessel having electrical heating means, the control comprising means for detecting boiling of liquid within the vessel, and means responsive to the boiling detection means for switching the heating means from a high power setting to a low power setting once the liquid in the vessel has boiled so as to permit continue boiling of the liquid for a predetermined period of time after boiling has been detected. Thus in accordance with this aspect of the invention, when liquid in the vessel boils, the heating means is switched automatically from its high power to its low power setting, to allow boiling to continue at a much lower power. The invention thus allows liquid to be boiled quickly, but at the same time allow boiling to continue at a reduced power, sufficient to maintain boiling but insufficient to cause the above mentioned problems . Preferably, the low power setting is less than or equal to X the high power setting, and more preferably less than or equal to X A the high power setting. For example, the high power setting may be 2kW and the low power setting 500W. 500W is still capable of maintaining the liquid in the vessel at a boil after it has been heated at 2kW and does not consume as much energy as a conventional airpot operating at 700W. The heating means may be provided as completely separate heating elements which are switched in and out by the switching means. Preferably, however, the heating means comprises a series of element sections which are connectable electrically in series . A high power setting can then be achieved by "picking off" a high power element section, and the low power setting obtained by connecting a lower resistance element in series with the high power track section. Most preferably, however, the heating means comprises element sections which are connectable selectively in series or in parallel by the switching means to effect the change in power setting. For example, if two heating element sections each rated at lkW are provided, they will provide 2kW if connected in parallel and 500W if connected in series . The heating means may take any convenient form, for example traditional sheathed heating elements attached to the underside of the vessel base. Preferably, however, the heating means is a so-called thick film heater. Such heaters typically comprise an electrically insulating layer laid down on a planar substrate with a resistive heating track deposited, more particularly printed on the substrate. The substrate may be an integral part of the vessel base, but preferably it is a separate member which is mounted to close an opening in the base of the vessel. The use of such heaters in airpots has already been proposed in WO97/04694, and examples of such heaters and their methods of manufacture are described in W096/17496, for example. With a thick film heater, the switching electrical connections can be made directly to terminal contacts provided on the track. The terminal contact may simply be formed as a printed silver contact pad, but more preferably a separate contact member is bonded to the track, for example during firing of the track. This permits an extremely good electrical connection to be made to the track through the contact member, which engages the track over a much larger area than would be the case if a contact were to be made directly to the track or a contact pad. Such an arrangement is described in GB9703224.7. Preferably, therefore the switching means acts selectively to lift and drop contacts onto terminal contacts of the track elements. Most preferably, these contacts are mounted on leaf springs which are moved by respective lever arms operated in response to operation of the boiling detection means. Preferably, the lever arms are provided on a single lever member on opposite sides of the lever pivot, so that when the lever member is operated, one lever arm lifts to break a connection to the heater tracks, for example to break a parallel connection between track sections, and the other drops to make another connection, for example to connect track sections in series. Most preferably, the lever member is bistable, being retained in its two positions until the application of an external force. As mentioned above, in operation, the heating means preferably defaults to a lower power setting after boiling. Preferably, therefore, the lever arms revert to their low power position after boiling, and reset means is provided to return the arms to their high power positions should high power boiling subsequently be required. The boiling detection means may take a number of forms. It could, for example, comprise a steam sensitive switch suitably arranged in the vessel or in the path of steam leaving the vessel. Preferably, however, it comprises a bimetallic actuator mounted in good thermal contact with a low volume sump provided in the base of the heater plate. This sump holds a relatively small volume of liquid whose temperature lags behind that of the main body of liquid in the vessel until the liquid boils at which time, the sump liquid is displaced from the sump by convection and turbulence, so that water at boiling temperature enters the sump. The temperature of the sump then rises to boiling and this rate change can be sensed by an actuator such as a bimetallic actuator mounted in good thermal contact with the sump. This form of boiling detection is a development of the detection method used in WO97/04694, where an actuator is mounted remotely from such a sump in order to introduce a time delay into the system, and is described in broad terms in co-pending application GB9703273.4 Such boiling detection means permits the integration of the boiling detection means and the power switching means into a compact, consolidated unit thereby reducing costs. Thus in the preferred embodiment, the actuator of the boiling detection means is arranged to operate the lever member of the switching means. Most simply, the lever member may comprise an arm which overlaps a movable portion of the actuator in such a way that when the heating means is initially in a high power setting, it will be moved by operation of the actuator, but when the heating means is initially in a low power setting, it will not be acted upon by the actuator. The mechanism described above switches the heating mens from a high power setting to a low power setting when liquid in the vessel boils. The vessel of the invention preferably also comprises means to allow the liquid in the vessel to boil for a longer or shorter period of time. This may be achieved in the preferred embodiment by having first and second thermally sensitive actuators arranged to operate at different times after liquid in the vessel boils, with means being provided to disable the operation of that actuator operating most quickly in the event that a longer boil is required. This is in itself a novel arrangement so from a further aspect the invention provides a control in or for a liquid heating vessel said control permitting liquid in the vessel to boil for a longer or shorter period of time, and comprising means for detecting boiling of liquid within the vessel, a first thermally sensitive actuator operable at a first predetermined time after boiling is detected to open a set of contacts to discontinue boiling, a second thermally sensitive actuator operable at a second, longer predetermined time after boiling is detected to open a set of contacts to discontinue boiling and means to disable the operation of the first thermally sensitive actuator in the event that a longer boil is required. In the preferred embodiment described above, boiling is discontinued by breaking one or more electrical connections to the heater track. Furthermore in the preferred embodiment, the first and second actuators are bimetallic actuators arranged at different thermal spacings from the sump of the heating vessel described above, so that heat takes longer to reach the second than it does the first. Accordingly, both the first and second actuators may be mounted in different positions on a thermally conductive plate mounted in thermal contact with the sump of the heater. Preferably the second actuator operates at the same temperature as the first, or more preferably at a higher temperature. These temperatures can be determined empirically, depending on the particular boiling times required, the mounting locations of the actuators and so on. Typically, however, the first actuator is arranged to operate at approximately 75°C and the second at 85°C. This avoids premature operation of the second actuator. To avoid duplication of parts, and thus reduce costs, both the first and second actuators preferably operate to open the same set of contacts in the control . In the preferred embodiment of the invention, the first actuator may be the actuator of the high power/ low power boiling mechanism, which thus performs two functions. This is extremely significant in terms of cost reduction, as once again it avoids duplication of parts. Thus in the event a short boil is required, the first actuator will act both to operate the high/low power lever member also act to open a set of contacts to discontinue boiling. In preferred embodiments, the set of contacts is opened by a pivotally mounted arm which is lifted by the first actuator when it operates so as to lift a power contact from the heating element, thereby discontinuing boiling. Preferably the arm is maintained in its operated condition by the first actuator such that when the first actuator resets, heating recommences automatically . To disable the operation of the first actuator to permit an extended boil, the pivotally mounted arm may itself be pivotally mounted on a long/short boil selector arm which is movable between long boil and short boil positions. In its short boil position, the long/short selector arm is positioned such as to allow the first actuator to lift the pivotally mounted lifting arm and discontinue boiling. In its long boil position, however, the long/short boil selector arm is positioned such that the first actuator will not act on the pivotally mounted lifting arm when it operates, but the second actuator upon operation will act to move the selector arm so that the pivotally mounted lifting arm will move into contact with the first actuator and then be pivoted around its pivot by continued movement of the selector arm. This provides a particularly compact mechanism whereby both actuators may operate on a single contact opening arm, to open a single set of contacts, thereby once again reducing costs . A liquid heating vessel or control of the present invention is preferably also provided with means permitting the liquid in the container to be maintained at a given temperature below boiling after it has boiled. Thus, in the preferred embodiment described above, the operation of the long boil/short boil actuators automatically switches the heating means to a simmer mode. To this end, the heating means may comprise a separate simmer element which is energisable when the vessel is in simmer mode. Preferably, however, the simmer element (which typically would have a rating of no more than 70W) is arranged such that in the vessel simmer mode, it is placed in a series connection with the main heating elements, as is described in our earlier WO97/04694. Thus in the preferred embodiment described above, the opening of the boiling contacts acts to place all three sections of the heating track in series . Preferably the simmer track is arranged such that when the main heater tracks are heating or boiling the water, the simmer element is shorted out. The preferred control of the invention is arranged such that the heating