what are the advantages of potentiometer over votlmeter ?

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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A potentiometer is a versatile device. It can be used for various purposes unlike voletmeter which is only used to measure potential difference. A potentiometer measures the emf of a cell very accurately. It is based on null deflection method. While measuring it does not draw current from the source of unknown emf. Whereas a classical volmeter draws some current, so it measures potential approximately(not accurately). In other words a potentiometer measures the actual emf of cell wheareas a voltmeter measures its terminal potential difference. The sensitivity of potentiometer is also very high. It can measure potential differences much smaller than voltmeters.

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