Explanation
Because cell is open circuit.
In meter bridge experiment, it is assumed that the resistance of the L shaped plate is negligible, but actually it is not so. The error created due to this is called end error. The remove this the resistance box and the unknown resistance must be interchanged and then the mean reading must be taken.
A potentiometer does not draw any current from the cells whose e.m.f is to he measured, whereas a voltmeter always draws some current. Hence, we can measure the e.m.f of a cell accurately by using a potentiometer.
Ammeter is always connected series with circuit.
The e.m.f of the standard cell must be greater than that of experimental cells, otherwise balance point is not obtained.
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