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Electric Thermostat

A thermostat or apparatus, similar to a thermometer in some cases, for closing an electric circuit when heated. It is used in connection with automatic fire alarms to give warning of fire. For this use a temperature of 52° C. (125° F.) is an approved one for settingone at, to complete the circuit. It is also applied to regulation of temperature, as in incubators.

(a) One kind of thermostat consists of a compound bar wound into a spiral and fastened at one end, to which a terminal of a circuit is connected. The bar may be made of two strips of brass and iron riveted together, and wound into a spiral. When such a bar is submitted to changes of temperature it bends in different directions, because brass expands and contracts more under changes of temperature than does iron. A contact point, to which the other terminal is connected, is arranged to make contact with the spiral at any desired degree of temperature, thus closing an electric circuit and ringing a bell, opening or closing a damper, or doing anything else to notify an attendant or to directly change the temperature.

If the brass forms the outside of the spiral, increase of temperature makes the bending of the spiral bring the coils still closer. If the brass forms the inside, increase of temperature makes the spiral tend to become less close. As shown in the cut, the brass should lie along the inside of the spiral.

Sometimes a straight compound bar is used, one of whose ends is fastened and the other is free. As the temperature changes such a bar curves more or less, its free end moving to and fro. Two contact screws are provided, one on each side of its free end. If the temperature falls it makes contact with one of these; if the temperature rises, it makes contact with the other. Thus it may close one of two circuits, one for a fall and the other for a rise in temperature.

It is well to introduce a third bar between the brass and iron ones, made of some material of intermediate coefficient of expansion.

(b) Another kind of thermostat comprises a vessel of air or other gas, which, expanding by heat, actuates a piston or other device and closes an electric circuit. Synonym: Electro-pneumatic Thermostat.

(c) Another form utilizes the expansion of mercury. The mercury is made part of an open electric circuit. As it expands it comes in contact with the other terminal of the circuit, thus completing it, when the current gives an alarm or does as is provided for in the apparatus employed.

Thermostats may be worked on either open or closed circuits; normally the circuit may be open as described and may close on rise of temperature, or it may be normally closed and open as the temperature rises.

Fig. 335. ELECTRIC THERMOSTAT.