Abstract: A visual indication is provided in a heater of an electric cooking appliance which incorporates a user-operable multi-position switch for switching the heater from one power output to another. The heater incorporates at least one heating element which is capable of emitting a significant amount of visible radiation, the at least one heating element being de-energized and subsequently re-energized as the heater is switched from one power output to another. Such a heater is of particular value where the heating element is capable of emitting fewer distinct levels of visible radiation than the number of distinct heat settings of the heater selectable from the multi-position switch. The heating element may be de-energized and subsequently re-energized only between adjacent heat settings in which there is no substantial change in the level of visible radiation.

Abstract: An automatic cooking method for cooking food wherein one of a timed or automatic cooking mode is selected prior starting the heating of the food upon depressing a cooking start key. The quantity or concentration of gas (e.g. carbon dioxide) or smoke emitted from the food every sampling time of a certain period is determined, and counting the time T1 taken for the quantity or concentration of gas or smoke to vary from a predetermined minimum value to a predetermined maximum value is also determined. The cooking is carried out to further heating the food for a time T2 calculated by multiplying the counted heating time T1 by a coefficient predetermined depending on the food.

Abstract: An electronic regulator (12) for electric heating apparatuses, such as baking ovens (11), is equipped with a microcontroller (20), which contains in fixed programmed form the process data for the individual regulating functions. For adapting the real data of a temperature sensor (19) to the ideal conditions, e.g. in the center of a baking oven muffle (14) is provided a separate, non-volatile, but programmable memory (EEPROM) (26). It can be programmed from a personal computer (34) by means of a read-in connection (32), said computer determining the parameters by comparing the values of the standard temperature sensor (19) and separate temperature sensors (36) located at ideal points. The memory (26) can be differently programmed for individual operating modes.

Abstract: An electric toaster that enables power to be disconnected from the heating elements upon completion of a toasting cycle regardless of the position of the main product supporting the carriage in the toasting compartment. The toaster has an electromagnetic latch and an auxiliary member carried by the main carriage which is movable relative to the main carriage. The auxiliary member is urged by biasing member to move in a direction away from the electromagnetic latch but able to be held thereby when the latch is activated. The auxiliary member also is arranged to close main power supply contacts to supply power to the heating elements when the auxiliary member is held by the latch, but to open said main supply contacts when the auxiliary member moves away from the latching means under action of the biasing member. The toaster may also include a sensor to sense abnormal conditions in the toaster to disconnect power from the heating elements when such abnormal conditions occur.

Abstract: Disclosed are a method and apparatus for high speed, selective detection of vapors of specific compounds, particularly nitrogen-containing compounds, utilizing a bypass branch and high speed gas chromatography for improved selectivity and sensitivity of detection. A system with two gas chromatographs (GC's) alternating in series with two pyrolyzers provides two time intervals of detection in a downstream detector, with the second time interval containing signals delayed and further separated relative to signals from the first time interval. The bypass branch, in diverting a portion of the flow of gas samples from passage through the second gas chromatograph and second pyrolyzer, avoids interferences in the first time interval of detection from non-GC-retained compounds decomposed in the second pyrolyzer. Also disclosed is an arrangement for rapid, precise heating of vapor concentrator tubes in which vapors are rapidly focussed and then injected into a GC, and for rapid, precise heating of the GC's.

Abstract: A control system, a control device therefor and methods of making the same are provided, the control system comprising a source of energy, an output producing unit that uses the energy to produce the output thereof, a control device having control structure for interconnecting the source of energy to the unit, the control device having a rotatable shaft that sets the control structure in different positions thereof in relation to the rotational set position of the shaft from an "off" position thereof, the control structure having the amount of the energy passing therethrough from the source to the unit being different for each set position thereof, indicating structure operatively associated with the shaft for visually indicating the set position of the shaft and, thus, the amount of energy being fed by the control structure to the unit, and sensing structure operatively associated with the shaft that senses the set position of the shaft and electronically displays that sensed position of the shaft whereby th

Abstract: A cooking apparatus such as a rice cooker includes means for obtaining a value or a combination of values indicative of an amount of food that is to be cooked, and a fuzzy inference section for operating on that value or combination of values to determine a level of heating power to be supplied to a heater element of the apparatus, during at least one part of the cooking process. The amount-indicating value can be obtained for example as a rate of change of temperature of the food while it is being heated to boiling point. The apparatus can further be configured to determine a plurality of cooking control parameters, by fuzzy inference operation, based on a specified degree of final hardness of the cooked food, which can be selectively inputted by the user.

