Explanation
Current through R is maximum when total internal resistance of the circuit is equal to external resistance.
By kirchhoff's current law
For power to be maximum
External resistance = Equivalent internal resistance of the circuit
In the given case cell is in open circuit (i = 0) so voltage across the cell is equal to its e.m.f.
Maximum current will be drawn from the circuit if resultant resistance of all internal resistances is equal to the value of external resistance if the arrangements mixed. In series R>> nr and in parallel, the external resistance is negligible.
To convert a galvanometer into an ammeter a low value resistance called shunt is to be connected in parallel to it.
Ammeter is used to measure that current through the circuit.
To convert a galvanometer into a voltmeter, a high value resistance is to be connected in series with it.
In balance condition, no current will flow through the branch containing S.
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