Question

why is the Potentiometer preferred to a voltmeter for measuring EMF of a cell?​

Answer 1

Answer:

The electromotive force (emf) of a cell is its terminal voltage when no current is flowing through it. The terminal voltage of a cell is the potential difference between its electrodes. A voltmeter cannot be used to measure the emf of a cell because a voltmeter draws some current from the cell.

For this reason potentiometer is used to measure EMF of a cell.