Discussion
A cell's measured voltage falls as soon as current is drawn from it. The explanation is
Answer: D
Correct answer: polarisation and the internal resistance drop
Drawing current makes the cell irreversible: products accumulate at the electrodes and oppose the reaction, and the current through the internal resistance produces a drop that comes off the terminal voltage. The terminal voltage is therefore \( E - Ir \) less the polarisation, and it approaches the true EMF only as the current approaches zero. This is exactly why a potentiometer is used for accurate EMF measurement.
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