ELECTROMECHANICAL TIMER

Abstract: An electromechanical timer (101) comprises a pair of switch contacts (109, 111), a rotary cam (115) determining the condition of the switch contacts, a snap-acting bimetal (124) arranged to move cyclically during operation of the timer, and first and second spring coils (129, 130) wound on the cam shaft (117) and arranged to unidirectionally couple the bimetal movements to the cam shaft so as to rotatably reset the cam and reset the switch contacts. The timer can be used with an electric toaster in which case the bimetal can be caused to cycle by being intermittently exposed to heat from the toaster by means of a shutter moved by the bimetal, and the rate of cycling of the bimetal can be adjusted to determine the time-out period of the timer by provision of a further shutter adjustably screening the bimetal from the heat of the toaster.

Description: Electromechanical timer * - Field of the Invention: i This invention concerns improvements relating to ^ 5 electrical apparatuses and controls therefor, and more particularly relates to a timer which is particularly well adapted for use in domestic electrical appliances and the like. Background of the Invention; 10 Some domestic electrical appliances, tumble driers for example, are provided with timers arranged to interrupt the power supply to an electrically powered part of the appliance, a heating element for example, after a set time period has elapsed. In some 15 appliances, an electronic clock circuit is provided within the appliance and is arranged to switch of the power supply once the. clock has counted over the set time period. such electronic timers require a separate DC power supply and are relatively expensive 20 and, furthermore, are not well suited for use in conditions of high temperature and high humidity as may be experienced in domestic appliances. In thermostatically controlled appliances, once the appliance has reached a working temperature, the 25 operation of the appliance is controlled by a substantially regular cyclical ON/OFF switching of the appliance power supply, and in DE-A-3703884 it is proposed to count these switching cycles electronically to effect a desired timer function. 30 This system only works correctly when the appliance .__ . has attained substantial equilibrium conditions where \ the cycle time does not vary substantially from cycle Λ to cycle, and furthermore is subject to the disadvantages of electronic timers that are outlined above. International Patent Application No. PCT/GB 91/00425 (Publication No. WO 91/15026) discloses an electromechanical timing apparatus which incorporates a thermostatic bimetal arranged sequentially to deflect on heating and then return to its original position as it cools. The cyclical movement of the bimetal as it heats and cools is transmitted via a rack and pawl to a linearly displaceable actuating member arranged so that, after a given number of cycles, the cumulative displacement of the actuating member causes it to act on a power supply switch. In addition, the timing apparatus includes means for enabling the selection of different timing periods as a function of the original position of the displaceable actuating member. The operation of the timing apparatus proposed in WO 91/15026 is unlikely to be unduly affected by use in conditions of high temperature and high humidity, unlike the device proposed in DE-A-3703884, but the device is however relatively complicated with consequential cost implications. Objects and Si-n-rniaT-y of the Invention: The present invention seeks to provide a low-cost general-purpose timer that can operate reliably in conditions of high temperature and high humidity. According to the present invention in one of its aspects an electromechanical timer comprises a cam that can be rotated to set the condition of a pair of switch contacts, a bimetallic actuator arranged to move in cyclical fashion and means coupling the bimetal movement unidirectionally to the cam such that the bimetal operation turns the cam so as to reset the switch contacts after a time period determined by the degree of initial rotation of the cam. According to another, more particular aspect of the present invention there is provided an electromechanical timer for controlling the period of time that electrical power is supplied to a load, the timer comprising switch means to be positioned in the power supply circuit to the load, a bimetallic element arranged to move cyclically when power is supplied via the switch means to the load, a rotary cam determining the condition of the switch means, cam positioning means for rotating the cam to set the condition of the switch means, and means for transmitting the cyclical movement of the bimetallic element unidirectionally to the cam so that, in operation of the timer, once the cam has been rotated to set the condition of the switch means the cyclical movement of the bimetallic element is transmitted to and causes unidirectional rotation of the cam in a direction-to reset the same and thereby cause the switch means to reset after a certain period of time corresponding to the degree of initial rotational displacement of the cam. In an exemplary embodiment of the invention which is described in detail hereinafter, the cam is attached to one end of a shaft and a unidirectionally- acting shaft rotating means is provided for transmitting the cyclical motion of the bimetallic element to the shaft. The unidirectionally-acting shaft rotating means can be any suitable means for converting the reciprocatory and generally linear movement of the bimetal into a unidirectional rotation of the cam and could for example be constituted by a rotary racking or ratchet arrangement which causes rotation of the shaft in response to movement of the bimetal in one sense but which accommodates the opposite movement of the bimetal without any corresponding rotation of the shaft. Alternatively, unidirectional clutching arrangements could be utilized with the advantage that the setting of the cam by rotation of the shaft need not affect the condition of the bimetal, and one such arrangement which is described hereinafter comprises a spring coiled around the shaft. The coil spring is arranged to be tightened onto and to frictionally engage the shaft by movement of the bimetal in one direction so that the bimetal acts on the spring and turns the shaft, and the opposite movement of the bimetal serves to loosen the coiled spring so that it slips on the shaft and no rotary drive is transmitted from the bimetal through the spring to the shaft. Regardless of the form of the unidirectionally-acting shaft rotation means, the shaft may be manually rotated so as to rotate the cam to various positions at which the switch means remains closed thereby providing variable timing periods commensurate with the extent of rotation of the cam. The bimetallic element preferably comprises a snap acting bimetallic blade, though it could alternatively comprise a creep bimetal. Snap-acting bimetals have more predefined operating characteristics than creep bimetals and therefore are preferred for the present application. A heater for the bimetal may additionally be provided, the cycling of the bimetal being achieved in a manner well known per se by intermittently powering the heater through a set of switch contacts controlled by the bimetal, though the bimetal may be of a type which is itself current sensitive in which case the heater could possibly be omitted. Alternatively or additionally, the cycling of the bimetal could be achieved by arranging for the bimetal to be exposed only intermittently to heat from a heat source, for example by use of a shuttering arrangement, and the movements of a shutter arranged to determine the intermittent heating of the bimetal could even be made to be dependent upon the movements of the bimetal itself. In accordance with yet another aspect of the present invention, therefore, there is provided an electromechanical timer comprising a pair of switch contacts, actuating means for determining the condition of said switch contacts, a bimetal arranged to cycle between hot and cold conditions in operation of the timer, and means