Abstract: The device comprises a sensor supplying a sensor signal which varies as a result of the placement of a cooking vessel in a heating zone of a heating appliance. The device includes evaluating circuit which, as a function of the sensor signal, supplies an output signal. The evaluating circuit produces the output signal as a function of the rate of change of the sensor signal to indicate the presence or absence of cookware in the heating zone of the heating appliance.

Abstract: A radiant heating control system for a computerized tape laying machine is described in which the radiant heater is controlled so as to reliably heat thermoplastic composite tape to a processing temperature above the melting point of the thermoplastic matrix irrespective of velocity or change in velocity of the tape as it travels from a spool of tape to a tape application point for application to a layup tool or the like.

Abstract: An automatic cooking method including the steps of starting the heating of the food upon depressing a cooking start key, detecting the amount of vapor emitted from the food every sampling time of a certain period, and counting the time T1 taken for the humidity to vary from a predetermined minimum value to a predetermined maximum value. The cooking is carried out by further heating the food for the time T2 calculated by multiplying the counted heating time T1 by a coefficient predetermined depending on the food.

Abstract: A rapid thermal processing system is accurately controlled during a deposition process, in which the emissivity of the substrate material changes as a function of the thickness of the deposited layer, by determining the expected emissivity as a function of time during deposition and applying the expected emissivity to a controller to produce a converted temperature which controls the radiant heat sources of the rapid thermal processing system. In one embodiment, the expected emissivity is used to convert a measured pyrometer temperature into a converted pyrometer temperature. The converted pyrometer temperature is applied to a feedback controller which controls the radiant heaters so that the converted pyrometer temperature equals the desired wafer processing temperature. In another embodiment, the expected emissivity is employed to convert the desired rapid thermal processing temperature into a converted desired rapid thermal processing temperature.

Abstract: A cooking appliance is disclosed which has a cooking control system responsive to selection of a given food to be cooked, for indicating the optimum cooking position of the food in the appliance. The appliance may be an oven, with the indicator indicating the optimum position of a shelf in the oven, or the optimum position of the food on the shelf.In a further disclosed feature, a cooking appliance may have an air circulation system, such as a blower, for circulating air about food to be cooked. A cooking control system is responsive to selection of a given food to be cooked for controlling the air flow about the food for optimum cooking thereof.In another disclosed feature, a cooking appliance which has a control system responsive to the selection of given food and the number of rack positions required, will control the rate of energy released from the oven heat sources for optimum cooking.

Abstract: An apparatus ensures that a controller memory is programmed to store safety critical information corresponding to a heating system in which it is installed. The heating system has a controller and a controller memory which controls the heating system based on input parameters provided to the controller and based on safety critical information stored in the controller memory. A program signal is generated when the controller memory is electrically disconnected from the heating system indicating that the controller memory must be programmed to include safety critical information. Operation of the heating system is inhibited in response to the program signal.

Abstract: An air heater with a safety control circuit is disclosed. A heat source is mounted within a hollow housing having an air inlet end and an air outlet end. The heat source is actuated in response to the presence of a heat control signal. The heat control signal is produced when both a predetermined minimum pressure differential between static air pressure outside the hollow housing and the air velocity pressure within the housing is reached and the temperature of a portion of the housing is lower than a predetermined maximum sensed temperature. Application of the heat control signal to the heat source can be manually initiated or interrupted.

Abstract: A temperature sensitive motor control for use in roofing membrane welding devices is described. The control circuit gives an output impedance responsive to variations in the temperature of the welding device and ambient temperature. A thermocouple provides a voltage signal responsive to changes in the temperature of the heating plate of the welding device. An ambient temperature sensitive circuit element acts in conjunction with the input signal to provide an output signal that is responsive to both ambient temperature and heating plate temperature. The output signal is applied to a voltage variable impedance element which outputs an impedance that is a function of both application temperature and ambient temperature. In one preferred embodiment of the present invention, the ambient temperature sensitive circuit element is a thermistor. In an alternate preferred embodiment of the present invention, the ambient temperature sensitive circuit element is a diode-comparator circuit.

