Suppose you wish to measure the emf of a battery. Consider what happens if you connect the battery directly to a standard voltmeter as shown in Figure 21.34 . (Once we note the problems with this measurement, we will examine a null measurement that improves accuracy.) As discussed before, the actual quantity measured is the terminal voltage V, which is related to the emf of the battery by V=emf−Ir, where I is the current that flows and r is the internal resistance of the battery.
The emf could be accurately calculated if r were very accurately known, but it is usually not. If the current I could be made zero, then V=emf, and so emf could be directly measured. However, standard voltmeters need a current to operate; thus, another technique is needed.
A potentiometer is a null measurement device for measuring potentials (voltages). (See Figure 21.35 .) A voltage source is connected to a resistor R, say, a long wire, and passes a constant current through it. There is a steady drop in potential (an IR) along the wire, so that a variable potential can be obtained by making contact at varying locations along the wire.