coupling the cyclical movement of the bimetal to said actuating means for causing the same to reset the switch contacts, shutter means being provided for determining the exposure of the bimetal to heat from a heat source and the bimetal movement being coupled to the shutter so that heat is intermittently supplied to the bimetal so as to cause it to cycle. Alternatively or additionally to having the timer period determined by the degree of initial rotation of the cam from its contacts-open position, the possibility also exists to vary the time period of the timer as a function of the rate of cycling of the bimetal by arranging for the heat input to the bimetal to be adjustable. By virtue of such a possibility, the timer could be arranged to be set by rotating the cam always by a predetermined amount, and to reset over a time period determined at least in part by the rate of cycling of the bimetal rather than by the amount that the cam has to rotate in order to achieve its reset condition. According to another aspect of the present invention, therefore, there is provided an electromechanical timer comprising a pair of switch contacts, actuating means for determining the condition of said switch contacts, a bimetal arranged to cycle between hot and cold conditions in operation of the timer, means coupling the cyclical movement of the bimetal to said actuating means for causing the same to reset the switch contacts, and means enabling the rate of cycling of the bimetal to be adjusted for determining the period timed by the timer. Various arrangements for achieving this result are possible. For example, in a timer embodying this aspect of the present invention and incorporating a heater for the bimetal, the heater current could be arranged to be adjustable for correspondingly adjusting the output of the heater, and/or means could be provided for adjustably shuttering the bimetal from the heat of the heater, such adjustable shuttering being additional to the shuttering abovementioned which might be provided for intermittently exposing the bimetal to heat from the heat source. In a timer embodying the invention and wherein the bimetal is arranged to cycle in response to heat from an external heat source, there being no heater provided in the timer and a shuttering arrangement being provided for intermittently exposing the bimetal to heat from the external source, there might for example be provided a further shutter which is adjustable for determining the amount of heat that can access the bimetal from the external source. The latter arrangement might for example be useful in an electric toaster wherein heat from the electric heating elements of the toaster could be adjustably shuttered to the bimetal in the timer so as to determine its rate of cycling and thus determine the ON period of the timer and the corresponding ON period of the toaster. Timers in accordance with the present invention preferably also include an overtemperature protection feature adapted to render the timer open circuit in response to an overtemperature condition in an associated appliance. Such an overtemperature protection feature might for example be constituted by the biassing of a contact-carrying spring conductor into an operative position by means of a fusible element designed to melt when exposed to an excessive temperature, the melting of the fusible element enabling the spring conductor to resile and carry the contact into an inoperative position. It is not essential to the present invention that the timer of the invention should incorporate an electrical switch. The timer could simply be in the form of a mechanical actuator intended to be used with an associated switch which can, for example, be a part of an appliance with which the timer is used. According to yet another aspect of the invention, therefore, there is provided a timer in which a bimetal is coupled to a rotary cam so that, in operation of the timer, reciprocal cyclical movement of the bimetal is converted into unidirectional movement of the cam for determining the period timed by the timer. Another aspect of the present invention provides a timer in which a bimetal is coupled to an actuator so that, in operation of the timer, reciprocal cyclical movement of the bimetal is converted into movement of the actuator for determining the period timed by the timer, and wherein means are provided for enabling the rate of cyclical movement of the bimetal to be adjusted for adjusting the period timed by the timer. Yet another aspect of the ~ present invention provides a timer in which, in operation, the movements of a cyclically operating bimetal determine the period timed by the timer, and wherein the cycling of the bimetal is caused by intermittently exposing the bimetal to a heat source by use of shutter means driven by the bimetal itself. The above and further features of the invention are set forth with particularity in the appended claims and, together with the advantages thereof, will become clear from consideration of the following detailed description of exemplary embodiments of the invention which is given with reference to the accompanying drawings. Description of the Drawings: Figure 1 is a schematic top plan view of a first electromechanical timer embodying the present invention, the timer employing a unidirectional clutch arrangement comprising coil springs to couple cyclical bimetal movements unidirectionally to a rotary cam; Figure 2 is a schematic circuit diagram showing the timer of Figure 1 connected in circuit with a power supply and a load to control the time period during which power is supplied to the load; Figure 3 is an exploded perspective view of the component parts of a second embodiment of the invention which is similar to that of Figures 1 and 2 in that it incorporates an intermittently powered heater, but differs therefrom in that a rotary ratchet arrangement provides the mechanical coupling between the bimetal and the rotary cam; Figures 4A, 4B and 4C are, respectively, perspective, side elevation and top plan views of the arrangement shown in Figure 3 when assembled; Figure 5 is an exploded perspective view of the component parts of a third embodiment which is similar in many respects to the arrangement of Figure 3 but has an intermittently moved shutter replacing the heater; Figures 6A-1, 6A-2, 6B, 6C-1 and 6C-2 are, respectively, a perspective view of the arrangement of Figure 5 when assembled, a view as in Figure 6A-1 with the cover removed, a side elevation view, and top plan views showing the shutter in alternative positions; Figure 7 is an exploded perspective view of the component parts of yet another embodiment; Figures 8A and 8B are perspective views showing the embodiment of Figure 7 in assembled form. Figure 8B showing the embodiment with its cover removed; Figures 9A, 9B and 9C are, respectively, a side elevation view of the embodiment of Figure 7 and top plan views showing the shutter in alternative positions; Figures 10A, 10B and IOC are perspective views illustrating one end of a toaster chassis into which the timer of Figures 7 to 9 may be incorporated; and Figures 11A to HE are diagrammatic views showing various stages in the timing cycle of a toaster fitted with the timer of Figures 7 to 9. Detailed Description of the Embodiments: Figure 1 shows a first exemplary electromechanical timer 101 embodying the present invention. The timer 101 comprises a first side plate 102 and a second side plate 103 bolted together by fixing bolts 104 and nuts 104A such that the side plates are aligned substantially parallel to one another. These side plates provide a structure to which the rest of the timer is mounted. Three electrical terminals, namely a live terminal 105, a load terminal 106 and a neutral terminal 107, are provided for connecting the timer between a power supply and a load in the manner shown in Figure 2. Each electrical terminal is mounted on the first side panel 102. Terminal 105 is connected to the fixed end of an upper cantilevered contact arm 108 which, as shown in Figure 2, has a downwardly projecting contact 109 positioned at its free end. Directly beneath the upper contact arm 108 is a lower cantilever contact