State working principle of potentiometer. Explain how the balance point shifts when value of resistor R increases in the circuit of potentiometer, given below.

Explanation

Principle of potentiometer, when a constant current flows through a wire of uniform area of cross-section, the potential drop across any length of the wire is directly proportional to the length.
Let resistance of wire AB be `R_(1)` and its length be 'I' then current drawn from driving cell -
`1 = (V)/(R+R1) " and hence"`
P.D. across the wire AB will be
`V_(AB) = 1R_(1) = (V)/(R+R1) xx (el theta)/(a)`
Where 'a' is area of cross-section of wire AB
`therefore (VAB)/(l) = (Ve)/((R+R1)a)` = constant = k
Where R increases, current and potential difference across wire AB will be decreased and hence potential gradient 'k' will also be decreased. Thus the null point or balance point will shift to right (towards, B) side.