Step 1: A potentiometer wire 100 cm long is standardized with a 1.018 V standard cell balancing at l_s = 50.9 cm. An unknown cell balances at l = 72.4 cm. Find its EMF. Step 2: Potential gradient k = E_s/l_s = 1.018/50.9 = 0.0200 V/cm. Step 3: Unknown EMF E = k·l = 0.0200 × 72.4 = 1.448 V. Step 4: Equivalently, E = E_s × (l/l_s) = 1.018 × 72.4/50.9 = 1.448 V. Step 5: Sanity check: the unknown balances beyond the standard cell's point, so E > 1.018 V ✓. Result: E ≈ 1.45 V. Because the measurement is at null (no current through the cell), this is the true EMF, not the terminal voltage — the potentiometer's great advantage over a voltmeter.