means will remain in simmer mode until the first actuator falls below its remake temperature, as might occur when the vessel is switched off and liquid in the vessel cools substantially, say overnight, or if cold water is added to the vessel. Preferably, however, in order to be able to reboil liquid in the vessel at any time, a reboil mechanism is also provided. Preferably this mechanism comprises a reset member acting on the first actuator to push it back to its initial operating state, thereby allowing the boiling contacts to remake and reboil the vessel contents . When in its simmer mode, the heating means is controlled essentially through a simmer control which senses indirectly the temperature of the liquid in the vessel, for example by sensing the temperature of the heater. Thus a thermally sensitive actuator may be provided in good thermal contact with the base of the vessel which selectively opens and closes a set of contacts to connect and disconnect the supply to the simmer element. Preferably, the simmer control is provided as a preassembled, precalibrated unit which is then assembled to a main control body. Preferably, the simmer control unit has a contact for making direct contact to a terminal contact of the simmer track, and most preferably it is mounted so as to be spring loaded against the heater, so as to give good thermal and electrical contact with the heater. Most preferably the spring loading is effected by leaf springs, one of which engages with a contact in the main control body such that when the simmer control unit is mounted to the control body, it is automatically connected into the power supply circuit . Preferably the simmer control is variable so that a user may vary the temperature of the liquid after boiling to suit its ultimate use. In the preferred embodiment, the simmer control unit comprises a snap acting bimetallic actuator of the type comprising a substantially rectangular bimetallic blade with a cut out defining a tongue on which is mounted a movable contact which engages a fixed contact. As is conventional, the blade is slightly domed during manufacture to introduce stresses into the blade such that it will move with a snap action upon operation. The blade is fixedly supported at one end in an insulating housing, that end also being in good thermal contact with the heater, for example through a thermally conductive member for example of copper so that upon reaching a predetermined temperature, the actuator will change its curvature with a snap action, so moving the movable contact away from the fixed contact and thereby interrupting the electrical supply to the simmer element . The operating temperature of the simmer control unit may be varied by changing the initial curvature of the bimetallic actuator. In the preferred embodiment, this is achieved by moving the edge regions of the supported end of the up or down from their neutral positions. This is preferably effected by resiliently biasing the side limbs against an adjustment member. The resilient biasing permits deflection of the side limbs beyond their neutral position, and at the same time provides a return force on the adjustment means. This arrangement is itself believed to be a novel and advantageous one, so from a further aspect, the invention provides a thermally sensitive control unit comprising a generally rectangular snap acting bimetallic actuator of the type having a generally U- shaped cut out to define a tongue extending from a first cross limb, two side limbs and a second cross limb joining the side limbs beyond the tip of the tongue, the tongue mounting a movable electrical contact, the actuator being mounted in a housing through the second cross limb, resilient biasing means acting against one face of the side regions of the second cross limb and a movable adjustment member acting on the other face of said side regions opposed to said biasing means. Preferably the adjustment member is a pivotally mounted lever arm. Most preferably, the lever arm is L shaped, one end of the arm acting on the actuator and the other being moved by suitable means, such as an adjustment screw. The lever arm is preferably arranged that the biasing means resiliently pivots the lever arm against the adjustment means and also into its pivotal seat . In the event that the vessel boils dry or is switched on without liquid in the vessel, the heater of - 13 - sections are configurable in a series configuration or a series/parallel configuration as discussed above. The most convenient arrangement is one in which all the connections to the track sections are broken. The dry switch on protection actuator is preferably arranged over a portion of the heater track which has a relatively small potential drop. This is advantageous in that it minimises the potential drop across the actuator, thereby discouraging shorting of the track through the actuator in the event that the track's protective overglaze should become damaged. Preferably, therefore the track is provided with a localised serpentine portion of a size generally similar to that of the actuator for receiving the actuator. This arrangement is of application to thick film heaters in general, so from a yet further broad aspect, the invention provides a thick film heater comprising a thick film heater track having a localised serpentine region for receiving a thermally sensitive actuator. Preferably, the localised serpentine portion comprises less than 10% of the total track length, so as to suffer a potential drop of about 25V on a 240V supply. A tight serpentine track pattern may be obtained by joining the ends of parallel track lengths by conductive bridges of silver or the like, as is described in our co-pending application no. 9703225.4. Such an arrangement allows substantially the entire surface of the actuator to be heated, thereby improving the response time of the actuator. The dry switch on protection actuator preferably acts on a lever arm which lifts the relevant contacts from the heater track. This lever arm may also be acted upon by a mechanism responsive to other operations such as dispensing liquid from the container or opening the lid of the container so as to disable the heating means in the event that liquid is dispensed from the vessel or the lid is opened. In the preferred embodiment, the control unit - 14 - sealingly clamps the heater in an opening in the base of a plastics vessel body. To this end, the heater is provided with a peripheral L shaped flange which receives a seal to be clamped against the vessel base. The vessel base is comprises a sealing surface and clamping means are provided to secure the heater to the vessel body and compress the seal. In order to obtain a reliable seal, the respective sealing surfaces are configured so as to compress the seal in both the longitudinal an radial directions. In order to prevent the seal effectively solidifying during compression, seal relief means are preferably provided to permit the seal to flow into a relief space. This ensures that full clamping may be effected, thereby improving the seal. Preferably the seal relief means comprises a projecting rib formed on the longitudinally facing sealing surface of the vessel base, and around which the seal may flow. Again this arrangement is an advantageous mounting arrangement for all liquid heating vessel, so from a yet further aspect, the invention provides a liquid heating vessel comprising a plate heater mounted to close an opening in a plastics body of the vessel, the heater plate having an L shaped sealing flange receiving a seal, the vessel body having sealing surfaces for engagement with the seal, and further comprising plate clamping means, the sealing flange and the sealing surfaces of the vessel body being so configured as to compress the seal both axially and radially, the vessel body sealing surfaces having means to permit the seal to deform into relief spaces to permit full compression thereof . Preferably, the plate is mountable to the control unit, and the combined control/heater unit then mountable as an assembly in the vessel body base. Preferably, this is achieved through a bayonet type fitting. Most preferably, the control unit is provided with mounting bosses which receive mounting pillars formed in the body base. Preferably at least one of the - 15 - bosses is positioned such that when the heater plate is mounted to the vessel, it is closely positioned with respect to a tag formed on the heater plate such that the plate will not be able to rotate with respect to the control unit, and thereby misalign the various cooperating features of the control and heater. This is also an advantageous feature in respect of all types of liquid heating vessels, so from a yet further aspect, the invention provides a liquid heating vessel comprising a vessel body, a heater plate mounted to a control unit of the vessel, the combined control unit/ heater plate assembly mounted in the base of the vessel, and means provided so as to prevent rotation of the heater plate relative to the control unit once the assembly has been mounted to the vessel. The various states of the control unit of the invention may be indicated by suitable indicator means coupled to the various actuators . A preferred indicator mechanism comprises a double headed rocker arm, each head selectively moving into or out of view in one of a pair of windows provided in an indicator panel of the vessel, depending on the state of the vessel. Such an arrangement is a particularly advantageous one so from a further aspect the invention provides a liquid heating vessel comprising an indicator panel having at least one pair of windows provided therein, a pivotally mounted rocker arm having two heads, each head selectively moving into or out of view in one of a pair of windows provided in an indicator panel of the vessel, so as to indicate an operational condition of the vessel. Preferably each rocker arm head is brightly coloured, for example with fluorescent paint or paper so as to give the impression of illumination of the window. The rocker arm is preferably biased into its respective positions by an overcentre spring mechanism. Preferably there is a lost motion coupling between the respective actuator of the control and the indicator mechanism. This is preferred as it avoids a further - 16 - force acting on the actuator as it operates. Rather the actuating force is transmitted to the indicator after the actuator's initial movement. It will be appreciated that the invention as described above can have different applications to an airpot type vessel. For example, with certain appliances such as travel jugs, used for heating water, it is desirable to be able to operate the appliance at more than one voltage e.g. 110V for the USA and 220V for Europe. Up to now, it has not been possible to provide a jug which will heat satisfactorily at both operating voltages . Also, it has been found to be desirable in other types of appliance merely to provide a variable heating output, for example depending on the liquid being heated, or the speed of heating required. The present invention in another aspect provides a heater which will allow both the above arrangements to be implemented in a simple manner. From this further aspect, first aspect, the invention provides a liquid heating vessel comprising at least first and second heating elements, and switch means for selectively connecting said first and second elements electrically in series or in parallel . Thus in accordance with this aspect of the invention, two (or more) heating elements may be connected either in series or in parallel. This is an arrangement described in particular