• Tags: potentiometer, EMF, internal-resistance
• Difficulty: Moderate

A potentiometer is a long uniform wire carrying a steady current from a driver cell. The potential drops linearly along its length: V(l) = (E_driverR_wire/(R_wire + r_driver)) * (l/L). Comparing EMFs: connect two cells alternately and find null points l_1 and l_2. Then E_1/E_2 = l_1/l_2. This is exact because at null, no current flows through the test cell — it measures true EMF (unlike a voltmeter). Finding internal resistance: measure null length l_1 (cell in open circuit). Then connect resistance R across the cell and find new null l_2. V = E - Ir = ER/(R+r). So E/V = l_1/l_2, giving r = R*(l_1/l_2 - 1). Precautions: EMF of driver cell must exceed test cell EMF; wire must be uniform; avoid heating (keep currents low); clean jockey contacts. Sensitivity increases with wire length and driver cell voltage.