Protection tubes should be replaced when cracked, as it is important that no foreign substance is allowed to freeze in the tube, so that the enclosed junction becomes a part of a solid mass joined in electrical contact with the outside protecting tube. Wires over the furnaces must be carefully inspected from time to time, as no true reading can be had on an instrument, if insulation is burned off and short circuits result.
If the standard calibrating instrument used contains a dry battery, it should be examined from time to time to be sure it is in good condition.
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.
FIG. 110.—Simple potentiometer.
A current from the dry cell Ba is constantly flowing through the main, or so-called potentiometer circuit, ABCDGEF. The section DGE of this circuit is a slide wire, uniform in resistance throughout its length. The scale is fixed on this slide wire. The current from the cell Ba as it flows through DGE, undergoes a fall in potential, setting up a difference in voltage, that is, an electromotive force, between D and E. There will also be electromotive force between D and all other points on the slide wire. The polarity of this is in opposition to the polarity of the thermo-couple which connects into the potentiometer at D and at G. By moving G along the slide wire a point is found where the voltage between D and G in the slide wire is just equal to the voltage between D and G generated by the thermo-couple. A galvanometer in the thermo-couple circuit indicates when the balance point is reached, since at this point the galvanometer needle will stand motionless when its circuit is opened and closed.