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.
FIG. 114.—Automatic cold-end compensator.
PLACING THE THERMO-COUPLES
FIG. 115.—Potentiometer ready for use.
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.