The circuit diagram of a potentiometer for determining the emf `epsilon` of a cell of negligible internal reistance.
(i) What is the purpose of using high resistance `R_(2)`? (ii) How does the position of balance point (J) change when the resistance `R_(1)` is increased ? (iii) Why cannot the point be obtained, (a) When the emf `epsilon` is greater than 2V, and (b) When the key K is closed.

Explanation

(i) The purpose of high resistance `R_(2)` is to protect the galvanometer by decreasing the current through it for positons, which are far away from the balance point.
(ii) When resistance `R_(1)` is increased, the current through the potentiometer wire circuit decreases. Due to it, the potential gradient across potentiometer wire decreases. As a result of it, the balance point shifts towards the end B.
(iii) (a) When emf `epsilon` is greater than 2 V, the balance point cannot be obtained on potentiometer wire because the potential drop across the potentiometer wire will be less than 2 V.
(b) When the key K is closed, the cell emf `epsilon` gets short circuited. Due to it, the balance point is not obtained on the potentiometer wire.