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The potentiometer is the most accurate device for measuring EMF because it draws zero current from the cell at balance. A uniform wire (length $L$, total resistance $R_w$) carries a steady current from a driver battery. The potential drops linearly along the wire: potential gradient $k = V_w/L$ (V/m).

For measuring EMF: connect the cell across the wire and find the balance length $l$ where the galvanometer reads zero. Then $\varepsilon = kl$. For comparing two EMFs: $\varepsilon_1/\varepsilon_2 = l_1/l_2$ (same gradient cancels). For internal resistance: measure balance length $l_1$ (open circuit) and $l_2$ (with resistance $R$ across the cell). Then $r = R(l_1 - l_2)/l_2$.

Key requirements: (1) The driver cell EMF must exceed the EMF being measured (otherwise no balance point exists). (2) The wire must be uniform (constant cross-section). (3) The driver current must remain constant during measurement (use a rheostat for adjustment).

Sensitivity increases with: lower potential gradient (longer wire or higher wire resistance), more sensitive galvanometer, and longer wire length. The potentiometer is superior to a voltmeter because it measures true EMF, not terminal voltage.