CA2140045A1

CA2140045A1 - Electric coffeemaker with keep warm control

Info

Publication number: CA2140045A1
Application number: CA002140045A
Authority: CA
Inventors: Michael A. Andrew; Walter G. Birdsell; Stephen D. Mowers
Original assignee: Michael A. Andrew; Walter G. Birdsell; Stephen D. Mowers; Black & Decker Inc.
Current assignee: Black and Decker Inc
Priority date: 1994-01-13
Filing date: 1995-01-05
Publication date: 1995-07-14
Also published as: BR9500161A; US5539856A

Abstract

An electric coffeemaker includes a first electric heater for heating water for brewing the coffee. A carafe is supported on a base portion of the housing of the coffeemaker. A second electric heater is mounted in heat transfer relation with the base portion for keeping the base portion warm when a coffee- laden carafe is positioned thereon. The operation of the second electric heater is controlled to reduce the heat delivered to the base portion when a relatively lesser quantity of coffee has been brewed and to increase the amount of heat delivered to the base portion when a relatively greater quantity of coffee has been brewed. The operation of the second heater may also be varied to satisfy specific consumer taste preferences.

Description

~4~)~45 , _ ~
BACKGROUND OF THE INVENTION
This invention relates to drip coffeemakers and in particular to a coffeemaker having a separate heater functioning to keep the coffee warm after it has been brewed. More specifically, the present invention relates to a control for the keep warm heater.
Conventional automatic drip-type coffeemakers have enjoyed widespread use and commercial acceptance. Drip coffeemakers are a very popular consumer small appliance. Such a coffeemaker typically includes a reservoir for containing water, a hot water generator to heat the water, a spreader for receiving the hot water and a container or basket ("brew basket") adapted to hold the coffee grind (generally in a filter) and to receive the heated water. The heated water is directed by the spreader onto the coffee grind contained in the brew basket. The brewed coffee is drained from the brew basket through an aperture located at the bottom thereof into a carafe supported on a base portion of the housing.
Many coffeemakers include heating means for maintaining the temperature of the coffee in the carafe at a relatively warm level after it has been brewed. Such means generally include utilization of the heater for the hot water generator.

21~0045 ~ -2-, , L ~

One of the problems associated with using the hot water generator heater as the heating means to keep the brewed coffee warm is that the heater is capable of delivering many times the watts required for the keep warm function. For example, the rated wattage of the heater for the hot water generator may be in the range of 950 through lOOO
watts whereas only about 50 to 75 watts may be required to obtain the necessary heat for the keep warm function. The use of the high wattage heater to perform the keep warm function results in increased operating costs for the user of the coffeemaker.
Some coffeemakers have used a separate heater rated at a lower wattage to perform the keep warm function. However, many higher quality recently introduced coffeemakers allow the user to select brewing cycles which will create lesser or greater amounts of coffee, for example, one to four cups or five through ten cups. In such instances, even the use of a second heater dedicated to the keep warm function is not totally satisfactory since the heat produced to satisfy the requirements for maintaining the desired temperature of ten cups of coffee will exceed the heat required to maintain the temperature of four cups of coffee.
Accordingly, it is an object of this ~3~ 2140045 .
invention to regulate the keep warm function heater so that the heat generated by the keep warm heater corresponds to the quantlty of coffee to be heated or may be varied to satisfy the specific taste of an individual consumer.
SUMMARY OF THE INVENTION
The foregoing object and other objects of the invention are obtained in an electric coffeemaker having a housing and a first electric heater lo mounted in the housing for heating water for brewing the co-ffee. A carafe is supported on a base portion of the housing. A second electric heater is in heat transfer relation with the base portion for keeping the base portion warm when a coffee-laden carafe is positioned thereon. The coffeemaker further includes means for varying the operation of the second electric heater to reduce the heat delivered to the base portion when a relatively lesser quantity of coffee has been brewed and delivered to the carafe and to increase the amount of heat delivered to the base portion when a relatively greater quantity of coffee has been brewed.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a perspective view of a coffeemaker embodying the present invention;
Figure 2 is an exploded, partially broken away perspective view showing details of the 21~0045 coffeemaker when viewed from the front;

Figure 3 is an exploded perspective view showing details of the coffeemaker when viewed from the rear;