arm 110 having an upwardly projecting contact 111 at its free end, and lower contact arm 110 is connected to load terminal 106 at its fixed end. Both of the contact arms 108 and 110 are substantially the same length and are made from an electrically conductive spring material. A flexible electrically-insulating strip 114 having a length shorter than the contact arms, is placed between the upper and lower contact arms 108, 110 so as to space them apart from each other, the thickness of strip 114 being such that when the contact arms 108, 110 are in an unflexed state, the pair of contacts 109, 111 are separated thereby forming an open circuit between electrical terminals 105, 106. The contact arms 108, 110 and the insulating strip 114 are held in position by upper and lower supporting blocks 113 and 112 respectively. Both these blocks are attached to the first side plate 102 and grip the contact arms substantially at the ends thereof which are connected to the electrical terminals 105 and 106. Immediately below the pair of contacts 109, 111, a cam 115 is mounted to one end of a rotatable shaft 117. A control knob 118 is mounted to the other end of the rotatable shaft 117, which extends parallel to the fixing bolts 104. The rotatable shaft 117 is supported in the support structure of the unit 101 by virtue of extending through two bearing holes which are provided in the first and second side plates 102, 103 . The cam 115 serves to determine the position of contact arm 110 which rests upon the surface of the cam. When contact arm 110 rests upon the lowest point of the cam 115, the contact arms 108, 110 are unflexed so that the contacts 109, 111 are separated thereby forming an open circuit between the electrical terminals 105 and 106. As the cam 115 is rotated by rotation of the control knob 118, the cam 115 lifts lower contact arm 110 and causes the contacts 109 and 111 to meet, slightly flexing the contact arms 108, 110 and thereby forming a closed circuit between electrical terminals 105 and 106. The cam 115 and rotatable shaft 117 are so arranged with respect to the pair of contacts 109, 111 that the point where the contacts 109, 111 engage is reached at an early stage in the rotation of the cam. Therefore, a substantial range of cam rotation positions exist between the point at which the contacts 109, 111 are out of engagement and the maximum rotation position of the cam which maintains the contacts 109, 111 in contact and establishes a closed circuit between electrical terminals 105 and 106. By rotation of the cam to any one of a plurality of predetermined positions which can be identified to the user by timing marks provided on the side plate 103 and are selectable by correspondingly positioning an arrow provided on the control knob 118, a correspondingly different amount of return cam rotation will be required to cause the contacts 109, 111 to separate again and, as will be explained in the following, this feature provides variable cut-out timing. A bimetallic switch unit 121 is mounted on the first side plate 102 of the electrical timing unit 101. The switch unit 121 incorporates a dished snap- acting bimetallic blade 124 formed with a U-shaped cut-out defining a central tongue 124A between spaced- apart side legs 124B. The switch 121 has a first terminal 122 connected to the load terminal 106, and a second terminal 123 which is connected via a thick- film resistor 126 mounted on a carrier 127 to the neutral terminal 107. The second terminal 123 is also connected within the body of switch 121 to a contact carrier 125 upon which is mounted a fixed contact which co-operates in switching operations of the switch 121 with a moving contact carried at the free end of the central tongue 124 of the snap-acting bimetallic blade 124, which in turn is connected through the bimetallic blade 124 to the first terminal 122 of the switch unit 121. The bimetallic switch unit 121 short circuits its terminals 122 and 123 when the bimetallic blade is in its low temperature state. In this condition, a current can flow through the switch unit 121 and through the series-connected heater 126. This causes the bimetallic blade 124 to heat up and eventually causes the switch to go open circuit. The resultant disruption of current flow in the heater 126 causes the bimetallic blade 124 to cool thereby causing the switch unit 121 to reset into its low-temperature closed-circuit state which again powers the heater 126. In this way the switch 121 will cycle between open and closed, hot and cold conditions and the cyclic behaviour of the switch will fairly rapidly stabilize to be substantially uniform with a constant periodicity. As the bimetallic switch 121 cycles, so the free end of the bimetal blade 124, that is the end remote from the fixing of the blade to the switch body, reciprocates generally in a direction perpendicular to the plane of Figure 1. The reciprocating movement of the bimetallic blade 124 is transmitted unidirectionally to the rotary shaft 117 by means of first and second spring coils 129 and 130 which are coiled around and frictionally engage the shaft 117. The first spring coil 129 is situated centrally on the rotatable shaft 117 and has a torque arm 128 formed as an extension of a central turn of the coil. The torque arm 128 extends generally tangentially from the shaft 117 and has a bifurcated end portion locating above and below the tip of the bimetallic blade 124 so that the torque arm 128 is moved with the reciprocating motion of the bimetallic blade. When the torque arm 128 is moved in an upwards direction as viewed in Figure 1, the coil 129 will slip around the shaft 117 because upwards movement of the torque arm 128 tends to unwind and loosen the coil. However, movement of the torque arm 128 in the downwards direction causes the coil 129 to tighten around the shaft 117, thereby gripping the shaft 117 and correspondingly rotating the same by an amount commensurate with the bimetal movement. The second spring coil 310 has a torque arm 131 formed from an end turn thereof. The torque arm 131 extends tangentially from the shaft 117 and is rigidly attached to the side plate 102 of the electrical timing unit. The coil 130 is arranged to permit rotation of the shaft 117 in the direction in which it is rotated by bimetal 124 interacting with coil 129 but to restrict shaft rotation in the opposite direction. When the shaft 117 is rotated by the bimetal 124, the shaft slips within the coil because the coil tends to be slightly unwound by the shaft rotation. However, any tendency of the shaft 117 to rotate in the opposite direction will be resisted by the coil 130 being wound more tightly around the shaft thereby gripping the shaft and preventing its rotation on account of the torque arm 31 being fixed to the side plate 102. The two coils 129, 130 are thus arranged to co¬ operate with each other in that when the first coil 129 is gripping and rotating the shaft 117 the second coil 130 is unwound by the shaft rotation and enables the shaft to slip within the coil 130, and when the first coil 120 is slipping around the shaft any tendency for the shaft to rotate is inhibited by the second coil 130 gripping the shaft 117. The electrical timing unit 101 is connected in use to an A.C. or D.C. power supply 132 and a load 133 via the electrical terminals 105, 106 and 107 as shown in Figure 2. The operation of the unit 101 when connected as in Figure 2, is described below. By means of the control knob 118, the cam 115 is rotated via the rotatable shaft 117 to close the pair of contacts 109, 111. This connects the supply to the load 133 and to the switch unit 121. The switch unit 121 will then cycle ON and OFF so causing the bimetal 124, via the arrangement of first and second spring coils 129, 130, to rotate the shaft 117 intermittently until the attached cam 115 re-opens the pair of contacts 109, 111 and terminates the supply of power to the load 133 and to the switch unit 121. The time period for which the load is energized is determined by the initial setting of the cam 115 and by the corresponding number of cyclic switching operations of the bimetal that are required to re-set it and open the contacts 109, 