embodiments of the invention above. In the situation where the vessel is required to operate at two voltages, two elements may be connected in parallel, when operating at a lower voltage, to provide a given power output, and in series to provide the same or a similar power output, depending on the particular resistance of each heating element. In the case where two heating powers are required, at the same operating voltage, the elements may be connected in parallel to give a high power output and in series to give a lower outpu . - 17 - Preferably the resistance of each element is substantially similar. This has the advantage that in either configuration, the watts density of the combined element will be a constant figure leading to a uniform heating effect. Furthermore, in the low power condition, the watts density of the series elements will be uniform and low, ideal for heating fatty liquids such as soup or milk which might burn onto the element in areas of high watts density. Although a reduction of heating power could be achieved by merely switching out one or more heating tracks, (as mentioned above) the advantage of a series/parallel switching arrangement is that it allows a reduction both in heating power and power density easily to be achieved. For example where two heating elements are provided of the same resistance, starting from the situation where they are connected in parallel, if one element were merely to be disconnected, there would be a 50% reduction in heating power, but no reduction in power density. However, if the elements are instead connected in series, there will be a 75% reduction in both the heating power and the power density. Furthermore, all the available track is being utilised, leading to a more uniform heating effect. Thus the invention permits a reduction both in heating power and power density to be achieved in a simple manner. It will be realised of course that it is not essential that the elements have the same resistance, as the above reductions will still be achieved, although a uniform power density across the heater as a whole may not . The invention is particularly applicable to heating vessels wherein the heating elements are provided on the underside of the base of the heating vessel. Two such types of vessel are known. In the first, the heating element is a conventional sheathed heating element which is secured on to the underside of the base of the - 18 - vessel, usually through a thermal diffusion plated aluminium or aluminium alloy. The second type of vessel is one in which the element is a so-called "thick film" element, which is deposited, e.g. printed, on to an insulating layer provided on the vessel base. Examples of both these types of heating elements are disclosed in WO 96/18331. The present invention lends itself particularly well to application with thick film elements, were more than one heating tracks may very easily provided on a substrate without significant additional cost. From a second aspect, therefore the present invention provides a liquid heating vessel comprising a thick film heating element on the underside of the base of the vessel, said heating element comprising at least two resistive heating tracks, and further comprising switch means for selectively connecting said heating tracks electrically in series or in parallel . The switch means may comprise any suitable mechanism. For example, the terminals of the respective heating tracks may be permanently connected to a separate switch unit. Preferably, however, the track terminals themselves form part of the switch means. In the preferred embodiment, therefore, the switch means comprises movable switch contacts which are movable to engage selected terminals of the heating tracks. This arrangement is advantageous in arrangements where the tracks are being connected in series/ parallel or in another manner, so from a yet further aspect, the invention provides liquid heating apparatus comprising a thick film heater comprising at least first and second thick film heating tracks, said tracks comprising respective terminals and switch means comprising switch contacts which are movable to engage selected said terminals. Preferably the switch comprises respective pairs of movable contacts. In the preferred embodiment, the switch comprises two double contact members and a - 19 - bridging member having contacts at either end. Each double contact comprises two contacts, connected electrically in parallel to each other and, and to one pole of an electrical supply. In one position of the switch, the contacts of the respective double contact members engage with terminals of the respective heating tracks, such that the tracks are connected in parallel through the double contact members. In that position of the switch, the contacts on the bridging member do not bridge any track terminals. In another position of the switch, the contacts of the bridging member engage respective terminals of the respective tracks, to connect the ends of the tracks, and one contact of each double contact engages the other terminal of each track. Preferably the movable switch parts are slidably mounted with respect to the heating elements, most preferably linearly slidable between the two operative positions. Of course, a rotary rather than a linear motion would also be possible. Furthermore, the switch parts may be lifted on and off the terminals, rather than slid on and off, for example with the mechanism described earlier. In one arrangement, for use with a slidable switch arrangement, respective pairs of movable contacts are aligned with one another, and equispaced. Also, the terminals of the respective track are arranged parallel to one another, at the same spacing as between the contacts. This is in itself believed to be a novel arrangement so from a further aspect, the invention provides a thick film heater comprising two thick film resistive heating tracks deposited on an electrically insulating substrate and having respective terminals, respective pairs of terminals being aligned parallel to each other, and the terminals within each pair being arranged at the same predetermined spacing from one another. Returning to the switch, the movable contacts are conveniently carried as a movable support which is - 20 - coupled to a knob extending, in use, outside the vessel for manipulation by a user. Preferably the contacts are spring contacts, for producing a suitable contact force onto the track terminals. The contacts may extend through apertures in the support, if necessary. The contacts may be formed as separate contacts suitably mounted onto leaf springs, or as a suitably formed part of the springs themselves . The respective double contacts may be permanently connected to a power supply for the vessel, for example by a spade terminal and flying leads which may be in turn connected to a thermally sensitive control unit for the vessel. However, in one arrangement, the switch is adapted to make sliding engagement with power supply terminals, for example those of a thermally sensitive control unit for the vessel . Such terminals are normally spring contacts biased into engagement with terminals of the heater track. In a particular arrangement, each double contact member is provided with a slider pad with which engages a respective power terminal, and over which the power terminal may slide as the switch is moved between its respective positions. The slider pads are suitably supported on the movable support of the switch. To avoid the possibility of arcing at the contacts, the switch may be made operable only when there is power supply to the appliance. For example, in the case of a cordless appliance, the operating handle may be positioned only to be accessible when the appliance is lifted from its base. This will extend the life of the appliance and means that substantial electrical contacts may not be needed. It will be appreciated that the various features as described above may be utilised in conjunction with other features of the invention as described earlier. Some preferred embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which:- - 21 - Figure 1 is a perspective view of a heater and control unit assembly embodying the invention, the assembly being inverted for ease of reference; Figure 2 is a section through the mounting for the assembly of Figure 1: Figure 3 is a view of the heater of the assembly of Figure 1 ; Figure 4 is a schematic view of the various switching components of the control; Figure 5 shows in greater detail the components of the low/high power mechanism of the control unit of the invention; Figure 6 shows in greater detail the components of the long/ short boil mechanism of the control unit; Figure 7 shows the reboil mechanism of the control unit ; Figure 8 is a view of the side of the control unit which in use will face the heater of Figure 3 ; Figure 9 shows a perspective view of the simmer unit of the control; Figure 10 shows an underneath plan view of the simmer unit of Figure 8; Figure 11 shows in greater detail the components of the dry boil mechanism of the control; Figure 12 shows a view of the base moulding for a vessel incorporating the invention; Figure 13 illustrates schematically a second liquid heating vessel embodying the invention; and Figure 14 illustrates a second switching unit and heater track arrangement in accordance with the invention; Figures 1 to 12 describe a first embodiment of the invention, which will now be described below. GENERAL ARRANGEMENT With reference to Figures 1 to 12, there is shown a - 22 - control unit 2 which is mounted against a thick film printed plate heater 4. The control unit 2 and heater 4 are assembled together before being mounted together to close opening 6 in the base 8 of a plastics airpot body (Fig. 3) . As can be seen for example in Figure 5, the control unit 2 and the heater 4 are assembled through a plurality of bayonet type fittings 10. To this end, the heater 2 is provided with four equispaced tangs 12 (Fig. 3) which engage under respective tongues 14 (Fig. 5) which extend circumferentially from four hollow mounting bosses 16 provided on the moulded housing 18 of the control unit 2. As can be seen in Fig. 5, the tangs 12 abut the rear wall 20 of the bosses 16 so as to locate the heater 4 rotationally with respect to the control unit 2, to ensure proper alignment of the various electrical connections between the control unit 2 and the heater 4 as will be described in further detail later. As can be seen in Figure 2, the mounting bosses 16 engage over mounting pillars 22 provided on the airpot body base 8 for receiving clamping screws 24 therethrough. It will be seen that a silicone rubber 26 seal 26 is arranged between an L shaped sealing flange 28 provided on the periphery of the heater 4 and an L shaped seat 30 provide on the airpot body 8 around the opening 6. The bottom surface of the boss 16 abuts the top of the mounting pillar 22 so as to control the clamping pressure applied to the seal 26 and prevent overtightening which might deform and damage the respective parts. As will be appreciated, the arrangement illustrated will compress the seal axially and radially so as to make a sealing contact along all the opposed surfaces of the sealing flange 28 and the seat 30. To allow the seal 26 to expand against these surfaces without effectively solidifying, a circumferential rib 32 is provided on one of the moulding seats 30. In this way an extremely satisfactory - 23 - seal can be obtained. It will also be noted from Figure 3 that the heater 4 has a number of semicircular recesses 40 in its outer periphery adjacent the bayonet tangs 12. The body mounting