Figure 4 is a side elevational view, partially in section, illustrating the manner in - which the coffeemaker is suspended from the lower surface of a cabinet;

Figure S is a fragmentary perspective view of the arrangement for suspending the coffeemaker from the cabinet;

Figure 6 is an exploded perspective view of the hot water generator and enclosure therefor employed in the coffeemaker;

Figure 7 is an elevational view further illustrating the hot water generator and its enclosure;

Figure 8 is a perspective view, somewhat similar to the view of Figure 1, illustrating the removable water reservoir of the coffeemaker;

Figure 9 is a side elevational view in section, illustrating the water reservoir mounted in the coffeemaker housing;

Figure 10 is an enlarged sectional view taken along line of Fig. 11;

Figure 11 is an enlarged top plan view illustrating details of the latch mechanism for the water reservoir;

Figure 12 is a schematic illustration of a first embodiment of a control for the heating elements of the coffeemaker; and Figure 13 is a schematic view of a second embodiment of the control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the various figures of the 21~004S

In referring to the various figures of the drawlng, like numerals shall refer to like parts.
Referring specifically to Figures 1, 2 and 3 there is shown a coffeemaker 10 particularly 5` suitable for mounting under a kitchen cabinet or the like in a manner that shall be more fully explained hereinafter. Coffeemaker 10 includes an outer housing 11 comprising a top wall 12, side walls 14 and bottom wall 16 and a rear wall 44.
Each of the side walls 14 inclùde a generally horizontally extending channel 30. Channel 30 is somewhat tapered so that its leading end 32 is wider than its trailing end 34.
Housing 11 is generally rectangularly shaped and includes an open front face which telescopically receives an inner housing and support structure 19 for coffeemaker 10.
Coffeemaker lO is a generally conventional automatic drip-type coffeemaker which has enjoyed widespread use and commercial acceptance.
Coffeemaker lO typically includes a reservoir 90 (illustrated in Figures 8 and 9) for containing water, a hot water generator 46 to heat the water, ? _ : . 2 1 ~ O O ~ S
_7_ (generally in a filter) and to receive the heated water. The heated water is directed by the spreader onto the coffee grinds contained in the brew basket. The brewed coffee is drained from the brew basket through an aperture located at the bottom thereof into a carafe 18 (shown in phantom in Figure 1) supported on base portion 20 of coffeemaker 10. Other than as shall be more fully explained hereinafter, the components of coffeemaker 10 are conventional.
Inner housing l9 includes a door 22. Door 22 is pivotally attached to supports 24 and 26 which are integrally formed with inner housing 19. Door 22 includes a generally rectangularly shaped opening 21 through which control knobs or buttons, such as start button 23, may be accessed by the user. Reservoir 90 is mounted in coffeemaker 19 behind door 22 when door 22 is in its closed position as illustrated in Figures 1 and 2. When the door is moved to an open position as illustrated in Figure 8, reservoir 90 can be either removed from or moved into compartment 29 formed in inner housing 19.

21~00~5 - _ 8-- be more fully described. Side walls 14 of outer housing 11 include a horizontally extending channel 30. Channel 30 has a leading end 32 and a trailing end 34. The width of leading end 32 is somewhat greater than the width of trailing end 34. As shown specifically in Figure 4, the upper wall 31 defining channel 30 has an opening 42 formed therethrough for a reason to be more fully explained hereinafter.
Coffeemaker 10 is designed for mounting beneath the lower surface 36 of a kitchen cabinet 33. Mounting bracket 35 is preferably employed to suspend coffeemaker 10 from kitchen cabinet 33.
Bracket 35 includes a main support frame 40.
Support frame 40 includes a horizontally extending rib 38 defining the upper face of bracket 35. A
finger 37 extends upwardly from the trailing end of rib 38. As illustrated in Figure 4, the upper face of rib 38 lies generally flush with the bottom surface of the lower surface 36 of kitchen cabinet 33. Similarly, finger 37 lies flush with the front surface 39 of cabinet 33.
Bracket 35 includes a rail member 43 which ` 2140045 .

