why is potentiometer preferred over voltmeter?

using Voltmeter u cant find emf..u wil get only terminal voltage

For operating voltmeter some current is required. So measured potential difference by using voltmeter will be less than exact voltage. But potentiometer doesn't require any extra current for running and hence gives more precise value of voltage.

Thus, potentiometer is preffered over voltmeter.

 Firstly is the difference in their principles on which they work. Potentiometer works on the principle of null deflection method whereas voltmeter works on the principle that when a current carrying coil is placed in magnetic field, it experiences torque.

Secondly, potentiometer does not any current from the cell or the circuit as the jockey branch has zero current whereas voltmeter draws some current from the circuit at the point where it has to measure the P.D.

therefore,voltmeter is not that accurate in measuring P.D. As is potentiometer

Hope it will help you

Voltmeter has a resistance and while measuring this some current is lost as heat so true voltage cant be read. While potentionmeter does not use resistance. Hence zero error.

 Potentio meter is preferred over volt meter for measurement of emf of cell  because potentio meter doesn't draw any current for itself from the primary circuit and hence gives more accurate measurement thereby acting as an ideal voltmeter. 

A potentiometer is preferred over a voltmeter due to the following reasons:

1. Potentiometer is a very sensitive instrument and measures emf of a cell very precisely

2. Potentiometer measures actual emf of a cell while voltmeter measures terminal potential difference of a cell

3. Potentiometer helps in measuring internal resistance of cell too, which is not possible in voltmeter.

Hence we can conclud ethat the actual reason behind using potentiometer is that it draws no current from cell measuring it 's emf while voltmeter can 't do so. , It has to draw some current from cell showing the cell 's terminal potential difference.

Hope you got it!!!

The voltmeter measures the voltage across the terminals of a cell when the cell is in closed circuit.(that is when current is flowing through the cell.) This voltage is in general not equal to the "emf" of the cell. In fact it is equal tp the emf MINUS the potential drop across the internal resistance of the cell. Thus voltmeter reading, V = Emf - i*r, where r is the internal resistance.

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 the emf of the cell.Further, 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 limit.