Explanation
In general, ammeter always reads less than the actual value because of its resistance.
In balance condition, no current will flow through the branch containing S.
The resistance of an ideal voltmeter is considered as infinite.
Due to the negligible temperature co-efficient of resistance of constantan wire, there is no change in it’s resistance value with change in temperature.
The resistance of voltmeter is too high, so that is draws negligible current from the circuit. hence potential drop in the external circuit is also negligible.
Reading of galvanometer remains same whether switch S is open or closed, hence no current will flow through the switch i. e., Rand C will be in series and same current will flow through them. IR = IG.
Pressing the key docs not disturb current in all resistance as the bridge is balanced. Therefore, deflection in the galvanometer in whatever direction it was, will stay.
When ammeter is connected in parallel to the circuit, net resistance of the circuit decreases. Hence more current is drawn from the battery, which damages the ammeter.
After connecting a resistance R is parallel with voltmeter its effective resistance decreases. Hence less voltage appears across it i. e., V will decrease. Since overall resistance decreases so more current will flow i.e., A will increase.
Resistance of voltmeters should be high.
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