g which is intended for alignment with and insertion into opening 42 of upper wall 31 defining channel 30. Channel 30 is tapered to permit rail 43 of bracket 35 to be readily inserted into the channel. The leading end 49 of rail 43 is somewhat smaller in width than the trailing end 51. Leading end 49 is first inserted into the leading end 32 of channel 30 and housing 11 is then moved relative to the rail to align latch member 41 with opening 42. When so aligned, finger 37 is flush with front surface 39 of cabinet 33. The coffeemaker is then anchored in place relative to cabinet 33. Bracket 35 in combination with channel 30 prevents unintended fore and aft movement, and unintended vertical upward or vertical downward movement of the coffeemaker relative to the cabinet. The interlocking relationship between bracket leading end 49 and the enclosed end of housing channel 30, together with the interlocking relationship between latch member 41 and housing opening 42 prevent any relative lateral movement between bracket 35 and housing 11.

, o generator 46 employed with coffeemaker 10. It should be understood, however, that hot water generator 46 may be used with coffeemakers of other designs.
Hot water generator 46, as shown in Figures 3 and 6, is generally mounted at the rear of inner housing 19 in a generally horizontal plane.
Generator 46 includes an inlet conduit 52 having an elastomeric coupling or elbow 50 formed therearound. Inlet conduit 52 is in communication with outlet 108 from water reservoir 90 as shown in Figure 9. Generator 46 further includes a relatively shallow V-shaped metallic tube 48 through which water flows from inlet 52 to an outlet 54. Outlet 54 is in communication with the inlet to spreader 27.
Hot water generator 46 further includes an axially extending electric resistance heater 58.
Heater 58 includes a thermostat 56 which senses the temperature developed within tube 48 of generator 46 and opens when the temperature exceeds a predetermined level. Electrical wires 61 and 61A connect heater 58 to a suitable source of 21~004S
.

V-shape, the temperature of the hot water delivered from generator tube 48 will be substantially consistent which, in turn, results in the brewed coffee having a consistent taste from one brewing cycle to the next.
Pump style hot water generators, such as generator tube 48 use the action of expansion and contraction of the generated steam in combination with the check valve in the outlet 108 from reservoir 90 to move heated water to the generator outlet 54 and spreader 27. The shallow "V" shaped generator tube 48 of the present invention uniquely takes advantage of the inherent characteristics of the generated steam to obtain substantially consistent high temperature water for brewing purposes.
Generally, in hot water generators employed in pump-type coffeemakers, the majority of water heating occurs by initial localized boiling, then the bulk of the water is heated by exposure to and condensing of circulating steam pockets. As steam is less dense than water, some control of the migration of the steam pockets formed in generator ,~. . 2l4''0n4s _l2_ will quickly cool to boiling temperature and violently collapse as they condense (change from a vaporous to liquid state). The violent collapse of the pockets will cause water to quickly flow from the direction of least hydraulic resistance to fill the void formed by the collapsed pockets.
Steam pockets formed in relatively hot water will cool slowly while moving where buoyant forces direct them. If given sufficient time, these latter pockets will also condense.
~ Iv~ shaped generator tube 48 directs the steam pockets towards the two extreme ends of the heated tube. The major heat transfer between resistance heater 58 and tube 48 occurs towards the center of the tube.
Steam migrating towards outlet 54 which contains freshly heated water is likely to remain in its vaporous state for a substantial portion of its passage to spreader 27. This provides a subtle force to move the water towards the spreader and a moving supply of heat should the temperature of the water cool much below 212F.
The more steam that is directed with the water to ~ ,. 2l40n~s ~_ _13_ whereat the temperature of the water is significantly colder, will condense more rapidly and the pockets will abruptly collapse. The voids left by the collapsed pockets will attempt to draw water from both ends of tube 48.
If a majority portion of the water flowing towards the voids comes from reservoir 9O, the temperature of the water in tube 48 tends to be colder and the pockets will collapse near the center of the tube. Conversely, if the temperature of the water in tube 48 is relatively warm, the steam pockets will migrate towards inlet 52 before collapsing. Thus, if the temperature of the water in tube 48 increases, the water flow from reservoir 9O will increase as the voids will be closer to inlet 52. In essence, by shaping tube 48 in a shallow V-shape, steam pockets will flow towards both inlet 52 and outlet 54 to provide a relatively high temperature water supply to spreader 27 at a smooth flow rate and with inherent self temperature regulation. Some design variations in the temperature of the water delivered to spreader 27 can be made by tailoring `~ 21~00~5 _14_ Tube 48 is housed in an enclosure 60 which is generally rectangularly shaped. Enclosure 60 comprises a first member 62 and a second member 64. Member 62 includes a first vertically extending section 76 and a second horizontally extending section 78. Section 78 terminates in a generally vertically extending flange 75. Vertical section 76 which may be considered a front wall includes a partial side wall 82 extending therefrom. A wall similar to wall 82 (not shown) extends from the opposite end of section 76.
Walls 82 include a generally semi-circular end portion.
Member 64 includes a generally vertically extending section 86 which is parallel to and spaced from section 76 of member 62. Attached to the lower end of section 86 is a generally horizontally extending section 88. Section 88 is parallel to and spaced from section 78 of member 62. Sidewalls 80 and 84 extend from the ends of section 86 and combine with walls 82 to form the side walls of the enclosure. Walls 80 and 84 have notches 85 and 87 formed respectively therein.

