Potentiometer
The potentiometer is a null measurement device. (a. ) A voltage source connected to a long wire resistor passes a constant current I through it. (b.) An unknown EMF (labeled script Ex) is connected as shown, and the point of contact along R is adjusted until the galvanometer reads zero. The segment of wire has a resistance Rx and script Ex=IRx, where I is unaffected by the connection, since no current flows through the galvanometer. The unknown EMF is thus proportional to the resistance of the wire segment.
Source
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"OpenStax College, College Physics. October 26, 2012."
http://cnx.org/content/m42362/latest/?collection=col11406/latest
OpenStax CNX
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