111. Described in the foregoing is an exemplary electromechanical timer embodying the present invention in which a cam determines the condition of a pair of switch contacts and the cyclical operation of an intermittently-heated bimetal is coupled unidirectionally to drive the cam so as to reset the switch contacts after a time delay determined by the initial setting of the cam. In the embodiment described the bimetal is a snap-acting switch actuator arranged intermittently to power a heater in close heat transfer relationship with the bimetal, and the movements of the bimetal are unidirectionally coupled to a cam shaft mounting the cam by way of a coil spring arrangement mounted about the cam shaft and arranged to tighten around the cam shaft for driving the same when urged in one sense by the bimetal and to loosen and slip about the shaft when urged in the opposite sense. Mention is, however, made in the foregoing of the possibility of using a unidirectional rotary ratchet arrangement to impart the movements of the bimetal to the cam shaft and in the following a number of embodiments employing such arrangements will be described. Referring to Figure 3, the timer arrangement shown therein comprises a moulded plastics body part 1 incorporating brass switch and terminal parts 2, 3, 4 and 5 and formed with recesses for operatively accommodating a thick film resistor 6, a bimetal blade 7, a cam 8 and a spindle 9, an actuator 10, a light coil spring 11, and a leaf spring 12. A cover 13 is adapted to be secured to the body part 1. The thick film resistor 6 constitutes a heater for the timer and has terminals 14 and 15 which are adapted to be inserted into openings 16 and 17 provided respectively in terminal 2 and switch part 4 and secured therein as, for example, by welding. The heater when assembled with the timer body part 1 underlies the opening 18 that is provided in the body part. The bimetal 7 comprises a dished blade of bimetallic material which is movable with a snap- action between oppositely dished configurations, and has a U-shaped cut-out defining a central tongue 19 between a pair- of side legs 20. The bimetal is further provided with an extended nose portion 21. The bimetal 7 . is mounted in the timer by virtue of the free end of the tongue 19 being spot-welded to the upwardly-bent portion 22 of switch part 4 where it is exposed at the upper surface of the plastics body part l of the timer, and a switch contact (not shown) which is mounted on the underside of the bimetal just behind the nose portion 21 co-operates with a contact 23 provided on switch part 5. By virtue of this arrangement a series electric circuit exists between terminals 2 and 5 via the thick film resistor 6 and the bimetal 7; when the bimetal-mounted contact is in electrical contact with the contact 23 provided on terminal part 5 (namely in the cold condition of the bimetal 7) and with a voltage source coupled between the terminals 2 and 5, the thick film resistor 6 will be powered thereby heating the bimetal 7 which will snap to its oppositely-dished, hot condition thereby breaking the circuit until the bimetal cools again. The bimetal will thus cycle repeatedly between its oppositely-dished, hot and cold configurations.- The actuator 10 has a generally cylindrical portion 4 formed with a bore into which the far end (as viewed in Figure 3) of the cam-carrying-spindle 9 is fitted. As shown in the enlarged showing of the near end of the portion 24 that is given in Figure 3, a series of ratchet teeth 25 are provided in the end surface of the actuator portion 24. Complementarily shaped ratchet teeth are provided in a facing part of the spindle and it will be understood that a clockwise rotation of the actuator 10 will cause the respective teeth to engage so that the spindle is driven by the actuator, but that an anti-clockwise rotation of the actuator 10 will simply result in the respective teeth slipping relative to each other. The light coil spring 11 is fitted onto the spindle so as to bias the toothed surface of the actuator towards the complementarily-toothed surface of the spindle, and the spring 11 also accommodates the axial movement of the actuator 10 on spindle 9 that accompanies anticlockwise movement of the actuator. The spindle 9 together with the actuator 10 and spring 11 are received within formations provided as shown in the timer- body part l and cover 13. The actuator 10 also has a portion 26 which, in the assembled timer, engages with the nose portion 21 of bimetal 7 so that, as the bimetal cycles between its hot and cold conditions and the nose portion of the bimetal moves up and down, so the actuator 10 is reciprocated on the cam spindle 9 and, by virtue of the provision of the unidirectional coupling provided by the interengaging teeth on the actuator and the spindle, causes unidirectional rotation of the spindle 9. The leaf spring 12 is spot-welded at its left- hand end (as shown in Figure 3) to the terminal 3 and carries a contact 27 on its underside near to its right-hand end. The contact 27 co-operates with a contact 28 which is provided on the terminal part 5 and is exposed through the upper surface of timer body part 1. The movement of the leaf spring 12 is determined by the setting of cam 8 which, as can be seen, has a raised portion designed to lift the right- hand end of the leaf spring so as to lift contact 27 out of electrical contact with the contact 28 on terminal part 5. The near end of spindle 9 extends from the timer body part 1 for the attachment of a setting knob thereto, and it will be understood that the greater the rotation of the cam 8 so as to distance the raised portion of the cam from engagement with the leaf spring 12, the longer will it take the bimetal 7 to perform the necessary number of cycles to rotate the cam sufficiently to open the contacts 27, 28. The timer thus enables different time periods to be set as regards the time period that the switch contacts 27, 28 will remain closed. The switch defined by the contacts 27, 28 thus provides for electrical connection between timer terminals 3 and 5 for a time period determined by the setting of the knob attached to the cam spindle 9. - Referring now to Figures 5 and 6, the timer arrangement shown therein is similar in many respects to the timer of Figures 3 and 4 but has no integral heater for causing cycling of the bimetal. Rather, the timer of Figures 5 and 6 is adapted for use with an appliance, such as an electric toaster for example, having an independent heat source and incorporates a shutter which is moved by the timer bimetal so as intermittently to shield the bimetal from the heat source and cause it to cycle. The bimetal in the arrangement of Figures 5 and 6 thus performs two mechanical functions, namely it drives the actuator to rotate the cam and furthermore it operates the shutter. In Figures 5 and 6 the same reference numerals are used as were used to designate like and similar parts in the timer arrangement of Figures 3 and 4, and only the significant differences between the two arrangements will be described in detail. The timer of Figures 5 and 6 thus has a snap-acting bimetal 7 with a nose portion 21 which engages portion 26 of actuator 10 so as to cause the actuator to reciprocate about cam spindle 9 and, by virtue of the unidirectional coupling of the actuator 10 to the cam spindle 9 that is provided by complementarily-shaped interengaging ratchet teeth as in the manner of the timer of Figures 3 and 4, rotate the cam spindle so as to open switch contacts 27, 28. In the timer of Figures 5 and 6, the bimetal 7 is mounted by the free end of its tongue 19 to a post 30 formed in the timer body part 1 and serves no electrical switching function. An extension 31 formed on the cover moulding 13 serves to stabilise the mounting of the bimetal. A further post 32 provides a pivotal mounting in the timer body part 1 for a heat-resistant shutter 33 having a through hole 34 which fits over the post 32. A small protrusion 35 formed on the underside of the cylindrical part 24 of the actuator 10 engages with a notch 36 in one side of the shutter 33 so as to cause the shutter to be pivoted back and forwards about the post 32 