Figure shows a potentiometer with a cell of emf 2.0 V and internal resistance 0.04 Ω Ω maintaining a potential drop across the potentiometer wire AB. A standard cell which maintains a constant emf of 1.02 V (for very moderate currents up to a few ampere) gives a balance point of 67.3 cm length of the wire. To ensure very low currents drawn from the standard cell, a very high resistance of 600 k Ω Ω is put in series with it which is shorted close to the balance point. The standard cell is then replaced by a cell of unknown emf E and the balance point found similarly turns out to be at 82.3 cm length of the wire.
Text Solution
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Sol. 
= 1.25 V
The high resistance to keep the initial current low when null point is being located. This saves the standard cell from damage.
This high resistance does not affect the balance point because then there is no flow of current through the standard cell branch.
The internal resistance of driver cell affects the current through the potentiometer wire. Since potential gradient is changed, therefore, the balance point must be affected.
(e) No, it is necessary that the emf of the driver cell is more than the emf of the cells.
(f) This circuit will not work well for measurement of small emf (mV) because the balance point will be very near to end A, and percentage error in EMF measured due to length measurement would be very large E = 
λ
= 
will be large if λ is very small.
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