• $summary_{type}$: application
• $word_{count}$: 160

The potentiometer uses a uniform wire carrying steady current. Potential drops linearly: V(l) = kl, where k = Irho/A (potential gradient). Comparing EMFs: $E_1$/$E_2$ = $\frac{l_1}{l_2}$ (null lengths). Key advantage: at null, no current flows through the test cell — measures true EMF (voltmeter always draws current, reading V = E - Ir < E). Internal resistance measurement: $l_1$ (open circuit null length), $l_2$ (with external R across cell). E/V = $\frac{l_1}{l_2}$, and V = $\frac{ER}{R+r}$, so r = R($l_1$/$l_2$ - 1). Sensitivity increases with: longer wire, higher driver EMF, lower wire resistance per unit length. The driver cell EMF must exceed the test cell EMF (otherwise null point is beyond the wire). JEE commonly tests: calculating r from given data, explaining why the potentiometer is more accurate than a voltmeter, and finding the potential gradient.