as the actuator 10 is reciprocated by the bimetal 7, the limiting positions of the shutter 33 being shown in Figures 6C-1 and 6C-2. The timer body part l has a through opening 37 which serves, in use, with the timer arrangement mounted adjacent to an independent heat source, for example the heating element of an electric toaster, to allow the heat of the heat source to access the bimetal 7. The shutter 33 is arranged to be moved into the position shown in Figure 6C-2, namely between the opening 37 and the bimetal 7, when the bimetal snaps from its cold to its hot condition, and to be moved back to the position shown in Figure 6C-1, where it allows heat from the heat source to access the bimetal 7, when the bimetal snaps back to its cold condition. The bimetal 7 will thus cycle so long as the heat source is energised, and the cycling of the bimetal, as with the timer of Figures 3 and 4, will return the cam 8 so as after a time delay determined by the initial setting of the cam to open the contacts 27, 28 by interaction of the cam 8 with the leaf spring 12. Figures 3 and 4 and Figures 5 and 6 thus illustrate two different electromechanical timer arrangements embodying the present invention wherein the cycling of a bimetal provides the driving force for resetting a cam and opening a pair of switch contacts, the first arrangement incorporating an intermittently powered heating element for causing the bimetal to cycle and the second arrangement incorporating a shutter which is moved so as intermittently to expose the bimetal to the heat of an independent heat source. In the respective cases, the intermittent energisation of the heating element and the movement of the shutter are each determined by the bimetal. The timer of Figures 5 and 6 is adapted for use with a separate heat source and incorporates a shutter which is moved by the bimetal movements so as intermittently to shield the bimetal from the heat source, thereby causing the bimetal to cycle between its hot and cold conditions. By provision of an additional shutter, which may, for example, be manually adjustable, the amount of heat accessing the timer from the heat source can be controlled so as in turn to control the time period required for the bimetal to perform a set number of cyclical operations necessary for opening the switch contacts. In application of such a timer to an electric toaster. for example, the additional, manually-adjustable shutter would enable the degree of browning achieved by the toaster to be set and this would not need to be reset for each toasting operation. Such an additional shutter could be formed as a part of the appliance in which the timer was fitted, or alternatively could be a part of the timer itself. It would furthermore be possible to utilize such a shutter with a timer as described hereinbefore with reference to Figure 1 or Figure 3 to control the heat transfer between the heater provided in the timer and the bimetal thereby to control the rate of cycling of the bimetal and correspondingly determine the timing period of the timer. By use of such a shutter to determine the rate of cycling of the bimetal, the timing period of the timer can be made dependent upon the position of the shutter and need not be determined by the initial setting of the cam. In the following there will be described in detail a modified form of the embodiment described above with reference to Figures 5 and 6, the modifications providing for double pole switching and including an overtemperature protection feature constituted by the biassing of a contact-carrying spring conductor into an operative position by means of a fusible element designed to melt when exposed to an excessive temperature, the melting of the fusible element enabling the spring conductor to resile and carry the contact into an inoperative position. Also described in the following is an exemplary electrical appliance, namely an electric toaster, which incorporates the subject timer. A description will first be given of the construction and operation of the timer of Figures 7 to 9 and then its incorporation into an electric toaster will be described with reference to Figures 10 and 11. Given the many similarities that exist between the timer of Figures 7 to 9 and the timer described with reference to Figures 5 and 6, the same reference numerals will be used hereinafter as were used to designate like parts in the embodiment of Figures 5 and 6 and only the significant differences between the two devices will be described in detail. Referring first to Figures 7, 8A and 8B, the timer has a snap-acting bimetal 7 with a nose portion 21 which engages portion 26 of actuator 10 so as to cause the actuator to reciprocate about cam spindle 9 as the bimetal cycles between its hot and cold conditions. A light coil spring 11 biasses the actuator 10 towards a double-sided cam 8 carried by the spindle 9 so as unidirectionally to couple the actuator 10 to the cam spindle 9 by virtue of the interengagement of the ratchet teeth 25 that are provided on the contiguous end faces of the actuator 10 and the cam part 8. The unidirectional coupling thus provided between the actuator 10 and the cam spindle 9 results in the cam 8 being rotated as a result of the cycling of the bimetal 7. With the cam 8 initially set in such a position that the contacts carried by the leaf springs 12-1, 12-2 engage with respective contacts provided in the timer body moulding 1, the cycling of the bimetal 7 will cause the cam 8 to be rotated into a position such as to open the two contact sets after a predetermined number of cycles corresponding to a substantially predetermined time delay. The cycling of the bimetal 7 is achieved by virtue of the shutter 33 which is moved by the movements of actuator 10 so as alternately to admit heat to the bimetal via the window 37 in the body moulding l and to interrupt such access. In these respects the timer as illustrated is the same as that describee with reference to Figures 5 and 6. The present timer of Figures 7 to 9 differs from the previously described timer of Figures 5 and 6 in a number of respects. Firstly, it incorporates double pole switching, there being two contact-carrying leaf springs 12-1, 12-2 each of which co-operates with respective pairs of terminal parts 3-1,5-1 and 3-2, 5- 2 secured in the body moulding 1. Secondly, a set of ratchet teeth 40 are provided on the end face of the cam spindle 9 which are directed oppositely to those provided on the actuator 10; these ratchet teeth 40 co-operate with formations (not shown) provided on the contiguous inner wall surface of the body moulding 1 to prevent the cam 8 from rotating backwards as the bimetal 7 rotates the cam spindle 9 in an anti¬ clockwise direction. Finally, an overtemperature protection function is provided in the timer by virtue of the inclusion of a short contact-carrying leaf spring 41 which is spot-welded at its right-hand end (as viewed in Figure 7) to the end of terminal part 5- 1 and extends under leaf spring 12-1. A melt pillar 42, formed for example of a plastics material having a predetermined melting temperature, sits in a hole 43 formed in the base of the body moulding 1, the pillar 42 having an enlarged head portion to prevent it from passing through the hole 43 and being of such a height that it projects through the hole on the underside of the body moulding 1. When the timer is attached to an appliance, an electric toaster for example, the pillar 42 is depressed by contact with the appliance and its head portion correspondingly elevates leaf spring 41 so as to bring its contact into a position where it can co-operate with the contact carried on the underside of leaf spring 12-1. On the occurrence of an overtemperature condition in the accompanying appliance such as to cause the melt pillar 42 to soften, the pillar will be deformed by the action of the spring 41 which will resile so as to move its contact away from the contact carried by leaf spring 12-1. Referring now to Figures 10A, 10B and IOC, the assembly of the timer described with reference to Figures 7 to 9 into an electric toaster will now be described as an example of the way in which the timer may be used with an electrical