pillars 22 pass through and engage in these recesses 40 during mounting, so that once the assembly is mounted to the vessel body, they will prevent rotation of the heater relative to the control unit 2. CONTROL UNIT Turning now to the features of the control unit 2 , as will be described in much greater detail below, the control unit 2 comprises a number of interrelated control mechanisms which are mounted in the control housing 18. These may be identified broadly as follows: 1. A high power/ low power boil mechanism 200, which will allow a user to boil liquid in the airpot at a high power or low power, and which will automatically switch to a low power setting after the liquid in the vessel has boiled; 2. A long boil / short boil mechanism 300 which will allow a user to select a long or a short boiling time, and automatically switch to simmer after the liquid has boiled; 3. A reboil mechanism 400 which will allow a user to reboil the liquid in the vessel after it has boiled; 4. A simmer unit 500 which will allow liquid in the vessel to be kept at a desired temperature after boiling; and 5. A dry switch on protection mechanism 600 which will disconnect the power supply to the heater in the event it overheats, for example if the airpot boils dry or it - 24 - is switched on without any water in it. Only the general locations of these mechanisms are indicated on Figure 1 and they will be described in greater detail below. Firstly, however, it is useful to describe the electrical detail of the heater 4 of the preferred embodiment and the various electrical connections made thereto with reference to Figures 2 and 4. HEATER The heater 4 is a thick film printed heater. Such heaters are well known in the art and are now beginning to be used for water heating purposes. Indeed, their use has already been proposed in the context of airpots in our earlier WO97/04694. Briefly, however, the heater structure is very similar to that described in WO97/04694, comprising a stainless steel plate 50 having a central sump 52 around which is deposited an insulating dielectric layer 54 on top of which is provided, a series of serpentine resistive heating track sections 56, 58, 60. A protective overglaze (not shown) is provided over the tracks. The plate 50 has a number of features around its periphery for mounting purposes, as have already been described above. The first and second track sections 56, 58 are arranged around the outside of the heater 4 and are each nominally rated at 1000W. These track sections are intended for heating liquid in the airpot to boiling. The other, inner, track section 32 is rated at about 80W and is intended to act as a simmer element when placed in series with the other two track sections. As can be seen, the respective track sections 56, 58, 60 are each serpentine, comprising a plurality of parallel track lengths, the respective ends of which are connected by bridges 64 of a low resistance material - 25 - such as silver. The silver bridges 64 may be printed in a suitable ink form over or under the track sections during manufacture of the heater and then fired with the heater tracks 62. This arrangement provides a particularly compact and even track arrangement, which means that a smaller heater plate may be used for a given heating power, thereby reducing costs. The first track section 56 is actually arranged in two serpentine portions 56a, 56b. The smaller, 56b of the two portions is intended to receive a dry switch on actuator 50 as will be described later. It occupies a relatively small area (about the same area as the dry switch on actuator) and accounts for only about 20% of the total resistance of the track section 56. This means that there is a relatively low potential drop across the portion 56b, thereby reducing the possibility of shorting through the actuator 50. The outer track sections 56, 58 are provided with respective regions 66, 68 which are so configured as preferentially to self protect by becoming open circuit in the event of a serious overheating of the respective element section. This feature is described in greater detail in WO 97/39603, to which reference should be made for further details. Essentially, however, should the heater overheat, the resistance of the overglaze will fall so as to allow a leakage current to flow between the adjacent track portions. This in turn generates further heat, which further increases the leakage current . This very quickly leads to the leakage current running away, to the extent that it exceeds the design rating for the track, which vaporises, so disabling the heater. It will be seen that in this embodiment the region 66 at least is situated at a spacing from one end of the track 56. This is intended to limit the overload current to a value which will not lead to fusing of the normal appliance fuse nor indeed the house fuse by leaving a length of resistive track between the region 66 and the terminal of the heater track. This - 26 - arrangement is itself inventive. The region 66 may be spaced from either or both ends of the track 56 to give the desired response. One end of the track section 56 is provided with a pair of spaced terminal contacts 70, 72 and the other end with a terminal contact 74. Similarly one end of the track section 58 is provided with two terminal contacts 76, 78 and its other end with a single terminal contact 80. That other end of the track section 58 is also connected to one end of the simmer track section 60, so that the terminal contact 80 acts as an interconnection between both track sections 58, 60. The other end of the simmer track section 60 is provided with a terminal contact 82. Electrical connections are made to each of these terminal contacts by respective contacts provided in the control unit 2 as will now be described in greater detail below. ELECTRICAL CONNECTIONS TO THE HEATER TRACK As can be seen most clearly from Figure 4, the control moulding 18 is provided with six apertures 84 therethrough to allow access to the various heater track terminal contacts 70-80. These apertures are built up on the heater facing side (see Figure 11) , so as to act to shield and contain any arcing produced at the. contacts. The control moulding 18 mounts six low force leaf springs 86, 88, 90, 92, 94 and 96, arranged in respective pairs. Each leaf spring has an electrical contact 70a, 72a, 74a, 76a, 78a, 80a, mounted on its free end for contacting the corresponding respective track terminal contacts 70-80 through the apertures 84. A first pair of leaf springs 86, 88, 56 are rivetted together at one end to a mounting boss 98 to which is also mounted a conductive strip 100 (Figure 1) running from the neutral terminal pin 102 of an electrical connector 104 for connection to an external - 27 - power supply. A further conductive link 106 runs from the neutral boss under the control moulding 18 as shown in Figure 8 for connection to the simmer unit 500. A second pair of leaf springs 90, 92 is mounted to a further boss 108 to which is also connected a conductive strip 110 running from the line terminal pin 112 of the connector 104. The third pair of leaf springs 94, 96 are commonly rivetted to a third boss 114. The leaf springs 86-96 are received and located within respective channels 116 moulded into the control moulding 18. The arrangement of the leaf springs 86-96 will allow the heating track sections 56, 58 to be placed selectively in series or in parallel, so as to vary the power of the element during heating of the airpot ' s contents to boiling, or to be placed together in series with the simmer track portion 60 to produce a low power simmer. This is achieved by the movement of a number of pivotally mounted lever arms which engage the various leaf springs 86-96. These lever arms form parts of the various respective mechanisms mentioned above, and for sake of simplicity, these lever arms will be described in the context of those mechanisms below. HIGH POWER/LOW POWER MECHANISM (FIG. 5! As stated above, this mechanism 200, will allow a user to choose to boil liquid in the airpot at either high power or low power, and will automatically switch the heater 4 to a low power setting after the liquid in the airpot vessel has boiled. Referring primarily to Figures 4 and 5, the mechanism comprises a snap acting bimetallic actuator 202 of the type described in GB1542252 which is mounted by its central tongue 204 on a "spectacles" shaped copper plate 206 which is clipped into a mounting slot 208 provided in the control moulding 18. The lower surface of the plate 206 will in use abut in thermal contact with the base of the sump 52 of the heater plate 50. - 28 - The nose 210 of a pivotally mounted double lever arm 210 is positioned over a peripheral portion 214 of the actuator 202. The double lever arm 212 is pivotally mounted in a V groove 216 provided in a mounting pillar 218 upstanding from the floor of the control moulding 18. The lever arm 212 is biased into, and retained in the V groove 216 by an Ω spring 220 (shown separately in Figure 5 for ease of understanding) mounted between respective seats 222, 224 provided on the rear of the nose 210 and the front face of the pillar 218. It will be appreciated that the lever arm 212 is bistable, being urged towards one or other of its pivotal positions by the Ω spring 220 will also act to retain the lever arm 212 in one or other of its operating positions. The double lever arm 212, which fits around the mounting pillar 218, has a pair of forwardly projecting arms 226, which extend under the leaf springs 86, 92. It also has a pair of rearwardly extending arms 228 which extend under the leaf springs 94, 96. It will be appreciated that depending on the position of the double lever arm 212, either the leaf springs contacts 70a, 78a will be lifted away from their respective corresponding heater terminal contacts 70,78 with the leaf springs contacts 72a, 74a in contact with the heater terminal contacts 72, 74, or vice versa. In the latter condition as illustrated in Figure 5, the heater track sections 56, 58 will be connected electrically in parallel, as the neutral side of the electrical supply will be connected through the leaf springs 86, 88 and contacts 70a, 80a to the terminal contacts 70, 80 of the track sections and the line side of the supply connected to the other terminal contacts 76, 78 of the track sections through leaf springs 90, 92 and contacts 76a, 78a. Thus the electrical supply will be connected across track section 56 through terminal contacts 70, 74 and across track section 58 through terminal contacts 78, 80. As each track section is rated at lkW, this will deliver 2kW heating power. - 29 - However, in the other condition with the leaf springs 86, 92 and contacts 70a, 78a raised, the track sections 56, 58 will be arranged electrically in series. A neutral connection is made to track portion 58 through leaf spring 88, contact 80a and terminal contact 80, and a line connection made to track portion 56 through leaf spring 90, contact 74a and terminal contact 74. The leaf springs 94, 96 then bridge the terminal contacts 72, 76 of the tracks 56, 58 to connect the track sections in series, and so deliver 500W heating power. As is shown in Figures 1 and 3, an upstanding arm 230 is provided on the rear of the double lever arm 212. This is linked to a push rod 232 which extends slidably into an opening 234 in the front face 236 of the control moulding 18. As shown in Figure 12, when the control unit is installed in the airpot, this face 236 will be arranged behind a window 238 in a base moulding 240 of the airpot . The window will be covered with a flexible diaphragm (not shown) to allow the end of the