Abstract: The invention relates to a temperature control method and to a temperature control circuit. In a measured value detecting member (2) a measuring voltage (U.sub.TH) is generated which is representative of the actual temperature. In an adjustment member (1), a desired voltage (U.sub.S) is generated which is representative of the desired temperature. A difference former (inverse adder 5) determines from the measuring voltage (U.sub.TH) and the desired voltage (U.sub.S) a deviation voltage (U.sub.D) which corresponds to the deviation between the desired temperature and the actual temperature. The heating power of the heating element (14) of the temperature control circuit is regulated proportionately in dependence on the deviation voltage (U.sub.D) by shifting the phase angle (.phi.). During each line half-wave (27), the deviation voltage (U.sub.D) is compared in a comparator (comparator 11) with the descending edge voltage of a line-synchronous delta voltage (U.sub.DS).

Abstract: Encapsulated optical fibres (FIG. 1A), for e.g. a submarine cable have their encapsulation removed by high velocity hot air blowers (6,7), leave free the acrylate coated fibres for splicing/jointing. An automated equipment moves the blowers progressively along the fibres to free between 0.75 and two meters.

Abstract: The present invention is directed to a heat treatment apparatus for performing heat treatment of a substrate. According to the present invention, a thermocouple is introduced into a heating furnace and discharged from the heating furnace by carrier means in such a state that one side ends of the wires forming thermocouple are connected to the substrate and another ends of the wires forming the thermocouple are connected to fixed terminals of a terminal mount. Movable terminals of a terminal driving mechanism are brought into contact with the fixed terminals of the terminal mount, to outwardly extract output signals of the thermocouple. Thus, temperatures of the temperature measuring substrate can be accurately measured while securing air-tightness in the heating furnace. Further, entanglement of metal wires forming the thermocouple is prevented in introduction/discharging of the substrate.

Abstract: Circuit protection devices which comprise a PTC conductive polymer element and a second electrical component which is thermally coupled to the PTC element and which, when a fault causes the current in the circuit to become excessive, generates heat which is transferred to the PTC element, thus reducing the time taken to "trip" the PTC element. The second component is for example a voltage-dependent resistor which is connected in series with the PTC element under the fault conditions and is thus protected from damage.

Abstract: The invention relates to a process for operating an apparatus having a cooking area for the heat treatment of food, the heated cooking area being at least partly actively cooled prior to the introduction of new food.The invention also relates to an apparatus for the heat treatment of food with a cooking area, in which an inadequate cooking area temperature can be indicated prior to starting a heat treatment, e.g. by means of an optical and/or acoustic signal generator and/or whose cooking area is at least partly actively coolable.

Abstract: The specification discloses a vehicle electrical control system capable of switching the alternator output between a uniform standard voltage and a uniform higher voltage. The control system includes a supplemental voltage regulator and a switching circuit for selectively switching the supplemental regulator into the system to regulate the conventional voltage regulator. Preferably, the control system further includes a timing circuit for powering a plurality of electrically heated windows in a timed sequence.

Abstract: A control system and control device for controlling a heating unit and method of making the control device are provided, the control device having a rod and tube temperature sensing unit for providing linear motion of a part thereof along a substantially straight linear path in relation to the temperature being sensed by the temperature sensing unit and having a potentiometer unit provided with an electrical output producing unit operatively interconnected to the part of the temperature sensing unit so as to produce a variable electrical signal in relation to the temperature being sensed by the temperature sensing unit, the electrical output unit being movable along an arcuate linear path by the part of the temperature sensing unit. The potentiometer unit has a member provided with an arcuate surface, the electrical output unit being disposed in sliding engagement with the arcuate surface whereby the arcuate surface defines the arcuate path of movement of the electrical output unit.

Abstract: A device for regulating the cooking process in a cooking vessel which is heated by a heating element provided with a control circuit. The temperature in the cooking vessel is measured and is utilized by the control circuit to regulate the heating element. The cooking time can be preset by an adjustable timer. The cooking time in the cooking vessel is made independent of the inertia inherent in the cooking system by causing the timer to start metering the preset cooking time only after the cooking temperature has been reached. The operation of the timer may be governed by the slope of the temperature-time curve during the heating phase to enable the timer to shut off the heating element prematurely, relative to the preset cooking time, as the temperature-time curve rises less steeply during the heating phase.

