The basic voltage comparisons can thus be made by first establishing
an accurately known voltage drop across a portion of the calibrated
the ratio of the calibrated resistances.
Referring to Figure 1-2, the calibrated resistance is represented by
the wire, (AB), shown with a scale along its length. This wire has uniform
resistance per unit of length and is equipped with a contacting slider, (C).
In operation, the rheostat is first adjusted to provide a constant current
in (AB) from the working battery. With the standard cell Es switched into
the circuit, let r be the resistance from (A) to (C) when the key is closed
and the galvanometer deflection is zero. Let r2 be the resistance to the
slider with the unknown EMF Ex in the circuit, the galvanometer deflection
at zero and the current in (AB) the same as before. For these conditions,
the current in the slide wire will be:
The potentiometer can be made direct-reading by dividing the slide
wire into, say, 200 divisions and adjusting the rheostat for 2.00 volts
across (AB). This can be done by the use of the standard cell. If the cell
has an EMF of 1.018 volts, the slider can be set at 101.8 divisions (cm)
from the scale zero (at point A) and the rheostat adjusted to obtain zero
galvanometer deflection when (K) is closed.
Now position 101.8 cm on the
slider scale represents 1.018 volts, and the value of any unknown EMF within
the range of the instrument can be read at balance directly from the scale
by multiplying the scale reading by .01. For example, 151.8 cm times .01
equals 1.518 volts.
Commercial potentiometers incorporate slide wire arrangements which
are usually in the form of single or multiturn rotating drums that afford a
long length of slide wire. To allow for greater accuracy of readout without
the necessity for an unduly long slide wire, the drum resistor is usually in
series with a multistep resistor, which can readily be set with a rotary
dial. The precision potentiometer, (fig 1-3), incorporates these features
in a single-range device that enables accurate readings to be made from 0 to