THE LEEDS AND NORTHRUP POTENTIOMETER SYSTEM The potentiometer pyrometer system is both flexible and substantial in that it is not affected by the jar and vibration of the factory or the forge shop. Large or small couples, long or short leads can be used without adjustment. The recording instrument may be placed where it is most convenient, without regard to the distance from the furnace. ITS FUNDAMENTAL PRINCIPLE.--The potentiometer is the electrical equivalent of the chemical balance, or balance arm scales. Measurements are made with balance scales by varying known weights until they equal the unknown weight. When the two are equal the scales stand at zero, that is, in the position which they occupy when there is no weight on either pan; the scales are then said to be balanced. Measurements are made with the potentiometer by varying a known electromotive force until it equals the unknown; when the two are equal the index of the potentiometer, the galvanometer needle, stands motionless as it is alternately connected and disconnected. The variable known weights are units separate from the scales, but the potentiometer provides its own variable known electromotive force. The potentiometer provides, first, a means of securing a known variable electromotive force and, second, suitable electrical connections for bringing that electromotive force to a point where it may be balanced against the unknown electromotive force of the couple. The two are connected with opposite polarity, or so that the two e.m.f.s oppose one another. So long as one is stronger than the other a current will flow through the couple; when the two are equal no current will flow. Figure 107 shows the wiring of the potentiometer in its simplest form. The thermo-couple is at _H_, with its polarity as shown by the symbols + and -. It is connected with the main circuit of the potentiometer at the fixed point _D_ and the point _G_. [Illustration: FIG. 110.--Simple potentiometer.]