The circuit shows in Fig . `6.33` shows the use of potentiometer to measure the internal resistance of a cell.
(a) When the key is open, how does the balance point change, if the driver cell decreases ?
(b) When the key is closed, how does the balance point change, if `R` is increased, keeping the current from the driver cell constant ?

Answer

a. Balance point will shift towards right. (b) Balance point will shift towards right.

Explanation

If current from driver cell decreases, then potential gradient decreases. So the balancing length should increase. Hence, balanced point shifs toward right.
b. If `R` is increased, current in the lower circuit decreases, due to which terminal potential difference of battery increases. To balance increased terminal potential difference, balancing length should increase, hence balance point shifts toward right.