21~0045 _15 Member 76 includes downwardly extending tabs 66 and 68 which pass through slots 70 and 7Z formed in member 88. The combination of the tabs and slots enable portions 62 and 64 to be readily joined. Flanges 74 and 75 are placed in facing relation and a suitable connector is inserted through slot 89 in flange 74 to join the two flanges together. The thus formed enclosure 60 encapsulates hot water generating tube 48.
Walls 76,78,86 and 88 form the top, bottom, front and rear walls of the enclosure and are spaced from the surface of hot water tube 48. The only contact points between the tube and the enclosure are at the ends where the tube passes through the side walls defined by portions 8~, 80 and 84. The foregoing maintains the contact area between the outer surface of tube 48 and the walls of enclosure 60 at a minimum. This results in the heat developed by the generator being maintained, for the most part, within the enclosure. By retaining the heat within enclosure 60, other parts of the coffeemaker are not subjected to such heat which will prolong their effective operating ` 21~004S

time before reclosing if heat is retained within the enclosure rather than emitted via radiation, convection and conduction to other components of the coffeemaker. This is of particular importance in coffeemakers of the type disclosed in the present application which utilize a separate heater to warm the coffee delivered into the carafe.
Referring now to Figures 2 and 8-11, it will be noted that reservoir 9O is inserted into a compartment 29 formed at the front of coffeemaker lO and within housing 19. Reservoir 9O is removable from compartment 29. The reservoir includes sidewall 96, top wall 92 and front wall 98. Top wall 92 includes a generally rectangularly shaped opening 94 through which water can be delivered when it is desired to brew coffee. Front wall 98 is concave shaped and includes an integrally molded handle lOO. The concave shape of wall 98 enables the user to comfortably grasp handle lOO for maneuvering the reservoir to and from the source of water such as a kitchen faucet. The foregoing is particularly ` 2140045 l7_ with water and is somewhat heavy and cumbersome to transport.
The top front face 99 of housing 19 includes a rectangularly shaped slot 104. Reservoir 9o includes a depressible latch 102. Latch 102 includes a latch button 103 and a return spring 105. As shown specifically in Figures 10 and 11, latch button 103 is urged upwardly by a force provided by spring 105. Button 103 has a pair of spaced spring fingers 107 which maintains latch 102 in compartment lOlA defined by flange 101. A vertically extending rib 109 insures that fingers 107 are forced outwardly in compartment lOlA.
Slot 104 has a downwardly extending finger or rib 106 provided at its forward end in the path of movement of latch 102 as reservoir 90 is inserted into compartment 91. Latch button 103 engages finger 106 and is depressed thereby to enable the reservoir to be inserted within the compartment.
After the latch moves forwardly out of engagement with finger 104, the latch is urged upwardly by spring 105.

