Figuqre shows a potentiometer with a cell of 20 V and internal resistance of 0.04Ω maintaining a potential drop across the resistor wire AB. A standard cell which maintains a constant emf 1.02 V gives a balance point at 67.3 length of the wire. To ensure very low currents drawn from the standard cell, a very high resistance of 600 kΩ is put in series with it which is shorted close to the balance point. The standard cell is replaced by a cell of unknown emf E and the balance point found similarly turn out to be 82.3 cm length of the wire. The value of E from the above given data is found to be 1.25 volt. We wish to determine the emf of, say, a themocouple (or the order of a few millivolt.
(1) To aclieve this measurement we should replace the high resistance of 600Ω with an infinite resistance.
(2) Increase the potential gradient of the potentiometer wire by a factor of at least 100.
(3) Decrease the potential gradient of the potentiometer wire by at least a factor of 100 by connecting suitable low resistance in series with the potentiometer wire.
(4) Decrease in potential gradient of potentiometer wire by a factor of more than 100 by connecting a suitable high resistance in series with the potentiometer wire.
