_ is the constant resistance coil _M_, on the other the nickel
coil _N_. _N_ is placed at or near the cold end of the thermo-couple
(or couples). Nickel has a high temperature coefficient and the
electrical proportions of _M_ and _N_ are such that the resistance
change of _N_, as it varies with the temperature of the cold end,
has the same effect upon the balancing voltage between _D_ and
_G_ that the movement of the point _D_, Fig. 114, has in the
hand-operated compensator.

Instruments embodying these principles are shown in Figs. 115 to
117. The captions making their uses clear.

[Illustration: FIG. 114.--Automatic cold-end compensator.]

PLACING THE THERMO-COUPLES

The following illustrations from the Taylor Instrument Company
show different applications of the thermo-couples to furnaces of
various kinds. Figure 118 shows an oil-fired furnace with a simple
vertical installation. Figure 119 shows a method of imbedding the
thermo-couple in the floor of a furnace so as to require no space
in the heating chamber.

[Illustration: FIG. 115.--Potentiometer ready for use.]

Various methods of applying a pyrometer to common heat-treatment
furnaces are shown in Figs. 120 to 122.

[Illustration: FIG. 116.--Eight-point recording pyrometer-Carpenter
Steel Co.]

LEEDS AND NORTHRUP OPTICAL PYROMETER

The principles of this very popular method of measuring temperature
are sketched in Fig. 123.

[Illustration: FIG. 117.--Multiple-point thermocouple
recorder--Bethlehem Steel Co.]

[Illustration: FIG. 118.--Tycos pyrometer in oil-fired furnace.]

The instrument is light and portable, and can be sighted as easily
as an opera glass. The telescope, which is held in the hand, weighs
only 25 oz.; and the case containing the battery, rheostat and
milliammeter, which is slung from the shoulder, only 10 lb.

[Illustration: FIG. 119.--Thermocouple in floor of furnace.]

[Illustration: FIG. 120.--Pyrometer in gas furnace.]

A large surface to sight at is not required. So long as the image
formed by the objective is broader than the lamp filament, the
temperature can be measured accurately.

[Illustration: FIG. 121.--Tycos multiple indicating pyrometer and
recorder.]

[Illustration: FIG. 122.--Pyrometer in galvanizing tank.]

Distance does not matter, as the brightness of the image formed
by the lens is practically constant, regardless of the distance
of the instrument from the hot object.

[Illustration: FIG. 123.--Lee