rod 232 to be pressed by a user. It will be appreciated that when the button is pushed, the double lever arm 212 will pivot anticlockwise (in the sense of Figure 1) about its mounting into the position shown in Figure 5, i.e. into the high power boiling condition. Also as shown in Figures 1 and 5, an arcuate arm 242 is integrally moulded to one side of the double lever arm 212. This has a U shaped saddle 244 which allows the passage of the push rod 232, and at its free end engages an indicator mechanism 250. The indicator mechanism 250 comprises a bistable rocker arm 252 pivotally mounted in a seat provided on the rear of the front wall 236 of the control moulding 18. The rocker arm 252 is biased into its seat and urged into one or other of its operative positions by a coil spring 256 which is anchored around a lug 258 on the rocker arm 252 and a post 260 in the front wall 236 of the moulding 18. The rocker arm 252 has symmetrically arranged upwardly and downwardly projecting heads 262 (only the upper one - 30 - of which is visible in Figure 5) , each of which with a planar indicator face 264 which is highly coloured, for example having a "dayglow" finish. These heads 262 are arranged to lie behind respective upper and lower windows 270 in the base moulding 240 of the airpot as illustrated in Figure 12. In the position shown in Figure 5, the upper face 264 will lie behind its window 270, which will thus appear illuminated, indicating high power heating. However, if the double lever arm 212 moves to its other position, the arm 230 will trip the indicator mechanism to its other position so that the face 264 moves away from its window 270 (which will then appear dark) , and the other head 262 will then move into position in, and apparently illuminate, its window 270 thereby indicating low power heating. The arcuate arm 242 connected to the double lever arm 212 engages the indicator arm 252 with a lost motion so that the actuator 202 does not have also to move the indicator mechanism when it operates, thereby reducing the forces on the actuator. The indicator arm 252 is actually picked up by the arm 240 after the double lever arm 212 has been tripped by the actuator 102. The mechanism 200 described above not only allows a user to select a high power boil at will, it also ensures that once liquid in the vessel boils, it will automatically reduce the power when boiling occurs. This is extremely important, since airpots are sealed containers, and if left to boil for any period of time at 2kW, they could prove very dangerous for a user. Assuming that the control is in the high power boil condition shown in Figures 3 and 4, when liquid in the airpot vessel boils, the bimetallic actuator 202 will operate by thermal conduction through the sump 52 of the heater 4 and the plate 206 within a predetermined time, say 5 seconds . This will cause the periphery of the actuator to snap upwardly, carrying with it the nose 210 of the double lever arm 212. The arm 212 will then - 31 - pivot overcentre with respect to the omega spring 220, causing the arms 226 to lift the leaf springs 86, 92 and allowing the arms 228 to lower the leaf springs 94, 96. Thus the heater tracks 56, 58 will be switched from a parallel to series connection, i.e. to a low power boil setting. At the same time the indicator 252 will change condition from its high power position to indicate the low power setting. Of course, if the airpot was being boiled at low power, the operation of the actuator 202 will have no effect on the heating power, as the nose 210 of the lever arm 212 will be out of engagement with the actuator 202. LONG BOIL / SHORT BOIL MECHANISM (FIG 6) The low power/high power mechanism described above only determines whether liquid in the airpot will be boiled at high power or low power. The long/short boil mechanism 300 described primarily with reference to Figures 1, 4 and 6, allows a user to select a short boil or a long boil time and automatically sets the airpot to simmer after the desired boiling time. The mechanism 300 comprises a selector arm 302 which is pivotally mounted in V grooves 304, 306 on respective spaced mounting pillars 308,310 provided in the control moulding 8. The selector arm 302 is bistable and is biased into the grooves 304, 306 and one or other of its operating positions by an Ω spring 312 which acts between respective seats 314, 316 provided on the selector arm 302 and mounting pillar 308. Extending in one direction from the selector arm 302 is a actuating arm 318 which engages with the peripheral rim 320 of a bimetallic actuator 322 which is mounted to the lower face of the spectacle shaped plate 206. Furthermore, at one end of a cross limb 324 of the selector arm 302 is provided a journal 326 for a pivotally mounted arm 328. The arm 328 clips into the - 32 - journal 326, and has a counterweight 330 formed at one end, and a leaf spring lifting arm 332 at its other end. The leaf spring lifting arm 332 extends under the mounting limb 334 of the selector arm 302 and has a lug 338 which engages under leaf spring 88. The leaf spring lifting arm 332 also has an abutment pillar 340 which engages the periphery of the bimetallic actuator 202. A leg 342 (Figure 6) extends downwardly from the mounting limb 334 of the selector arm 302 and engages a push rod 344 which extends through an aperture 346 in the front face of the control moulding 18. This will be covered by a flexible diaphragm in the same way as the boiling power push rod 232 and will allow a user to select a long boil time, as will be explained further below. An arcuate arm 350 extends away from the cross limb 334 and engages in a boiling time indicator 52 identical in construction and operation to the boiling power indicator described above, and which will indicate whether the airpot is in a long boil or short boil mode. In the condition shown in Figure 6, the mechanism is in a long boil mode, and one indicator face 354 will be arranged in a window 356 in the airpot base moulding 240 (see Fig. 12) . Finally, a further arcuate arm 360 (Figure 6) , which is pivotally mounted in bearings 362 in the control moulding 18 has a nose 364 which rests on the leaf spring 88. The other end 366 of the arm engages with a yet further indicator mechanism 368 which is identical in construction and operation to the other indicator mechanisms described above and which will indicate whether the airpot is either boiling or simmering, as will be described further below. The end 366 is provided with an integral counterweight 370 to assist the mechanism 268 to return to its baking position. Assuming that the mechanism is in the short boil condition, the selector arm 302 will be the condition - 33 - shown in Figure 4, with the abutment pillar 340 of the leaf spring lifting arm 332 resting on the periphery of the bimetallic actuator 202. When the actuator 202 operates when liquid in the vessel boils, the pillar 340 will be lifted by the actuator 202, so pivoting the arm 332 in the journal 326 and lifting the leaf spring 88. Should the mechanism initially be set in the long boil condition by pressing the long boil push rod 324, then the selector arm 302 will be pivoted up from its short boil position, so that the abutment pillar 240 no longer engages the actuator 202. Rather, the long boil actuating arm 318 moves into engagement with the bimetallic actuator 322 under the plate 206. The bimetallic actuator 322 is arranged thermally remote from the sump of the heater and is heated through conduction through the mounting plate 206. A choke region 370 in the plate slows down the transfer of hat to the actuator 322, so that it will not operate until some time after the actuator 202 operates. Thus when the control is set to long boil, the actuator 202 will operate first, but this will have no effect on the mechanism, as it will not move the leaf lifting arm 332. After a predetermined further time, however, the long boil actuator 322 will operate, pivoting the selector arm 302 anti-clockwise in the sense of Figure 4 and moving it overcentre with respect to the omega spring 312. With this movement, the pillar 340 on the leaf spring lifting arm 332 moves into engagement with the periphery of the bimetallic actuator 202 (which will be raised, having already operated) and continued movement of the selector arm 302 will cause the leaf lifting arm 332 to pivot anticlockwise about the pillar 340 to lifting the leaf spring 88. As will be recalled, when liquid in the vessel boils, the actuator 202 operates the high/low power mechanism to connect the heater track sections 56,58 in series to provide 500W heating power, if they are not already so connected. If the airpot is initially set - 34 - for a high power, short boil, then the leaf spring 88 will be lifted at substantially the same time as the high/low power mechanism switches the track sections 56,58 into series. If the control is set for a high power, long boil then the actuator 202 will act to connect the track sections 56, 58 in series some time before the long boil actuator 322 operates. When the leaf spring 88 is lifted by the leaf spring lifting arm 332, it breaks the electrical connection between contact 80a and heater terminal contact 80, which places the three track sections 56, 58, 60 in series to provided a low simmer power. The electrical circuit to the heater is provided from the neutral mains connector pin 102 through the simmer control unit 500 to the simmer track terminal contact 82, through the simmer track section 60, the heater track section 58 to the terminal 76, through the bridging leaf springs 94, 96 to terminal contact 72 of the track section 56, through the track section 56 to the terminal contact 74 and through leaf spring 90 to the line connector mains pin 112. It will be appreciated that when during heating, electrical contact is made to the terminal contact 80, the simmer element 60 is effectively shorted out, as both of its ends will be at the same, neutral, potential. As long as the contact remains broken at terminal 80, the heater will cycle solely depending on the setting of the simmer control unit 500, whose construction and operation will be described further below. Also, the operation of the mechanism above returns the selector arm 302 to its short boil position. Furthermore, the lifting of the leaf spring 88 also moves the indicator 368 to its simmer indicating position via the arm 360. If it is desired to reboil liquid in the airpot after the heater begins to simmer, then a reboil mechanism 400 is provided as will now be described. - 35 - REBOIL MECHANISM (FIG. 7) Once liquid in the airpot has boiled and the heater begins to simmer, it will continue to simmer until either the actuator 202 falls below its reset temperature (which may happen if the airpot is turned off, or if cold water is added) or a user operates a reboil mechanism 400. The preferred actuator reset temperature is about 50°C, which allows the actuator to reset if, as is typical, a volume of liquid in the vessel at room temperature (say 20°C) is added to a similar volume of liquid at about 80°C. The reboil mechanism 400 comprises a reboil lever arm 402 pivotally mounted in the moulding 18. The reboil lever arm 402 has a nose 404 which overlies the periphery of the bimetallic actuator 202. The lever arm 402 also acts on a push rod 406 which extends through a further aperture 408 in the front face 236 of the control moulding 18, which is accessible through the window 238 provided in the airpot bottom moulding 240, which window 238 is covered by a diaphragm or the like for pushing by a user. When the push rod 406 is pressed, it will cause the nose 404 to pivot down onto the