:
21~0045 `_ _l8_ button 103 to compress spring 105. The foregoing lowers latch 102 beneath finger 106 to enable the user to withdraw reservoir 90 from compartment 29.
Resistance heater 58 for hot water generator 46 may deliver anywhere from 800 to 1000 watts to heat the water flowing through tube 48. Some models of coffeemakers utilize the same heater for heating the hot water to also maintain the temperature of the brewed coffee in the carafe at a desired temperature for a period of time after the coffee has been brewed. The heat developed by an 800 - loO0 watt heater far exceeds the heat necessary to maintain the temperature of the coffee in a carafe at a satisfactory level for consumption purposes.
Some models of present day coffeemakers utilize a separate heater, for example 50 to 80 watts, for maintaining the temperature of the coffee in the carafe at the desired level, known as a "keeps warm" heater. However, even the use of a separate heater for the "keep warm" function has not been totally satisfactory, particularly when used with coffeemakers having the capability ` 2140045 , , 9 schematically disclosed a first embodiment of a control for adjusting the heat generated by the "keep warm" heater so that it is compatible with the quantity of coffee retained in the carafe or variable to suit the particular tastes of a consumer.
Control 110 is connected to a suitable source of power represented by lines 112 and 114 via electrical plug 111. Line 112 is the "hot" line and line 114 is the "neutral" line. Electrical power flows through line 112 to terminal 112A and then through conductor 116 to main power switch 118. If switch 118 is closed electricity flows through conductor 115 to junction 117. Conductor 115 is electrically connected to conductor 130 which, in turn, is connected to diode 120. Diode 120 is electrically connected to terminal 126 of switch 122. Switch 122 further includes terminals 124, 128 and 130. When switch 122 is placed across terminals 124 and 126, the flow of electricity is interrupted. When switch 122 is placed across terminals 126 and 128, current is supplied via conductor 131 to terminal 112B. If _20_ through conductor 133 to terminal llZC. Terminal 112C is in continuous electrical connection with terminal 112F. Terminal 112E is in electrical contact with terminal 112B. Terminal 112E is electrically connected through conductor 139 to electrical heater 140. Terminal 112F is electrically connected through conductor 141 to heater 138. Heaters 138 and 140 are electrically connected in parallel. In turn the heaters are electrically connected via conductor 143 to neutral terminal 114D which, in turn, is connected to neutral terminal 114C. Terminal 114C is electrically connected to terminals 114B and 114A
to complete the electrical circuit. Conductor 117A
connects terminals 114A and B.
Terminal 112G is electrically connected to terminal 112D. Conductor 119 is electrically connected to conductor 115. Conductor 119 is connected to conductor 149 through a parallel circuit alternatively comprising normally open switch 142 or diode 144. In turn, terminal 112G is connected via conduit 147 to thermostat 56, a thermal overload fuse 134 and hot water generator ` 21400~5 `~ _21_ complete the electrical circuit.
In use, when the user desires to brew coffee, and with reference to the control disclosed in Figure 12, the user connects cord 111 to the source of electrical power and closes switch 118.
Electrical power flows through conduits 112, 116, and 118 to junction 117. If the user is brewing a relatively full pot of coffee, switch 142 is closed and electrical power is supplied via conduit 119, switch 142 and conduit 149 to terminal 112C. Power then flows through terminal 112G, thermostat 56, thermal overload protector 134 to heater 58 which generates the heat to raise the temperature of the water passing through hot water generator 46 to a desired temperature level.
If the user is brewing a lesser batch of coffee, for example one to four cups, then switch 142 is maintained open and electrical power flows through diode 144 which provides half wave rectification. The half wave rectified electrical power flows through conduit 149 to terminal 112C
and thereafter passes to heater 58. If heater 58 is rated at 9OO watts under full power, its rating -` 2140045 _22_ Once the coffee has been brewed, the user may not immediately drink all the coffee and thus coffee remains in the carafe to be kept warm by the "keep warm" heater. In the preferred embodiment heater 138 is rated at 52 watts and heater 140 is rated at 28 watts.
If a relatively small quantity of coffee remains to be warmed in the carafe , the user moves switch 122 to the low position. With switch 122 connected across the terminals 124 and 126, heater 140 is effectively removed from the electrical power circuit. Thus, only heater 138 operates to provide heat for the "keep warm"
function.
However, if a greater quantity of heat is required due to the quantity of coffee in the carafe being at a higher level, then the user may opt to place switch 122 in either the medium or high positions. If switch 122 is placed in its medium position, it is placed across terminals 126 and 128 as illustrated in Figure 12. In this position, the electrical power flows through diode 120 and thence through terminals 126 and 128 to . 21400~5 - _23_ heater 138 is rated at 52 watts and heater 140 rated at 28 watts then the total wattage output when diode 120 is in the circuit is approximately 66 watts.
Finally, if the user desires the maximum level of heat for the "keep warm" function, switch 122 is placed in its high position whereby the switch connects terminals 128 and 130. In this position, diode 120 is effectively removed from the circuit and full wave voltage supply is furnished to heater 140. Current flows through contacts 130 and 128, 112B and 112E to heater 140.
Further, electrical power is furnished to heater 138 through contacts 112D and 112F. The fully rated wattage for example 80 watts is used for the "keep warm" function.
Referring now to Figure 13, there is disclosed an alternative embodiment to the control described in Figure 12 for regulating the operation of the "keep warm" function.
Control 210 includes conductors 212 and 214 which are connected to a source of electrical power via supply cord 211. Conductor 212 is - 2140~
_24_ delivers electrical power to terminals 212A which, in turn, provides power to main power switch 216.
When closed, switch 216 delivers electrical power via conductor 217 to buss 218.
Buss 218 is electrically connected to a light 219, which when lit, indicates that electrical power is furnished to the coffeemaker. Buss 218 is also electrically connected to terminals 220A, 222A, and 224A. Terminal 220A in turn is connected to terminal 220B; terminal 222A is connected to 222B and terminal 224A is connected to conductor 224 and to normally open switch 226.
Terminal 220B is connected via conductor 242 to a thermostat 240 and a "keep warm" heater 238.
In the preferred embodiment heater 238 is rated at 80 watts. Heater 238 is connected via conductor 244 to terminal 214C which is a neutral terminal connected to terminal 214A and conductor 214.
Switch 226 is connected to terminals 228 and 228B and then via conductor 230 to heater 238.
Thus, electrical power is alternatively delivered to heater 238 via the circuitry associated respectively with conductors 230 and/or 242.

