Question

A voltmeter of reading up to $1000mV$ is calibrated by a potentiometer$.$ An emf of $1.08V$ is balanced across $5.4m$ length of the wire$.$ When voltmeter reading is $0.9V,$ the balancing length is $4.55m.$ The error in the voltmeter reading is$:$

• A. -0.01mV
• B. 0.5 mV
• C. 0.25 mV
• D. No error

Answer 1

The correct option is A -0.01mV

It is given that an emf of $1.08V$ is balanced across a wire of length of $5.4$ meters.

The potential gradient =$1.08/\mathrm{5..4}V/m$

It is also given that when the voltage reading is $0.9V,$ the balancing length is $4.55$ meters.

Therefore$,$the required voltage$=108/5.4$ $\times 4.55$ $=0.91V.$

Hence, the error $=0.9-0.91=-0.01mV$

Hence,

option $\left(A\right)$ is correct answer.