galvanmeter has a coil of resistance 200 Ω with a full scale deflection at 20 μA. The value of resistance

Question

A galvanmeter has a coil of resistance \(200 \ \Omega\) with a full scale deflection at \(20 \ \mu \mathrm{A}\). The value of resistance to be added to use it as an ammeter of range \((0-20) \mathrm\ {mA}\) is:

(1) \(0.40 \ \Omega\)

(2) \(0.20\ \Omega\)

(3) \(0.50 \ \Omega\)

(4) \(0.10 \ \Omega\)

Answer 1

Correct option is : (2) \(0.20 \ \Omega\) 

\(\mathrm{G}=200 \ \Omega\)

\(\mathrm{i}_{\mathrm{g}}=20 \ \mu \mathrm{A}\)

\(\mathrm{i}=\mathrm{i}_{\mathrm{g}}\left(\frac{\mathrm{G}}{\mathrm{S}}+1\right)\)

\(\Rightarrow 20 \times 10^{-3}=20 \times 10^{-6}\left(\frac{200}{\mathrm{~S}}+1\right)\)

\(\Rightarrow \frac{200}{\mathrm{~S}}=999\)

\(\Rightarrow \mathrm{S} \approx 0.2 \ \Omega\)