1 See voltage divider.
2 An instrument for measuring, comparing, or dividing small potential differences. A typical example of its use is the measurement of the e.m.f. (
E1
) of a cell by comparing it with the e.m.f. (E2
) of a standard cell. In this case a circuit is set up as illustrated, in which AB is a wire of uniform resistance and S is a sliding contact onto this wire. An accumulator X maintains a steady current through the wire. To measure the e.m.f. of a cell C, it is connected up as shown in the diagram and the sliding contact moved until the e.m.f. of C exactly balances the p.d. from the accumulator, as indicated by a zero reading on the galvanometer G. If the length AS is then l1
, the value of E1
is given by E1/E2 = l1/l2
, where l2
is the length AS when the standard cell is used as the cell C. 
Potentiometer