Abstract: A device for the control of the cooking process in a steam pressure cooker filled with water and the food to be cooked and heatable by means of a heating element controlled by a control circuit wherein the temperature of the pressure cooker is detected and utilized by way of the control circuit for the adjustment of the heat output of the heating element and in which by means of an adjustable timing element a cooking time can be preselected.

Abstract: This invention relates generally to thermal control apparatus wherein a change in temperature is transmitted from a sensing bulb through a capillary to a control device. More particularly this invention is related to a novel, fail-safe control device for use in conjunction with a temperature sensing bulb, capillary, and control device in an oven or other heating facility whereby a loss of pressure within the bulb, capillary, control device system will cause the heating device to be shut off and thus will prevent the oven or other heating facility from being overheated.

Abstract: A temperature controller for monitoring the temperature of a rotating heating system and regulating the temperature relative to a desired temperature. A transmitter within the rotating system coupled to a remote stationary controller via slip rings produces a magnitude varying current control signal within a calibrated range at a magnitude proportional to the sensed temperature. A pair of voltage dividers within the transmitter establish the level and range of current magnitude relative to a constant current source. The remote stationary controller compares the current control signal to a set-point and synchronously supplies power to the heater elements of the rotating system, so long as the sensed temperature is below the desired temperature.

Abstract: The output of a first bridge circuit including as one component element a thermistor for detecting the temperature of a pan is compared with a preset value in a first operational amplifier. The first operational amplifier provides a low level output when the detected temperature is below a predetermined temperature. When this low level output is supplied to a second operational amplifier, the output thereof goes to a high level to turn on a transistor so as to energize a relay for passing current to a cooking heater. When a cooking end temperature is reached, the internal resistance of the thermistor is reduced, and as a result the output of the first operational amplifier goes to a high level, causing the output of the second operational amplifier to go to a low level. Thus, a warming heater is energized while the cooking heater is de-energized.

Abstract: In a foodstuffs heating apparatus wherein a plurality of different heating function modes are selectively effected and heating temperature and heating time are settable, when an oven heating function is selected from the plurality of function modes by a selected cooking card, the function of selector switches adapted to select heating function modes other than the oven heating function may be changed from the heating function mode selection to the heating temperature setting, so that the setting of heating temperature may automatically be effected by utilizing the cooking card.

Abstract: A control system for selectively turning on and off various electrical devices used in the operation of a spa is disclosed. The control system includes a power supply for generating a 5 volt DC voltage from a 120 volt AC external power source, a timing circuit, a clearing circuit and a plurality of control circuits, one for each device to be controlled. Each control circuit, which is driven by the 5 volt DC voltage, includes a relay which is adapted to be coupled to the electrical device to be controlled, a momentary manually operated switch, a pair of flip-flops, a gate, a transistor, and an incadescent light bulb. In the operation of the circuit the transistor is turned on and off by the flip-flops and the output of the transistor is used to energize or deenergize the relay. Because the circuits operate at low DC voltage, the switches through which these voltages are transmitted can be located at or near the spa without causing potential safety problems.

Abstract: An electrical circuit electrically connects a timer motor for a timer in parallel with a thermostat means, the thermostat being electrically connected in series with a heating mechanism. With such a connection, heat may be applied to a drying apparatus while the timer is not running and when the thermostat opens to shut off the heat, the timer motor will run to operate the timer to advance the timer through the time cycle.The timing mechanism or timer for the control system includes at least one set of switching structures responsive to a cam, the switching structure including a terminal block, and first and second electrical contact blades cantilevered on opposite sides of the terminal block and cooperating electrical contacts to open and close electrical circuits.

Abstract: A timing control apparatus and circuit for use in a toaster or other device, such as an oven, includes electronic solid-state circuitry including a thermistor operating in conjunction with a metal oxide semiconducter (CMOS) wherein when the predetermined voltage or heat buildup is attained, a condenser is charged to positive to activate the timing circuit, which, in turn, activates a SCR, shorting a full wave bridge to deactivate the heating elements and close a normally open solenoid, which unlocks a mechanical latch and permits the toast to be ejected. The device employs low operating current, expends no energy except at the moment of triggering the solenoid and is capable of being used as original equipment or to retrofit existing units.

