Question

Under what circumstances would an ordinary voltmeter give a accurate reading of call emf (what would have to be true about the cell)?

Answer 1

Potential difference across the terminals of voltmeter placed across a cell:

V = E - I r

where

E = EMF of the cell

I = Current passing through the cell

r = Internal resistance of the cell

Under open circuit conditions,

current through the cell, I = 0

Then,

V = E - 0 * r

   = E

Hence, it can be concluded that in the circumstances of open circuit, an ordinary voltmenter gives accurate reading of emf of the cell.