appliance. As is well known, an electric toaster includes a bread lifter mechanism which can be manually depressed against the action of a spring bias and latches in its depressed condition. The depression of the bread lifter mechanism serves to initiate the supply of electrical current to heating elements provided in the toaster and, after a generally predetermined time period determined by a timer device, the latch is released to release the bread lifter and discontinue the current supply to the heating elements. The timer device conventionally can simply be a bimetal serving to operate switch contacts which control the operation of a latch-release solenoid or may be a much more complex and correspondingly more expensive electronic device. Conventional toaster timers are renowned for their inability to provide consistent toasting times and to compensate for different bread conditions such as slice thickness etc. and are not considered to be very satisfactory. The timer of the present invention is believed to have the capability to overcome or at least substantially reduce these problems. Figure 10A is a perspective view of one end of a toaster chassis with the bread lifter mechanism removed. A wall 50 is upstanding from the base of the toaster chassis and is formed with guide slots 51 and 52 for the bread lifter mechanism which, as will be described hereinafter with reference to Figure 10B, slides on a rod received in and extending between the openings 53 and 54 that are provided in the parts 55 and 56 respectively. The bread lifter mechanism has a hooked part which co-operates with a latch mechanism generally designated 57 and comprising a rocker member 58 pivotally mounted on the axis 59 and having connected thereto an arm 60 which extends across to the location of a trip arm 61 which is pivotally mounted on the axis 62 with a light spring bias in the anti-clockwise direction. As will be explained hereinafter, when the bread lifter mechanism is depressed the abovementioned hooked part thereof engages with the rocker member 58 and, by virtue of the spring biassing of the bread lifter in the upwards direction, attempts to pivot the rocker member 58 in a clockwise direction. Such pivotal movement of the rocker member 58 is prevented, however, by the interaction of the end of the arm 60 with the trip arm 61 until such time as the timer releases the trip arm 61 as will be described hereinafter. Notable also in Figure 10A is an opening 63 formed in the wall 50 for enabling heat from the heating elements of the toaster to access the bimetal blade of the timer when it is fitted in place. A shutter 64 is pivotally mounted adjacent the opening 63 and can be manually adjusted to control the amount of heat accessing the timer. Referring now to Figure 10B, parts of the bread lifter mechanism are shown. A rod 65 provides a sliding mounting for an end part 66 of the bread lifter mechanism, the parts of the bread lifter mechanism which contact the bread slices to be toasted extending therefrom on the other side of the toaster chassis end wall 50 and not being shown. The end part 66 of the bread lifter mechanism carries a pivotally mounted part 67 formed with a hook portion 68 adapted for co-operation with the rocker member 58 of the latch mechanism 57 as will be explained hereinafter. An arm 69 formed as an integral part of the pivotally mounted part 67 extends between a pair of stops 70 and 71 released from the end part 66 of the bread lifter mechanism and defines limits of pivotal movement of the part 67 in clockwise and anti-clockwise directions, and a latch-setting limb 72 also extends from the pivotally mounted part 67. The arrangement of the part 67 with its integral arm 69 and limb 72 both extending on the same side of its pivot axis 73 causes the part 67 to have a gravitational bias towards its most anti-clockwise position and the purpose of the latch-setting limb 72 is to pivot the part 67 clockwise into its clockwise limiting position when the bread lifter mechanism is manually depressed and thereby enable the hook portion 68 to engage with the rocker member 58, such clockwise pivoting of the part 67 being achieved by virtue of the abutment of the end of limb 72 with the top of trip arm 61 as the bread lifter mechanism is depressed to its fullest extent, the subsequent manual release of the bread lifter depressing means and the small upwards movement of the bread lifter mechanism that then follows causing the hook portion 68 to engage with the rocker member 58 thereby preventing the bread lifter mechanism from moving any further in the upwards direction. A spring member 74 is affixed to a part released from the end part 66 of the bread lifter mechanism and, as will be explained hereinafter, serves to activate the timer mechanism when the bread lifter mechanism is depressed. A reset arm 75, formed as an integral part of the end part 66, serves to reset the timer at the end of its operating cycle when the bread lifter mechanism is released and this too will be explained more fully hereinafter. Figure IOC shows how the timer previously described herein is mounted on the end wall 50 of the toaster chassis with its base moulding 1 in contact with the wall 50 so that the melt pillar 42 of the timer is depressed inwardly of the timer and biasses the leaf spring 41 into an operating position where the contact that it carries can operatively make and break with the contact carried on the underside of leaf spring 12-1. The mounting position of the timer furthermore is such that the bimetal 7 is exposed to heat from the toaster elements through the window 63 in the toaster chassis end wall 50, the degree of such exposure being adjustable by adjustment of the shutter 64. A double-sided rotary cam 76 is fitted onto the end of cam spindle 9 of the timer where it projects from the timer body so as to rotate unidirectionally with the cam spindle 9 in an anti-clockwise direction (as viewed in the drawings) . The detailed form of the rotary cam 76 is shown in the detailed views of Figures lOC-l and 10C-2 and it can be seen that on the side of the cam which is furthest from the timer there are provided a pair of ramp formations 77 and 78 which are adapted to co-operate with the trip arm 61 of the bread lifter latching mechanism for pivoting the same in a clockwise direction about its pivotal axis 62 for releasing the latch. The end portion 79 of the trip arm 61, which is shown in detail in Figure 10C-3, serves as a cam follower and bears against the outer surface of the rotary cam 76 with a light spring pressure. Figure 10C-4 is. a detailed view in the direction of the arrow B shown in Figure IOC and illustrates the latched position of the latching mechanism wherein the trip arm 61 is in its most anti¬ clockwise position with its end portion 79 bearing against the flat outer surface of the rotary cam 76 between the formations 77 and 78 and its lower edge 80 engaged with a notch 81 formed in the end of the arm 60 so as to prevent it from pivoting in a clockwise direction and enabling the hooked portion 68 of the end part 66 of the bread lifter mechanism to slip out of engagement with the rocker 58. Figure 10C-5 shows what happens when the cam 76 rotates so that one of the formations 77 and 78 lifts the trip arm 61 thereby, enabling the arm 60 to disengage therefrom and pivot in a clockwise direction so as to release the latch and release the bread lifter mechanism. The formations that are provided on the inner side of the rotary cam 76, that is the side nearest to the timer, consist of a first pair of diametrically opposed levers 82 and 83 which are adapted to co¬ operate with the timer activating spring arm 74 that is provided on the end part 66 of the bread lifter mechanism, and a second pair of diametrically opposed levers 84 and 85 which are adapted to co-operate with the timer reset arm 75 of the bread lifter mechanism, the levers 82 and 83 and the levers 84 and 85 being angularly spaced apart from each other on the shaft of the cam by an angle of 90°. The levers 82 and 83 are adapted to be driven downwards, so as to rotate the cam 76 in an anti-clockwise direction (as viewed in Figure 10C) , by engagement of