CBSE Class 12 · 2011 · All India · Magnetic Effects of CurrentLong Answer

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(a) State the principle and the working of a moving coil galvanometer, giving its labelled diagram.
(b) Justify the statement: increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity.
(c) Outline the necessary steps to convert a galvanometer of resistance

R_{G}

into an ammeter of a given range.

Solution

(a) A current coil in the radial field feels torque

N I A B

, balanced by

k ϕ

, so

ϕ = \frac{N A B}{k} I \propto I

. (b) Current sensitivity

= \frac{N B A}{k}

, voltage sensitivity

= \frac{N B A}{k R}

. Doubling

N

doubles the current sensitivity but roughly doubles the coil resistance

R

too, leaving

\frac{N B A}{k R}

unchanged. (c) Choose shunt

S = \frac{I _{g}}{I - I _{g}} R_{G}

for the required full-scale current

I

, connect it in parallel with the galvanometer, and connect the combination in series in the circuit.

CBSE Class 12 Board · 2011 · All India · Subjective

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CBSE Class 12 · 2011 · All India · Magnetic Effects of CurrentLong Answer

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