(a) For the circuit shown in the figure, how would the balancing length be affected, if (i) `R_1` is decreased, (ii) `R_2` is increased, the other factors remaining the same in the circuit ? Justify your answer in each case.
(b) Why is a potentiometer preferred over a voltmeter ? Give reason.

Explanation

(a) (i) When `R_1` is decreased, the current flowing in the potentiometer wire AB and consequently the potential gradient along the potentiometer wire increases. As a result, the balancing length decreases i.e., the balancing point shifts towards left (towards point A).
(ii) When `R_2` is increased, the current flowing through `R_2` decreases and as a result the potential difference across the terminals of the cell, whose internal resistance is to be determined, increases `[V = epsi- Ir]` . As a result, the balancing length increases, i.e., the balancing point shifts towards right (towards the point B).
(b) A potentiometer is preferred over a voltmeter for measuring potential differences and emf of cells because measurement is done here in balance condition, when no current is being drawn from the given cell.