the lowermost end of spring arm 74 in the recess 86 that is provided in the surface of each of the respective levers as the bread lifter mechanism is depressed; this can be seen in Figure IOC where the lowermost end of arm 74 is in line to descend into engagement with the lever 82 as the bread lifter mechanism is depressed. The arrangement is such that the cam 76 is rotated through 90° when the bread lifter mechanism is depressed. The levers 84 and 85 are adapted to co-operate with the timer reset arm 75 in the upwards or return movement of the bread lifter mechanism after the latch has been released; the lower end of the arm 75 is turned through 90° to define an engagement portion which engages with and lifts respective ones of the levers 84 and 85 as the bread lifter mechanism moves upwardly, the respective lever 84 or 85 having previously been brought into a position to be engaged by the arm 75 on its next upwards movement by the 90° rotation of the cam 76 that is achieved by engagement of the spring member 74 on its downwards stroke with one of the levers 82 and 83. Figures 11A to HE illustrate different stages in one full cycle of operation of the toaster of Figures 10A, B and C fitted with the timer of Figures 7, 8A and 8B and 9A, 9B and 9C, the views shown being diagrammatic views in the direction of the arrow A shown in Figure IOC. The sequence of views show in Figure HA the situation when the timer contacts are open and the bread lifter mechanism is beginning to be depressed to initiate a cycle of operation, in Figure 11B the situation when the descending timer activating spring member 74 engages one of the levers 82 and 83 on the cam 76, in Figure HC the situation where the descending spring member 74 has rotated the cam 76 through 90° thereby closing the timer contacts so as to supply electrical power to the toaster heating elements and initiate a timing cycle, in Figure HD the situation where the cycling of the bimetal 7 in the timer has rotated the cam 76 through a sufficient angular distance (in this example 75°) to cause the timer to time out, and in Figure HD the situation where the rising reset arm 75 engages one of the levers 84 and 85 so as to rotate the cam 76 back into a position where the timer contacts are again open and the timer is ready for a subsequent operation cycle. Referring more particularly to Figure HA and assuming that the timer is in a condition where the cam 8 which, remember, is double sided with two "high" portions spaced angularly by 180° is in a position to hold the leaf springs 12-1 and 12-2 out of electrical contact with their respective contacts in the timer base moulding 1, it can be seen that depression of the bread lifter mechanism 66 will bring the lowermost end of timer actuating spring member 74 into contact with the recess 86 provided in the lever 82. The descending end of the timer reset lever 75 is able to clear the lever 85 in this condition of the timer and does not interfere with the lever 83 in its continued downwards movement. Figure 11B shows the engagement of the lower end of spring member 74 with the lever 82 and Figure HC shows how the continued downwards movement of the spring member 74 rotates the cam 76 through an angle of 90°, thereby moving the cam 8 within the timer from a "high" to a "low" position and lowering the leaf springs 12-1 and 12-2 so that the timer contacts close and electric current begins to flow through the heating element of the toaster. The toaster then heats up and heat from the toaster elements accesses the bimetal 7 of the timer thereby causing the bimetal to cycle, each cycle of the bimetal causing the cam spindle 9 in the timer to rotate through a predetermined angular distance which in the current embodiment is 15°. The rotation of the cam spindle 9 as the bimetal cycles causes the cam 76 to rotate correspondingly and, after a predetermined angular rotation which in the current embodiment is 75°, the wedge formations 77 and 78 move into a position where one of them releases the trip arm 61 so as to release the latching of the bread lifter mechanism in its downwards position and enable it to return in an upwards direction. This situation is shown in Figure HD where it can additionally be seen that the rotation of the cam 76 that accompanies the cycling of the timer bimetal brings the lever 83 into a position where it bears upon the spring member 74 which, however, simply flexes to accommodate this interference. The latching mechanism having thus been released, the bread lifting mechanism 66 moves upwardly as shown in Figure HE and the end of the rising reset arm 75 catches the lever 84 and rotates the cam 76 through a further 15° so as to rotate the cam spindle 9 and the cam 8 carried thereon into a condition where the leaf springs 12-1 and 12-2 in the timer again rest upon a "high" point of the cam 8 and the timer contacts are again opened ready for the next cycle of operation of the toaster. The foregoing description provides an example of the way in which a timer mechanism in accordance with the present invention may be utilized with an electric toaster. The timers described hereinbefore with reference to Figures 1 to 6 could be utilized similarly. Furthermore, whilst the double-sided cam arrangement described in the foregoing is a convenient way of providing for operative interaction between the bread lifter mechanism of an electric toaster and the cam mechanism within the timer, alternative arrangements could be utilized which could for example include a solenoid controlled through the timer contacts for determining the operation of the bread lifter mechanism latching arrangement, in which case the arrangement for setting and resetting the timer could be simplified, though at the cost of requiring the provision of a solenoid. Additionally, whilst the description herein has concentrated upon the incorporation of the subject timer into an electric toaster of the kind incorporating a bread lifter mechanism, it will be appreciated that the timer is not restricted to use with this kind of electric toaster and could be used with toasters having other bread release mechanisms. Furthermore the timer could be used with other domestic and industrial appliances. The present invention having been described in the foregoing by reference to a number of embodiments, it is to be appreciated that the described embodiments are exemplary only and that modifications and variations within the routine skills of one skilled in the art could be made thereto without departure from the spirit and scope of the invention as set forth in the appended claims. For example, whereas the embodiments hereinbefore described have all included switch means incorporated into the timers, it would be possible to configure these timers simply as mechanical actuators adapted to determine the operation of a switch in an associated appliance. The electric toaster hereinbefore described could thus incorporate a switch, arranged to be operated by the bread release mechanism for example, and the timer might simply provide a means for determining the resetting of the bread lifter mechanism and correspondingly resetting the associated switch. Another example of a possible modification would be to configure the timers of Figures 5 and 7 so that, as with the timers of Figures 1 and 3, they incorporated a heater for causing the bimetal to cycle; with such an arrangement, selection of the time period of the timer could be achieved by arranging for the heater output to be adjustable for example by provision of an adjustable resistance in the heater circuit, and/or by arranging for the heat access to the bimetal from the heater to be adjustable for example by provision of an adjustable shutter between the heater and the bimetal, and/or by arranging for the timer operation to be initiated with the rotary cam set to different initial positions requiring different numbers of bimetal cycles to reset the cam. Yet a further possibility would be to replace the rotary cam in any of the described embodiments by an alternative form of linear actuator coupled to the bimetal by suitable means converting the bimetal movements into unidirectional movement of the actuator.