periphery of the actuator 202 so as to reset it. As it does so, the leaf spring lifting arm 332 is lowered to remake the contact at heater terminal contact 80, so shorting out the simmer track portion 60 and once again connecting the track sections 56, 58 in series to provide low power heating. At the same time, the simmer indicator arm 360, which rests on the leaf spring 88, returns to its heating position under the action of its counterweight 370, thereby changing the condition of the indicator 268 which moves from a simmer to a boiling condition. Should high power boiling then be required, the high power push rod 232 may be pressed to pivot the lever arm 210 to reconnect the track sections 56, 58 in parallel . - 36 - SIMMER UNIT (FIGS. 8, 9 & 10) The simmer unit 500 is a completely separate, precalibrated, unit which is mounted in a recess 502 formed in the base of the control moulding 18 from below, as shown in Figure 7. The simmer unit is retained in the recess by moulded clips 504 provided on the control moulding 8. As described above, a conductive link 106 extends into the mounting recess 502 to connect the neutral side of the power supply to the simmer unit 500. As shown in Figures 8, 9 and 10, the simmer control unit 500 comprises a stack of components held together by rivets 506. The stack includes, from the bottom up, a base plate 508, a lower insulator body 510, a contact mounting plate 512, a spring plate 514, and an upper insulator body 516. The base plate 508 is of a thermally and electrically conductive material such as copper, and has a downwardly projecting leg 520 for engaging a portion of the simmer track 60 when the simmer unit 500 is mounted to the heater. This leg 520 therefore senses the temperature of the track 60 which will be indicative of the liquid temperature within the vessel. The base plate 508 also has a downwardly extending electrical contact 522 which engages with the terminal contact 82 of the simmer track 60. Finally it has a forwardly projecting portion 524 which acts as a backstop for the movable contact 526 of a snap acting bimetallic actuator 528. The actuator 528 is arranged in thermal contact with the plate 508 so as to receive heat from it and is slightly domed so as to reverses its curvature when heated above a certain operating temperature, as is well known in the art. In fact, the actuator 528 has a rearwardly projecting tongue 530 which is rivetted by rivets 532 to the plate 508. The actuator 528 has a contact 526, mounted on a movable tongue 534 defined by - 37 - a U shaped cut out 536 of the actuator 528, which in use makes and breaks electrical contact with a fixed contact 538 formed on a projection 540 of the contact mounting plate 512. One end of the side limbs 542 of actuator 528 are clamped between a bow spring 544 and the arms 546 of an L shaped adjustment lever 548 which is pivotally mounted in V grooves 550 provided in the lower insulator body 510. The upper end 552 of the lever 548 is biased by the action of the bow spring 544 against an adjustment screw 554 which is mounted in a threaded boss 556 formed in the upper insulator body moulding. The combined action of the bow spring 544 and the lever arm 546 acting on the side limbs 542 of the actuator 528 determine its operating characteristics, and provide an adjustment mechanism as will be discussed further below. The contact mounting plate 512 is located between the upper and lower insulator bodies 510, 516 and is clamped against the spring plate 514 which has four resilient leaves 560 extending therefrom. These leaves 560 which act against the bottom of the recess 502 act not only to bias the simmer unit 400 into good thermal contact with the heater 4, but one of them also engages with the conductive link 106 which extends into the mounting recess 502. The lower insulator body 510 comprises a pair of upwardly extending hollow bosses which pass up through holes provided in both the contact mounting plate 512 and the spring plate 514 so as both to locate the stack components with respect to one another, but also electrically to insulate the contact plate 512 and the spring plate 514 from the rivets 506 which hold the stack together and which pass right through the stack from the base plate 508 (with which they are electrically connected) to the upper surface of the upper insulator body 516. As stated above, the upper insulator body 516 has a - 38 - threaded boss 556 which receives an adjustment screw 554. The adjustment lever 548 is pivotally biased against the adjustment screw 554 by the action of the bow spring 544 through the actuator 528. As can be seen from Fig 1, when the simmer unit 500 is installed in its recess 502 in the control moulding, the head of the adjustment screw 554 extends through the base of the recess 502 and is engageable with a complementary shaped head 562 of a rod 564. The other end of the rod 564 engages with an adjustment screw 566 mounted in an aperture 568 the front face of the control moulding 18 and which can be manipulated by a user, for example with a screw driver. It will be appreciated that as the adjustment screw 554 is moved backwards and forwards by rotation of the rod 564, the lever 548 will pivot and move the side limbs 542 of the actuator 528 against the action of the bow spring 544. This will change the operating point of the actuator 528, so meaning that a higher or lower sensed temperature will be needed to operate the actuator 528. Thus the desired water simmering temperature can be set simply by the user by adjusting the external screw head 566. The spring biasing arrangement described above allows the geometry of the blade to be varied in two directions from its neutral, as manufactured position, thereby affording a greater range of available adjustment. When the control unit 2 switches to its simmer mode by operation of the mechanism 300 as described above, the simmer unit 500 will cycle the heater by opening and closing the contacts 526, 538, which break the neutral side of the supply to the element. DRY SWITCH ON PROTECTION (FIGS. 8 AND 11) In the event that the vessel boils dry or is switched on without any liquid in it, a dry switch on protection mechanism 600 is also provided in the preferred - 39 - embodiment . This mechanism, which is described primarily with reference to Figures 4, 8 and 11 comprises a snap action bimetallic actuator 602 which, as shown in Figure 6 is mounted by its central tongue 604 on the face of the control moulding 18 which faces the heater 4. In fact, the tongue 604 of the actuator is mounted to a stainless steel leaf spring 606 which is clicked into a mounting pillar 608 on the control housing 18. This spring 606 acts resiliently to bias the actuator 602 against the track portion 56b (Fig. 2) when the control unit 2 is mounted to the heater 4. The intended position of the actuator 602 is indicated in phantom on Fig. 2. A nose 610 of a dry switch on lever arm 612 which is pivotally mounted in a support 614 provided in the moulding 18 extends through the base of the control moulding 18 to engage the actuator 602. The dry switch on lever arm 612 has two generally U shaped limbs 616, 618 (Figure 9) which have respective lugs 620, 622, 624 and 626 which engage under the respective leaf springs 86, 88, 90, 92. In the event that the heater track section 56b overheats say if the airpot is turned on without water in it, the actuator 602 will operate, to push on the nose 610 of the dry switch on lever arm 612, so pivoting the dry switch on lever arm 612 to lift all four leaf springs 88, 90, 92, 94. This breaks the electrical connections to the terminal contacts 70, 80, 74, 78 of the heating tracks 56, 58, which completely disconnects the power supply to these tracks, irrespective of the setting of the control and thus irrespective of which of these contacts may already have been made. It will also be appreciated that this will also disconnect the power supply to the simmer track section 60. Once the heater 4 cools, the actuator 602 will automatically reset, allowing the contacts to be re-made, and the heater will then cycle on and off until it is switched off or liquid is added to the airpot to prevent the overheating. - 40 - As can also be seen in Figure 11 in particular, a pair of lugs 630, 632 extend from the top of the dry switch on lever arm 612 and engage with hooks 634 provided on a further generally L-shaped lever arm 636. This lever arm 636 is pivotally mounted in bearings 638, 640 in the control moulding 18. The other end of the lever arm 636 is provided with a coupling 642 for connection to a further push rod (not shown) which is connected to an interlock mechanism in the airpot which is arranged such that should the lid of the airpot be opened, or should a user attempt to dispense liquid from the vessel, the push rod will pull up on the lever arm 536 (in the sense of Figure 11, so rotating the dry switch on lever arm 612 to open the four contacts and disable the heater. This will help to prevent injury to anyone using the airpot, as liquid in the airpot will not be boiling when the airpot is opened or dispensing occurs . Turning now to Figure 13 and 14, a second embodiment of the invention is described. This embodiment is much simpler than the first, being intended only to have a high power and low power setting. With reference to Fig. 1, a hot water jug 702 comprises an open bottomed plastics body 704 with a planar thick film heater 706 suitably secured (by means not shown) to close the bottom of the body 704. Under the base of the vessel is arranged a combined thermally sensitive control and cordless electrical connector unit 708, (illustrated schematically only in phantom lines) of the type disclosed in WO 95/34187, available commercially as the applicant's U18 control unit . Arranged between the control unit 708 and the planar heater 706 is a switching unit 710, whose detail is described further below. The base of the vessel is covered by a cover 712, and the vessel rests on a base 714 having a connector - 41 - 716 for engagement with the electrical connector in the control unit 708. A power lead 718 connects the connector of the base unit to a power supply. Turning to Fig. 14, as is conventional, the planar heater 706 comprises a stainless steel plate 720, on which is deposited an insulating glass layer 722. Two thick film heating tracks 724,726 are provided on the insulating layer 722 in a conventional manner. Each track has resistance of 44Ω and is of the same length and watts density. The length of track needed to produce the desired resistance (and thus power) can be calculated knowing the properties of the track material in a known manner . Track 724 has respective terminals 724a, 724b and track 726 has respective terminals 726a, 726b, which are in the form of conventional printed silver contact pads. The tracks 724,726 are arranged over the substrate in a suitable pattern. The terminal pairs 724a, 726a and 724b, 726b are arranged parallel to each other and at the same spacing. Two locators 730 (only one of which are shown) are fixed to the heater, e.g. by welding. The switching unit 710 comprises a moulded plastics slider body 734 having a grip 736 at one end, which in use extends though an opening in the base cover 712, as shown in Fig. 1. A pair of guide slots 738 are provided in the other end of the slider body 734 for engagement over the locators 730. The slider body 734 is provided with upstanding slotted lugs 740,742,744 to receive components of the switching unit 710. In particular, the first lug 740 has a slot 746 which receives a BeCu leaf spring 748 having contacts 750,752 at either end. The contacts may be portions