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_ 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.]