Abstract: A rapid response thermal switch for isolated load control includes a bimetallic switch electrically connected to a controller and operatively connected to a switch follower which controls the operation of a second switch electrically connected to a load so that the load is electrically isolated from the controller. While energization of the bimetallic switch by the controller causes the bimetallic switch to move away from its switch follower, the switch follower is biased to follow the bimetallic switch. The initial movement of the switch follower allows the load current controlling switch to close, energizing the load. The bimetallic element, heated by the controller currents flexes to cycle itself on and off while maintaining the load controlling switch in the closed position. When the bimetallic element sufficiently cools after deenergization, it returns the bimetallic switch to its closed position.

Abstract: The temperature of an electrically heated element is controlled by applying a current until the element reaches a threshold temperature, preferably determined by the expanded length of the elements, and then applying a predetermined voltage-current condition for a selected period of time. This is followed by repeated applications of such voltage-current conditions in response to the cooling of the heating element back to the threshold temperature.

Abstract: A cooking apparatus provided with a card type switch actuator including a card holder having at least an openable cover member and a bottom wall disposed in opposition to the cover member in its closed state, a plurality of push-button switches mounted on the bottom wall of the card holder, a plurality of cards of a substantially same size prepared for various control purposes, respectively, and a chamber provided in the card holder for accommodating a selected one of the cards at a predetermined position. Each of the push-button switches has a push-button head projecting in the chamber from the bottom wall, and each of the cards is provided with through-holes of a size slightly greater than that of the push-button head at locations corresponding to the positions of those push-button switches which are not to be actuated depending on the control purpose assigned to the card.

Abstract: A coffeemaker is provided with a power circuit and a control circuit. The power circuit is connected to a household source of electric power and it is activated by a relay switch. The control circuit includes a relay coil for the switch, and a triac that may be triggered into conduction when a turn on signal from an electronic timer module is applied to the control terminal of the triac simultaneously with the application of direct current control circuit voltage across the triac. Thereafter, the triac remains in its conductive state so long as the direct current flowing through the triac remains above a minimum level. Subsequent removal of the turn on signal from the control terminal of the triac has no effect on the operation of the power circuit which includes the coffeemaker heating means so long as the direct current flowing through the triac remains above a minimum level.

Abstract: A control system for a heating unit that has controls for comparing a variable electrical signal that has a value in relation to the output temperature of the heating unit and a selected electrical signal that has a value in relation to a selected temperature setting to control the operation of the heating unit to tend to produce an output temperature substantially equal to the selected temperature, the controls including a control device having a rod and tube temperature sensing unit for providing linear motion of a part thereof in relation to the output temperature of the heating unit and having a potentiometer provided with an electrical output producing part operatively interconnected to the movable part of the rod and tube unit so as to produce the variable electrical signal in relation to the output temperature of the heating unit.

Abstract: A control instrument is used for the manually continuously adjustable supply of electric power to electric hot plates in the form of individual power pulses. The control instrument contains a switch, operated by a bimetallic member, whose heater is controlled by an electronic circuit and supplies current to the heating means in individual half-waves. An automatic initial cooking device with an electronic timing member is provided which, during the initial cooking phase, reduces the power supply to heater in a predetermined ratio and consequently correspondingly increases the power supplied to the electric hot plate. The automatic initial cooking device is automatically switched on by the control instrument knob. By rotating the control knob beyond a median power limit the automatic initial cooking device is switched off.

Abstract: A system comprising a heat generating load, a power control element for controlling power supply to the heat generating load, a temperature detecting circuit for producing an ON signal when the temperature of the load detected by a temperature sensor is lower than a set temperature and an OFF signal when the temperature has reached the set temperature, and a control circuit for feeding on ON or OFF signal to the power control element in response to the signal from the temperature detecting circuit. The control circuit includes a hysteresis control circuit which, once the temperature detecting circuit detects that the temperature has reached the set temperature, permits the power control element to receive OFF signals for a suitable period of time even after the temperature has subsequently lowered to a level at which the signal from the temperature detecting circuit changes to an ON signal.

Abstract: A control circuit for supplying power to the heater of an electrically heatable windshield of an automotive vehicle while simultaneously providing the requisite current for the standard electrical system of the vehicle includes a resistive heating element connected in series between the vehicle alternator output and the high voltage terminal of the vehicle battery, so that current conducted through the heating element is supplied to both the battery and the load circuit. A control circuit, responsive to the alternator output voltage and the battery voltage, controls a voltage regulator to regulate the alternator output voltage within a first narrow band corresponding to the rated battery voltage and within a second narrow band corresponding to a given power rating for the heater.

