Explanation
Pressing the key does not disturb current in all resistances as the bridge is balanced. Therefore, deflection in the galvanometer in whatever direction it was, will stay.
If the voltmeter is ideal then given circuit is an open circuit, so reading of voltmeter is equal to the e.m.f. of cell i.e., 6V.
If ammeter is used in place of voltmeter (i. e., in parallel) it may damage due to large current in circuit. Hence to control this large amount of current a high resistance must be connected in series.
Manganin or constantan are used for making the potentiometer wire.
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