Superiority of potentiometer over voltmeter?
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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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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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