Meter bridge: A 1-meter uniform wire replaces two arms of the Wheatstone bridge. Unknown resistance X = R * l/(100-l), where l is the balancing length. End corrections may apply if junctions have resistance. Most accurate when l is near 50 cm (balanced sensitivity).

Potentiometer: A long uniform wire carrying steady current. Potential is proportional to length: V = kl where k = V_0/L is the potential gradient. Comparing EMFs: epsilon_1/epsilon_2 = l_1/l_2 (null method — no current drawn from the cell). Internal resistance: r = R(l_1/l_2 - 1), where R is external resistance across the cell, l_1 is open-circuit balance, l_2 is with R connected. Advantage over voltmeter: measures true EMF (not terminal voltage), infinite effective resistance.