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. [Illustration: FIG. 111.--Standard cell potentiometer.] The voltage in the slide wire will vary with the current flowing through it from the cell _Ba_ and a means of standardizing this is provided. _SC_, Fig. 111, is a cadmium cell whose voltage is constant. It is connected at two points _C_ and _D_ to the potentiometer circuit whenever the potentiometer current is to be standardized. At this time the galvanometer is thrown in series with _SC_. The variable rheostat _R_ is then adjusted until the current flowing is such that as it flows through the standard resistance _CD_, the fall in potential between _C_ and _D_ is just equal to the voltage of the standard cell _SC_. At this time the galvanometer will indicate a balance in the same way as when it was used with a thermo-couple. By this operation the current in the slide wire _DGE_ has been standardized. [Illustration: FIG. 112.--Hand adjusted cold-end compensator.] DEVELOPMENT OF THE WIRING SCHEME OF THE COLD-END COMPENSATOR.--The net voltage generated by a thermo-couple depends upon the temperature of the hot end and the temperature of the cold end. Therefore, any method adopted for reading temperature by means of thermo-couples must in some way provide a means of correcting for the temperature of the cold end. The potentiometer may have either of two very simple devices for this purpose. In one form the operator is required to set a small index to a point on a scale corresponding to the known cold junction temperature. In the other form an even more simple automatic compensator is employed. The principle of each is described in the succeeding paragraphs, in which the assumption is made that the reader already understands the potentiometer principle as described above. As previously explained the voltage of the thermo-couple is measured by balancing it against the voltage drop _DG_ in the potentiometer.