(i) State the principle of working of a potentiometer.
(ii) In the following potentiometer circuit AB is a uniform wire of length 1 m and resistance `10Omega`
Calculate the potential gradient along the wire and balance length AO (=l)

Explanation

(i) When a constant current flows through a wire of uniform cross section the fall of potential is directly proportional to the length of wire .
(ii) Total resistance of primary circuit =15 + 10 =`25Omega`
Current in wire `AB,I= 2//25 =.08A`
P.D . across wire =.08 `xx` 10 = 0.8V
Potertial gradient `=0.8//100 = 0.008 V cm^(-1)`
Resistance of secondary circuit `=1.2 + 0.3 = 1.5 Omega`
Current I = `1.5 //1.5 = 1A`
Current in 0.3 `Omega` resistor = 1A
P.D. across `0.3 Omega` resistor = 0.3 V
Length `AO= (0.3V)/(0.008) = 37 .5 `cm