Q1(j). Why a voltmeter does not give the EMF

Answer in brief

EMF is the potential difference at zero current. A voltmeter draws current, so it reads V = E − Ir and polarises the cell; a potentiometer (null method) draws no current at balance.

Key points

• EMF = potential difference of the cell when no current flows (reversible condition); only this value gives ΔG = −nFE.
• A moving-coil voltmeter draws a current I ⇒ IR drop across the internal resistance r: it reads V = E − Ir < E.
• The current also runs the cell reaction: concentration polarisation and overpotentials appear ⇒ the reading drifts down.
• Daniell cell, E = 1.10 V, r = 5 Ω: a 100 Ω meter reads 1.048 V; a 10¹² Ω meter 1.100 V.
• Correct method: Poggendorff compensation; at the null point I = 0 and Ex = Es lx/ls (Weston standard cell).