st 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

Automatic temperature control instruments are similar to the Brown
indicating high resistance pyrometer with the exception that the
pointer is depressed at intervals of every 10 sec. upon contact-making
devices. No current passes through the pointer which simply depresses
the upper contact device tipped with platinum, which in turn comes
in contact with the lower contact device, platinum-tipped, and the
circuit is completed through these two contac