A HIGH TEMPERATURE CONTROL CIRCUIT
This is an electronic circuit useful in the control of electric air-bath furnaces having a long time lag. The control is accomplished by means of a relay that shunts a portion of the resistance in series with the furnace winding. The thermometric device is a thermocouple connected in series with a mirror-type galvanometer and a variable voltage supplied by a potentiometer. The setting of the potentiometer selects the temperature at which the device will control. A light beam falling on the mirror is reflected to one of two photocells. One photocell closes the shunt by means of the described circuit and increases the current in the furnace. This condition is maintained until the beam moves across the second photocell, when the shunt is again opened. The device will control temperatures over 1000 °C. within a precision of ± 2 °C. over any desired length of time.