Figure shows the circuit diagram of a potentiometer for determining the emf E of a Cell of negligible internal resistance.
(i)What is the purpose of using high resistance R2?
(ii) How does the position of balance point (J) change when the resistance R1 is decreased?
(iii) Why cannot the balance point be obtained
(1) when the emf E is greater than 2V and
(2) when key (K) is closed?

The purpose of high resistance R₂  is to reduce the current through the galvanometer. When jockey is far from balance point, this saves the galvanometer and the cell (of emf ε) from being damaged. 

(ii) When resistance R₁  is decreased, the potential gradient of potentiometer wire increases, so balance point (J) shifts to longer length of wire. 

(iii) (1) The balance point is not obtained because maximum emf across potentiometer wire is 2V. 

(2) When key (K) is closed, the terminal potential difference of cell is zero; so balance point cannot be between Aand B. (Since V = kl => l = 0 for V = 0)