as been standardized.

[Illustration: FIG. 112.--Hand adjusted cold-end compensator.]

DEVELOPMENT OF THE WIRING SCHEME OF THE COLD-END COMPENSATOR.--The
net voltage generated by a thermo-couple depends upon the temperature
of the hot end and the temperature of the cold end. Therefore, any
method adopted for reading temperature by means of thermo-couples
must in some way provide a means of correcting for the temperature of
the cold end. The potentiometer may have either of two very simple
devices for this purpose. In one form the operator is required
to set a small index to a point on a scale corresponding to the
known cold junction temperature. In the other form an even more
simple automatic compensator is employed. The principle of each is
described in the succeeding paragraphs, in which the assumption is
made that the reader already understands the potentiometer principle
as described above.

As previously explained the voltage of the thermo-couple is measured
by balancing it against the voltage drop _DG_ in the potentiometer.

As shown in Fig. 111, the magnitude of the balancing voltage is
controlled by the position of _G_. Make _D_ movable as shown in Fig.
112 and the magnitude of the voltage _DG_ may be varied either from
the point _D_ or the point _G_. This gives a means of compensating
for cold end changes by setting the slider _D_. As the cold end
temperature rises the net voltage generated by the couple decreases,
assuming the hot end temperature to be constant. To balance this
decreased voltage the slider _D_ is moved along its scale to a new
point nearer _G_. In other words, the slider _D_ is moved along
its scale until it corresponds to the known temperature of the cold
end and then the potentiometer is balanced by moving the slider
_G_. The readings of _G_ will then be direct.

[Illustration: FIG. 113.--Another type of compensator.]

The same results will be obtained if a slide wire upon which _D_
bears is in parallel with the slide wire of _G_, as shown in Fig.
113.

AUTOMATIC COMPENSATOR.--It should be noted that the effect of moving
the contact _D_, Fig. 113, is to vary the ratio of the resistances
on the two sides of the point _D_ in the secondary slide wire. In
the recording pyrometers, an automatic compensator is employed.
This automatic compensator varies the ratio on the two sides of
the point _D_ in the following manner:

The point _D_, Fig. 114, is mechanically fixed; on one side of
_D