GB338880A - Improvements in thermostatic devices for electrically heated fabrics - Google Patents

Abstract

338,880. Negromanti, A. May 18, 1929. Thermostats .-An electrically-heated fabric is provided with means by which any extensive or local overheating causes a break in the heating circuit. The heating resistance 1 is arranged in a circuit including a battery 2 and thermal switch 3, and a control wire 5, the resistance of which increases with temperature, arranged close to the wire 1 over its whole length and insulated from it. The circuit of the wire 5 includes the battery 2 and a heating resistance 4 adjacent to the thermal switch 3. The switch 3 is open when cold, but by the passage of a heating current in the resistance 4 the switch closes the circuit of the main heater 1. Any overheating causes an increase in the resistance of the control wire 5 so that the current in the resistance 4 is no longer sufficient to keep the switch 3 closed. In a modification, Fig. 2, not shown, the thermal switch is closed when cold and its circuit includes two wires with free ends which follow the fabricheating wire but are insulated from it. The two wires are covered by a metallic oxide which when cold is a bad conductor but when overheating occurs in the fabric becomes a good conductor and allow current to pass through the heater of the thermal switch which is thereby opened and the main circuit broken. In another modification, Fig. 3, not shown, the controlling circuit and the heating circuit are arranged as the four arms of a Wheatstone bridge. When the temperature equilibrium of the bridge is disturbed by overheating, a current flows in the diagonal of the bridge and operates either a thermal switch of an electromagnetic relay. The length of the arms of the bridge may be changed by a slider to enable different voltages to be used in the heating circuit, and the temperature limits of the device may be varied by varying the ratio of the arms of the bridge.