Claims: CLAIMS : 1. An electromechanical timer comprising a cam that can be rotated to set the condition of a pair of switch contacts, a bimetallic actuator arranged to move in cyclical fashion, and means coupling the bimetal movement unidirectionally to the cam such that the bimetal operation turns the cam so as to reset the switch contacts after a time period determined by the degree of initial rotation of the cam. 2. An electromechanical timer for controlling the period of time that electrical power is supplied to a load, the timer comprising switch means to be positioned in the power supply circuit to the load, a bimetallic element arranged to move cyclically when power is supplied via the switch means to the load, a rotary cam determining the condition of the switch means, cam positioning means for rotating the cam to set the condition of the switch means, and means for transmitting the cyclical movement of the bimetallic element unidirectionally to the cam so that, in operation of the timer, once the cam has been rotated to set the condition of the switch means the cyclical movement of the bimetallic element is transmitted to and causes unidirectional rotation of the cam in a direction to reset the same and thereby cause the switch means to reset after a certain period of time corresponding to the degree of initial rotational displacement of the cam. 3. A timer as claimed in claim 1 or 2 wherein the cam is attached to a shaft and unidirectionally-acting shaft rotating means is provided for transmitting the cyclical movement of- the bimetallic element to the cam. 4. A timer as claimed in claim 3 wherein said unidirectionally-acting shaft rotating means comprises a rotary racking arrangement driven by the bimetal movements. 5. A timer as claimed in claim 3 wherein said means for unidirectionally transmitting the movement of the bimetallic element to the cam comprises first means coupling the bimetallic element to the cam shaft so that said first means is moved alternately in opposite directions by the cyclical movement of the bimetallic element but only rotates the shaft when moving in one of these directions, and second means coupled to the shaft to allow rotation of said shaft in said one direction but to inhibit rotation of the shaft in the opposite direction. 6. A timer as claimed in claim 5 wherein said first and second means comprise spring coils wound around the cam shaft and arranged either to tighten onto and bind with the shaft or to loosen and permit relative movement between the coil and the shaft depending upon the direction of the movement of the bimetallic element. 7. A timer as claimed in any of the preceding claims wherein the cam is rotatable to a range of positions at which the switch means is set, the cam thereby requiring different amounts of rotation in order to reset the switch means and the timer thereby providing in use a selectably variable time period during which power is supplied to the load. 8. A timer as claimed in any of the preceding claims and further comprising a heater for intermittently heating said bimetallic element in order to cause it to cycle. 9. A timer as claimed in any of claims 1 to 7 wherein the bimetallic element is arranged to be exposed intermittently to heat from a heat source so as to cause it to cycle. 10. A timer as claimed in claim 9 wherein a shutter is provided in the timer for determining the exposure of the bimetallic element to heat from the heat source, and means are provided for moving the shutter so as intermittently to expose the bimetallic element to the heat. lϊ. A timer as claimed in claim 10 wherein the movements of the shutter are arranged to be dependent upon the movements of the bimetallic element. 12. A timer as claimed in claim 9 or 10 or 11 wherein means are provided for enabling the amount of heat from the heat source to be adjusted thereby to adjust the rate of cycling of the bimetallic element. 13. A timer as claimed in claim 12 wherein an adjustable shutter is provided between the heat source and the bimetallic element for adjusting the amount of heat that can access the bimetallic element. 14. An electromechanical timer comprising a pair of switch contacts, actuating means for determining the condition of said switch contacts, a bimetal arranged to cycle between hot and cold conditions in operation of the timer, means coupling the cyclical movement of the bimetal to said actuating means for causing the same to reset the switch contacts, and means enabling the rate of cycling of the bimetal to be adjusted for determining the period timed by the timer. 15. A timer as claimed in claim 14 wherein the bimetal is arranged to be exposed to heat from a heat source for causing it to cycle, and adjustable shutter means are provided for determining the amount of heat from the heat source that can access the bimetal and thereby determining the rate of cycling of the bimetal. 16. A timer as claimed in claim 14 or 15 wherein the bimetal is arranged to cycle by virtue of being exposed intermittently to heat from a heat source, shutter means being provided for shielding the bimetal from the heat source and the bimetal being coupled to the shutter means so that the movements of the bimetal cause the shutter means to move so as intermittently to expose the bimetal to the heat from the heat source. 17. An electromechanical timer comprising a pair of switch contacts, actuating means for determining the condition of said switch contacts, a bimetal arranged to cycle between hot and cold conditions in operation of the timer, and means coupling the cyclical movement of the bimetal to said actuating means for causing the same to reset the switch contacts, shutter means being provided for determining the exposure of the bimetal to heat from a heat source and the bimetal movement being coupled to the shutter so that heat is intermittently supplied to the bimetal so as to cause it to cycle. 18. A timer in which a bimetal is coupled to a rotary cam so that, in operation of the timer, reciprocal cyclical movement of the bimetal is converted into unidirectional movement of the cam for determining the period timed by the timer. 19. A timer in which a bimetal is coupled to an actuator so that, in operation of the timer, reciprocal cyclical movement of the bimetal is converted into movement of the actuator for determining the period timed by the timer, and wherein means are provided for enabling the rate of cyclical movement of the bimetal to be adjusted for adjusting the period timed by the timer. 20. A timer in which, in operation, the movements of a cyclically operating bimetal determine the period timed by the timer, and wherein the cycling of the bimetal is caused by intermittently exposing the bimetal to a heat source by use of shutter means driven by the bimetal itself. 21. A timer is claimed in claim 18 or 19 or 20 and incorporating switch means controlled by the timer for determining the supply of electrical current to a load. 22. An electrical appliance incorporating a timer as claimed in any of claims 1 to 17 and 21 and wherein the switch means in the timer is arranged to determine the operation of the appliance. 23. An electrical appliance as claimed in claim 22 except that the switch means forms part of the appliance, and the timer has no switch means determining the operation of the appliance and rather is arranged so that the cam in the timer determines the operation of the switch means in the appliance. 24. An electrical appliance incorporating a timer as claimed in claim 18 or 19 or 20, the appliance further incorporating switch means arranged to be controlled by the timer for determining the operation of the appliance. 25. An electrical appliance as claimed in claim 24, the appliance being a toaster incorporating a bread release mechanism arranged to operate switch means in the toaster for determining the operation of the--. toaster, and the timer being mechanically coupled to the bread release mechanism for determining the operation of the same.

Inventors: FLETCHER KEITH, WISE TOM, FLETCHER, KEITH, WISE, TOM
Applicants: FLETCHER KEITH, OTTER CONTROLS LTD, WISE TOM, OTTER CONTROLS LIMITED
Application year: 1993
Main classification: H01H43
IPC codes: H01H37, H01H43