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potentiometer
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| (A) A potential difference is set
up along a resistance wire XY by a drive cell (Ed). The
cell under test (Et) is switched into the circuit and the
length of potentiometer wire required to zero the current through
the galvanometer (G) is determined, first under the protection of
the series resistance (R), and then without it. This process is repeated
for a standard cell (Es) of accurately known emf. The ratio
of the lengths of galvanometer wire tapped off then equals the ratio
of the emfs of the cells when no current is flowing, that is, while
non of the cell's power is expended in overcoming its internal resistance.
(B) The potentiometer (potential divider) as a voltage control device. |
Typically, a potential drop is established in a long wire by a battery, and a sliding contact used to tap a variable proportion of this drop, the lengths needed to balance the standard and unknown potentials being noted in turn. The ratio of these lengths is the ratio of the potentials. The same arrangement is also used to vary an applied voltage, for example, in the thin-carbon-film potentiometers used as volume controls in radios, etc.
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