Question

A galvanometer with its coil resistance $25 \, \Omega$ requires a current of $1\, mA$ for its full deflection. In order to construct an ammeter to read up to a current of $2 \,A$, the approximate value of the shunt resistance should be :

• A. $2.5 \times 10{-3} \Omega$
• B. $1.25 \times 10{-2} \Omega$
• C. $1.25 \times 10{-3} \Omega$
• D. $2.5 \times 10{-2} \Omega$

Answer 1

Answer: (B)

Given: $G=25\, \Omega, I_{ g }=1\, mA ,\, I_{ m }=2\, A$
So, divisions on full deflection is
$n=\frac{I_{ m }}{I_{ g }}=\frac{2}{0.001}=2000$
We know that, shunt resistance is given by $S=\frac{G}{n-1}$
Therefore, $S=\frac{25}{2000-1} S=\frac{25}{1999}=1.25 \times 10^{-2}\, \Omega$