of the leaf spring 748 itself (e.g. a silver plated portion) or a separately mounted contact member. The contacts 750,752 extend through apertures 754 provided in the slider body 734. The lug 742 comprises slots 758,760 for receiving - 42 - one end of respective BeCu double contact members 762, 764. Each double contact member 762,764 is generally "H" shaped, with the upper ends 766 of each outer limb of each member 762,764 being received in the respective slots 758,760. Of course, the ends 766 of the respective members 762,764 are spaced from each other so that they do not short circuit the heater. Respective electrical contacts 768,770,772,774 are provided on the other ends of the outer limb of each member 762,764. These may be separate contact members mounted to the member 762 or an integral part thereof. The contacts extend through respective apertures 776 in the slider body 734. A 'T' shaped extension 778 extends from the cross member of each "H", the cross limb of the "T" forming a slider pad 780 for slidably receiving spring contacts 782,784 of the control unit 708. These contacts 782 are leaf springs pushed onto the terminal of the control unit 708. A tab 784 extending from each slider pad 780 engages in slotted lug 744 to secure the member 762 in position. The respective spring contacts 782 are connected through the control unit 708 to respective poles of a power supply. The slider body is located against the heater by suitable means and is guided in its movement by the guide slots 738 and locators 730. Operation of the device will now be described. In the position shown in Fig. 14, contact 750 of bridging member 748 engages contact pad 726b of heater track 726, and contact 752 of the bridging member 748 engages contact pad 724a of heater track 724. Also, contact 768 of the double contact member 762 engages contact pad 726a of track 726 and contact 772 of double contact member 764 engages contact pad 724b of track 724. The other contacts 770,774 of the double contact members 762,764 do not contact any terminal part of the track 726,724. In this condition, therefore, the tracks 724,726 - 43 - are connected in series, power flowing from spring contact 782 through double contact member 762, contact 768, contact pads 726a, track 726, contact pad 726b, bridging contact 750, bridging member 748, bridging contact 752, contact pad 724a, track 724, contact pad 724b, contact 772, double contact member 764 and spring contact 784. With a supply voltage of 220V, the total heating power generated in the heater will be 220 2 /88 = 550 W. This condition would be suitable for low power, slow heating of e.g. milk or other fatty products. When the slider is moved in the direction of arrow A, bridging contacts 750,752 move out of contact with contact pads 726b, 724a respectively, and the contacts 772,768 of the double contact members 762,764 move into engagement with these pads. At the same time the other contacts 774,770 of the double contact members 762,764 move into engagement with the contact pads 724b and 726a respectively. In this position of the slider, the tracks 724,726 are connected electrically in parallel, the contacts of the double contact members connecting the respective ends of each track. In this condition, for the same supply voltage, the heating power will be 2 x (220 2 /44) = 2200 W, i.e. four times greater than that of the series arrangement. Furthermore, the power density is also increased by a factor of four. Such conditions are suitable for rapidly heating water. The slider pads accommodate the movement of the slider relative to the control spring contacts 782,784 to maintain continuity of electrical supply. It will thus be seen that this embodiment provides for at least two levels of heating in a liquid heating appliance and for easy switching between the two. The embodiment described is of course only illustrative, and variations on the arrangement shown may be made without departing from the scope of the invention. For example, while only two heating tracks are shown in the above embodiments, the principle of the - 44 - invention may apply to two or more tracks. Also, while the switch unit is shown with two operative positions, it could have a third position in which no circuit is made with the heater tracks, effectively turning the heater off. Furthermore, if a greater range of possible power outputs is required, it would be possible for example to switch one or other of the heating tracks in or out completely in addition to parallel/ series switching. Also the second embodiment may be adapted to incorporate additional features of the first embodiment. As has been explained, the invention will find particular application in contexts where operability at different supply voltages is required, or more likely where different heating powers are required. In the latter case the invention has the advantage of being able to provide rapid, high power heating or slower, lower power heating also at a reduced power density which will reduce the likelihood of fatty products such as milk burning. It also means, for high power heating, an audible signal is produced indicating that the appliance is heating rapidly. From the above description, it will also be seen that the first described embodiment of the invention provides an integrated control unit which performs a multiplicity of functions. It will allow water to be boiled quickly or slowly and for a short period or for a longer period. It will also ensure that once liquid in the vessel has boiled, it will continue to boil (if so desired) at a low power. Moreover it incorporates overheat protection means which are also associated with safety features provided in the airpot itself. It does this with a minimum number of components which keeps costs to a minimum.

Claims: - 45 - Claims 1. A liquid heating apparatus comprising a thick film heater, said heater comprising at least two resistive heating tracks having respective terminals, and further comprising switch means comprising switch contacts which are movable to engage selected terminals of said heating tracks . 2. Apparatus as claimed in claim 1 wherein said tracks are connectable selectively in series and in parallel by said switch means . 3. Apparatus as claimed in claim 1 or 2 wherein the resistance of each heating track is similar. 4. Apparatus as claimed in claims 1, 2 or 3 wherein said heating tracks are provided on the underside of a member forming a base portion of a heating vessel. 5. Apparatus as claimed in any preceding claim wherein the movable switch contacts are slidably mounted with respect to the heating tracks . 6. Apparatus as claimed in any of claims 1 to 5 which said movable switch contacts are liftable on and off said track terminals. 7. A liquid heating vessel having electrical heating means having a high power setting and a low power setting, and control means permitting a user to select said high power setting. 8. A liquid heating vessel having electrical heating means having a high power setting and a low power setting, and control means comprising means for detecting boiling of liquid within the vessel, and means responsive to the boiling detection means for switching - 46 - the heating means from its high power setting to its low power setting once the liquid in the vessel has boiled so as to permit continue boiling of the liquid at the lower power setting for a predetermined period of time after boiling has been detected. 9. A control for a liquid heating vessel having electrical heating means, the control comprising means for detecting boiling of liquid within the vessel, and means responsive to the boiling detection means for switching the heating means from a high power setting to a low power setting once the liquid in the vessel has boiled so as to permit continue boiling of the liquid for a predetermined period of time after boiling has been detected. 10. Apparatus as claimed in of claims 7 to 9 wherein the low power setting is less than or equal to % the high power setting, and more preferably less than or equal to A the high power setting. 11. Apparatus as claimed in any of claims 7 to 10 wherein the heating means comprises a series of element sections which are connectable electrically in series. 12. Apparatus as claimed in any of claims 7 to 11 wherein the heating means comprises element sections which are connectable selectively in series or in parallel by the switching means to effect the change in power setting. 13. Apparatus as claimed in any of claims 7 to 12 wherein the heating means is a thick film heater. 14. Apparatus as claimed in claim 13 wherein switching electrical connections are made to terminals provided on the track. - 47 - 15. Apparatus as claimed in any of claims 1 to 6 or claim 14 wherein the terminal is a contact member bonded to the track. 16. Apparatus as claimed in 14 or 15 wherein the switching means acts selectively to lift and drop spring mounted contacts onto terminals of the heater tracks. 17. Apparatus as claimed in claim 6 or claim 16 wherein the switching means comprises double lever arm having arms arranged on opposite sides of a pivot such that on operation, one arm pivots in one direction and the other pivots in the opposite direction such that respective contacts may be lifted from or dropped onto the terminals substantially simultaneously. 18. Apparatus as claimed in claim 8 or 9 or in any of claims 10 to 17 as dependent directly or indirectly upon claims 8 or 9 further comprising reset means for resetting the heating means to a high power boil condition after boiling. 19. Apparatus as claimed in claim 8 or 9 or in any of claims 10 to 18 as dependent directly or indirectly upon claims 8 or 9 wherein the boiling detection means comprises a bimetallic actuator. 20. Apparatus as claimed in claims 17 and 19 wherein said actuator acts to move said double lever arm from its high power to its low power position. 21. Apparatus as claimed in any preceding claim further comprising means to allow liquid in the apparatus to boil for a longer or shorter period of time. 22. Apparatus as claimed in claim 21 wherein said means comprises first and second thermally sensitive actuators - 48 - arranged to operate at different times after liquid in the vessel boils, and means to disable the operation of that actuator operating most quickly in the event that a longer boil is required 23. A control in or for a liquid heating vessel said control permitting liquid in the vessel to boil for a longer or shorter period of time, and comprising means for detecting boiling of liquid within the vessel, a first thermally sensitive actuator operable at a first predetermined time after boiling is detected to open a set of contacts to discontinue boiling, a second thermally sensitive actuator operable at a second, longer predetermined time after boiling is detected to open a set of contacts to discontinue boiling and means to disable the operation of the first thermally sensitive actuator in the event that a longer boil is required. 24. Apparatus as claimed in claim 22 or 23 wherein the first and second actuators are bimetallic actuators arranged at different thermal spacings from a sump portion of the heating means . 25. Apparatus as claimed in claim 22, 23 or 24 wherein the second actuator operates at the same temperature as the first, and more preferably at a higher temperature . 26. Apparatus as claimed in any of claims 22 to 25 wherein operation of the first actuator is also effective to switch the heating means from a high power to a low power setting. 