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.]

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_ h