COMPENSATING LEADS.--By the use of compensating leads, formed of the same material as the thermo-couple, the cold junction can be removed from the head of the thermo-couple to a point 10, 20 or 50 ft. distant from the furnace, where the temperature is reasonably constant. Where greater accuracy is desired, a common method is to drive a 2-in. pipe, with a pointed closed end, some 10 to 20 ft. into the ground, as shown in Fig. 128. The compensating leads are joined to the copper leads, and the junction forced down to the bottom of the pipe. The cold junction is now in the ground, beneath the building, at a depth at which the temperature is very constant, about 70 deg.F., throughout the year. This method will usually control the cold-junction temperature within 5 deg.F. Where the greatest accuracy is desired a compensating box will overcome cold-junction errors entirely. It consists of a case enclosing a lamp and thermostat, which can be adjusted to maintain any desired temperature, from 50 to 150 deg.F. The compensating leads enter the box and copper leads run from the compensating box to the instrument, so that the cold junction is within the box. Figure 129 shows a Brown compensating box. [Illustration: FIG. 128.--Correcting cold-junction error.] If it is desired to maintain the cold junction at 100 deg.: 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 deg. is reached, when the thermostat will open the circuit and the light is extinguished. The box will now cool down to 98 deg., when the circuit is again closed, the lamp lights, the box heats up, and the operation is repeated. [Illustration: 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 deg.F. of the correct temperature--and the green light when the temperature is too high. [Illustration: 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. [Illustration: FIG. 131.--Automatic temperature control.] AN AUTOMATIC TEMPERATURE CONTROL PYROMETER