21~0~45 _25 generator heater 58 and conductor 246 to neutral terminal 214B. Terminal 214B, in turn, is connected to terminal 214A and neutral conductor 214.
In operation, when switch 216 is closed, electrical power is delivered via conductors 212 and 214 to buss 218. Buss 218 provides electrical power to conductor 244 through terminals 222A and 222B. Electrical power is then delivered to hot water generator 58 to provide hot water for brewing process.
Further, when it is desired to utilize the "keep warm" function, switch 226 may either be closed or open. If switch 226 is open, power is delivered from buss 218 through terminals 220A and 22OB to thermostat 240 and thence to heater 238.
Thermostat 240 can be set at a selected temperature. Thermostat 240 senses the temperature at base 20 and may be set to obtain a lower temperature when a lesser quantity of coffee remains in the carafe or a selectable lower temperature to satisfy the individual taste of a consumer. The contacts of thermostat 240 will open _26_ warm" function as a consequence of the carafe being relatively filled with coffee, switch 226 is closed. Electrical power is then delivered through terminals 228A and 228B, and conductor 230 to heater 238. The wattage available from the heater will then be continuously delivered for the "keep warm" function. Thermostat 240 is effectively shorted from the circuit.
With either of the two controls described above, operation of the "keep warm" heater is selectively controlled so that the heat generated by the heater is compatible with the quantity of coffee remaining in the carafe for which the "keep warm" function is required.
While preferred embodiments of the present invention have been described and illustrated, the invention should not be limited thereto but may be otherwise embodied within the scope of the following claims.

Claims

1. An electric coffeemaker comprising:
housing means;
a first electric heater mounted in the housing means for heating water for brewing the coffee;
a carafe supported on a base portion of the housing means;
a second electric heater in heat transfer relation with the base portion of the housing means for keeping the base portion warm when the coffee-laden carafe is positioned thereon; and means for varying the operation of the second electric heater to reduce the heat delivered to the base portion when a relatively lesser quantity of coffee has been brewed and delivered to the carafe and to increase the amount of heat delivered to the base portion when a relatively greater quantity of coffee has been brewed.

2. An electric coffeemaker in accordance with Claim 1 wherein the means for varying include a third electric heater which is selectively energized when the relatively greater quantity of coffee has been brewed.

3. An electric coffeemaker in accordance with Claim 1 wherein the means for varying include means for sensing the temperature of the base portion and for selectively connecting the second heater to the source of electric power to lower the effective operating temperature of the second heater when the relatively lesser quantity of coffee has been brewed.

4. An electric coffeemaker in accordance with Claim 3 wherein the means for sensing comprises a variable thermostatic switch.