The figure shows a potentiometer using a cell `E` of emf `2.0 V` and internal resistance `0.40 Omega` connected to a resistor wire `AB`. A standard cell of constant emf of `1.02 V` gives a balance point a t `67.3 cm` length of the wire. A very high resistance `R = 100 k Omega` is put in with the standard cell. This resistance is shorted by inserting switch `S` when close to the balance point. The standard cell is then replaced by a cell of unknown emf `E` and the null point turns out to be `82.3 cm` length of the wire.
(a) What is the value of `E` ?
(b) What is the purpose of using the high resistance `R` ?
( c) Is the null point affected by this high resistance ?
(d) Is the null point affected by the internal resistance of the cell `E` ?
( e) Would this method work if
(i) the internal resistance of cell `E` were higher than the resistance of wire `AB` and
(ii) the emf of cell `E` were `1.0 V` instead of `2.0 V` ?

Answer

(a) 1.25 V,
(b) saving of glvanometer from damage and to prevant the cell discharging fast
( c) No, (d) Yes, ( e) No, (f) No