Question

Superiority of potentiometer over voltmeter?

Answer 1

An ordinary voltmeter cannot measure the

emf accurately because it does draw some current to show the deflection. As per definition of emf, it is the

potential difference when a cell is in open circuit or no current through the cell. Therefore voltmeter can

only measure terminal voltage of a give n cell.

Potentiometer is based on no deflection method. When the potentiometer gives zero deflection, it does

not draw any current from the cell or the circuit i.e. potentiometer is effectively an ideal instrument of

infinite resistance for measuring the potential difference.

Answer 2

The voltmeter measures the voltage across the terminals of a cell when the cell is in closed circuit, i.e. current is flowing through the cell. This voltage measured is not equal to the emf of the cell. It is equal to E - I r, where r is the internal resistance of the cell. In contrast, the voltage measured using potentiometer is the voltage across the terminals of the cell when current is not flowing through it. Because at null point there is no current through the cell. This voltage is exactly same as the emf of the cell. 


The accuracy of a potentiometer can be increased to a great extent by increasing the length of the potentiometer wire.The accuracy of a voltmeter cannot be increased beyond a certain limit. 

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