Abstract: An off-peak, electric, central air heating system for buildings incorporating a heat storage material that is used to store heat generated electrically during off-peak electrical power periods and to deliver heat on demand. The heat storage material is an alkaline metal or alkaline earth metal salt such as Na.sub.2 SO.sub.4, NaCl, CaCl.sub.2 and KCl individually or in combination with one another. Generation of heat during the off-peak electrical power periods is automatically controlled by an electronic time clock and time delay circuitry, and by selective variable temperature control. The heating unit is constructed with a casing having an inner jacket and an outer jacket with an air space between the two jackets. The storage material is surrounded by the inner jacket and the air space provides insulation as well as an air passage between the two jackets.

Abstract: The invention relates to an electric storage heater assembly which includes a core and a resistance heater for the core. A low tariff period switch and a charging switch are in series with the resistance heater. A triggering unit, which includes an integrator in one embodiment, and an integrator and comparator in a second embodiment, operates the charging switch. A feed circuit for charging the integrator is responsive to the indoor temperature and the unregulated heat loss from the core. A scanning circuit which produces the opposite effect is responsive to the regulated heat loss from the core and the outdoor temperature. The scanning circuit can be connected to either the integrator in one embodiment or to the comparator in the other embodiment.

Abstract: A charging apparatus for an electric storage heater includes a resistance heater in series with a low tariff time switch and a charging switch. While the low tariff time switch is closed, usually at night, an integrator controlled by a differential amplifier controls the charging switch in accordance with the total charge accumulated by the integrator which reflects the use of energy during the preceding day. The charge of the integrator at the end of the day is the result of it being intermittently charged and discharged by a voltage divider circuit which has one or more temperature responsive resistances and transmits a voltage to one input of a differential amplifier for the integrator which can be above or below a reference voltage on the other input of the amplifier.

Abstract: An electric storage heater assembly which includes a core and a resistance heater for the core. A charging switch is in series with the resistance heater and an electric comparator for operating the switch has two inputs connected to first and second controls which are responsive to the instantaneous temperature of the core and the final temperature of the core for the preceding charging period. The first comparator input is also connected to auxiliary controls which sense indoor and outdoor temperatures and provide signals which are summed with the signal which represents the core temperature.

Abstract: Apparatus for controllably heating a vehicle window having a heating grid is described including an oscillator and a signal detector, with a signal circuit coupling the oscillator and the signal detector having a DC voltage component at one or more points thereof. One or more humidity detectors are coupled to the signal circuit to control the amplitude of the oscillations supplied from the oscillator to the signal detector, advantageously by voltage divider action, and capacitor means provides DC isolation between the humidity detector(s) and any DC voltage component in the signal circuit, and also between humidity detectors when a plurality are used. Advantageously the humidity detector(s) are directly connected to the heating grid, and varying DC voltage levels at the point(s) of connection to the grid are isolated by the capacitor means. Particular arrangements of the humidity detectors on the window and connections to the heating grid are described.

Abstract: An electric storage heater assembly which is charged at night when energy rates are low and utilized during the day when the rates are higher. A fan unit is controlled by a thermostat to extract heat from the heater assembly. The previous days experience relative to the total time the fan operates is a measure of the heat to be stored during the off hours. An integrator with input and output switching elements activated by a clock controlled time switch is used in connection with a three position room thermostat, the positions being for high and low temperatures and neutral. The input switching element has positive and negative terminals and during day operation, the time the thermostat is in its low temperature position is summed through the positive terminal. The time the thermostat is in its high temperature position is subtracted from the accumulated value of the integrator through the negative terminal.

Abstract: Temperature control system for furnaces used in heat-treating metallic articles, such as boronizing steel.

Abstract: A grid of heating conductors for defrosting applied to the window of a vehicle is connected to a source of electrical energy through a relay-operated switch. This switch is closed either in response to a drop in the resistance between two conductors or electrodes mounted in spaced relation on an insulating support such as the window (the drop occurring by accumulation of moisture on the support and between the conductors) or in response to a drop in temperature at or near the window. For this purpose the resistance between the two conductors over the surface of the support may be paralleled by a thermostatic switch which closes with drop in temperature, both this resistance and the thermostatic switch being in the input to a trigger circuit controlling the relay to energize it in response to a decline in resistance at that input. Alternatively the trigger circuit may include thermistors which sense the drop in temperature and set the trigger circuit to the condition which energizes the relay.