27. Apparatus as claimed in any of claims 22 to 26 wherein boiling is discontinued by breaking one or more electrical connections to the heater. - 49 - 28. Apparatus as claimed in any of claims 22 to 27 wherein the first and second actuators operate to open the same set of contacts in the control . 29. Apparatus as claimed in claim 28 wherein the set of contacts is opened by a pivotally mounted arm, said arm being pivotally mounted on a long/short boil selector arm which is movable selectively between long boil and short boil positions, the arrangement being such that when the long/short selector arm is in its short boil position, operation of the first actuator will pivot the pivotally mounted arm so as to open the set of contacts, and when it is in its long boil position, the pivotally mounted arm is moved away from the first actuator such that operation of the first actuator will not cause the pivotally mounted arm to move, but the second actuator upon operation will act to move the selector arm back towards its short boil position, and in so doing will first move the pivotally mounted arm into contact with the first actuator and thereafter cause the arm to pivot to open the contacts. 30. Apparatus as claimed in claim 28 or 29 wherein the contacts remake automatically when the first actuator resets. 31. Apparatus as claimed in any preceding claim further comprising means permitting the liquid in the apparatus to be maintained at a given temperature below boiling after it has boiled. 32. Apparatus as claimed in claim 31 as dependent upon any of claims 22 to 30 wherein the operation of the first or second actuator to discontinue boiling automatically switches the heating means to a "simmer" setting for maintaining this lower temperature. 31. Apparatus as claimed in claim 32 wherein switching - 50 - means is arranged such that the heating means will remain in its simmer setting until the first actuator resets . 32. Apparatus as claimed in claim 31 further comprising means to reset the first actuator. 33. Apparatus as claimed in any of claims 29 to 32 wherein the lower temperature is maintained by a simmer control which senses indirectly the temperature of the liquid in the vessel. 34. Apparatus as claimed in claim 33 wherein the simmer control is a preassembled, precalibrated unit which is then assembled to a main control body housing other components of the control. 35. Apparatus as claimed in claim 34 wherein the simmer control unit has a contact for making direct electrical contact to a terminal contact of a thick film heater track. 36. Apparatus as claimed in claim 34 or 35 wherein the simmer control unit is mounted spring loaded against the heater. 37. Apparatus as claimed in claim 34, 35 or 36 wherein the simmer control unit is adjustable to allow the liquid temperature to be varied. 38. Apparatus as claimed in claim 37 wherein the simmer control unit comprises a generally rectangular snap acting bimetallic actuator of the type having a generally U-shaped cut out to define a tongue extending from a first cross limb, two side limbs and a second cross limb joining the side limbs beyond the tip of the tongue, the tongue mounting a movable electrical contact, the actuator being mounted in a housing through - 51 - the second cross limb, resilient biasing means acting against one face of the side regions of the second cross limb and a movable adjustment member acting on the other face of said side regions opposed to said biasing means. 39. A thermally sensitive control unit comprising a generally rectangular snap acting bimetallic actuator of the type having a generally U-shaped cut out to define a tongue extending from a first cross limb, two side limbs and a second cross limb joining the side limbs beyond the tip of the tongue, the tongue mounting a movable electrical contact, the actuator being mounted in a housing through the second cross limb, resilient biasing means acting against one face of the side regions of the second cross limb and a movable adjustment member acting on the other face of said side regions opposed to said biasing means . 40. Apparatus as claimed in claim 38 or 39 wherein the adjustment member is a pivotally mounted lever arm. 41. Apparatus as claimed in claim 40, wherein the lever arm is L shaped, one end of the arm acting on the bimetallic actuator and the other being movable by adjustment means. 42. Apparatus as claimed in any preceding claim further comprising overheat protection means. 43. Apparatus as claimed in claim 42 wherein the overheat protection means comprises a thermally sensitive actuator mounted on one side of a main control body for engagement against a portion of the heating means . 44. Apparatus as claimed in claim 43 wherein the actuator is lightly spring loaded against the heating means . - 52 - 45. Apparatus as claimed in claim 42, 43 or 44 wherein the overheat protection means acts to break all the necessary electrical connections to the heating means to discontinue heating of liquid within the vessel irrespective of the operative condition of the heating means . 46. Apparatus as claimed in claim 45 wherein said heating means comprises a series of thick film heating tracks selectively connectable together in a number of configurations . 47. A liquid heating vessel having first and second thick film heating tracks, electrical connections being made selectively to terminal contacts of the respective tracks so as to energise the tracks in at least two configurations, and comprising overheat protection heating means operable to break those electrical connections to the tracks necessary to disable the heater irrespective of the configuration of the heater tracks . 48. Apparatus as claimed in claim 46 or 47 wherein all the connections to the track sections are broken. 49. Apparatus as claimed in any of claims 44 to 48 wherein the overheat protection actuator acts on a lever arm which lifts the relevant contacts from the heater track. 50. Apparatus as claimed in claim 49 wherein said lever arm is also operable by a mechanism responsive to liquid being dispensed from the apparatus or opening a lid of the apparatus . 51. Apparatus as claimed in any of claims 46 to 50 wherein the actuator of the overheat protection means is arranged in thermal contact with a section of the heater - 53 - track which has a relatively small potential drop thereacross . 52. Apparatus as claimed in claim 51 wherein said section of the heater is a localised serpentine portion of a size generally similar to that of the actuator. 53. A thick film heater comprising a thick film heater track having a localised serpentine region for receiving a thermally sensitive actuator. 54. Apparatus as claimed in claim 52 or 53 wherein said serpentine track pattern comprises at least two parallel track lengths by bridges of a high conductivity material . 55. Apparatus as claimed in any preceding claim wherein said control is or said switch means forms part of an integrated control mountable as a unit to the heating means. 56. Apparatus as claimed in claim 55 wherein the control unit sealingly clamps the heater in an opening in the base of the vessel body. 57. Apparatus as claimed in claim 58 wherein the heating means is provided with a peripheral L shaped flange which receives a seal to be clamped against the vessel base, the vessel base comprises a sealing surface and clamping means are provided to secure the heater to the vessel body and compress the seal, the clamping arrangement is such as to compress the seal in both the longitudinal and radial directions. 58. Apparatus as claimed in claim 57 wherein seal relief means are provided on the vessel body to prevent complete solidification of the seal during compression. - 54 - 59. A liquid heating vessel comprising a plate heater mounted to close an opening in a plastics body of the vessel, the heater plate having an L shaped sealing flange receiving a seal, the vessel body having sealing surfaces for engagement with the seal, and further comprising plate clamping means, the sealing flange and the sealing surfaces of the vessel body being so configured as to compress the seal both axially and radially, the vessel body sealing surfaces having means to permit the seal to deform into relief spaces to permit full resilient compression thereof . 60. Apparatus as claimed in any of claims 55 to 59 wherein the heating means plate is preassembled to the control unit for mounting to the vessel by means of a bayonet type fitting. 61. Apparatus as claimed in claim 60 wherein the control unit is provided with hollow mounting bosses receiving mounting pillars formed on the vessel, at least one of the bosses being positioned such that when the heater plate is mounted to the vessel, it prevents rotation of the heater with respect to the control unit. 62. Liquid heating apparatus comprising a vessel body, a heater plate mounted to a control unit of the vessel, the combined control unit/ heater plate assembly mounted in the base of the vessel, and means provided so as to prevent rotation of the heater plate relative to the control unit once the assembly has been mounted to the vessel . 63. Apparatus as claimed in any preceding claim further comprising indicator means for indicating one or more conditions of the heating means. 64. Apparatus as claimed in claim 63 wherein said indicator mechanism comprises a bistable double headed - 55 - rocker arm, each head selectively moving into or out of view in one of a pair of windows provided in an indicator panel of the heating vessel. 65. A liquid heating vessel comprising an indicator panel having at least one pair of windows provided therein, a bistable pivotally mounted rocker arm having two heads, each head selectively moving into or out of view in one of a pair of windows provided in an indicator panel of the vessel, so as to indicate an operational condition of the vessel . 66. Apparatus as claimed in claim 64 or 65 wherein each rocker arm head is brightly coloured, so as to give the impression of illumination of the window. 67. A liquid heating vessel comprising a plastics vessel body, a plate heater arranged in the base of the vessel body and a control unit mounted below the heater and clamping the heater to the vessel body, said vessel body having a plurality of mounting bosses over which locate a plurality of bores provided on the control unit . 68. A liquid heating vessel comprising a thick film heating element on the underside of the base of the vessel, said heating element comprising at least two resistive heating tracks, and further comprising switch means for selectively connecting said two heating tracks electrically in a desired electrical configuration. 69. A thick film heater comprising an electrical resistive track provided on an insulating substrate, two predetermined sections of said track having a predetermined current carrying capacity being bridged by a glass material (as defined herein) , the configuration of the track and the glass material being chosen such that at a predetermined temperature, the leakage current - 56 - between the track sections rises to the extent that it causes a current to flow through one or both of said sections which is substantially above its current carrying capability, said section being positioned away > from at least one end of the track so as to limit the said current . 70. A heater as claimed in claim 69 wherein the glass material is applied as an overglaze.

Inventors: ELLA JOHN, MEEKS ALAN MICHAEL GEORGE, TAYLOR JOHN CRAWSHAW, ELLA, JOHN, MEEKS, ALAN, MICHAEL, GEORGE, TAYLOR, JOHN, CRAWSHAW
Applicants: ELLA JOHN, MEEKS ALAN MICHAEL GEORGE, STRIX LTD, TAYLOR JOHN C, ELLA, JOHN, MEEKS, ALAN, MICHAEL, GEORGE, STRIX LIMITED, TAYLOR, JOHN, CRAWSHAW
Application year: 1998
Main classification: A47J27
IPC codes: A47J27, H05B3