If it is desired to maintain the cold junction at 100°: the thermostat is set at this point, and the lamp, being wired to the 110- or 220-volt lighting circuit, will light and heat the box until 100° is reached, when the thermostat will open the circuit and the light is extinguished. The box will now cool down to 98°, when the circuit is again closed, the lamp lights, the box heats up, and the operation is repeated.
FIG. 129.—Compensating box.
BROWN AUTOMATIC SIGNALING PYROMETER
In large heat-treating plants it has been customary to maintain an operator at a central pyrometer, and by colored electric lights at the furnaces, signal whether the temperatures are correct or not. It is common practice to locate three lights above each furnace-red, white and green. The red light burns when the temperature is too low, the white light when the temperature is within certain limits—for example, 20°F. of the correct temperature—and the green light when the temperature is too high.
FIG. 130.—Brown automatic signaling pyrometer.
Instruments to operate the lights automatically have been devised and one made by Brown is shown in Fig. 130. The same form of instrument is used for this purpose to automatically control furnace temperatures, and the pointer is depressed at intervals of every 10 sec. on contacts corresponding to the red, white and green lights.
FIG. 131.—Automatic temperature control.