Question

A galvanometer coil has a resistance of $10 \, \Omega$ and the ammeter shows full scale deflection for a current of $1\,mA$. The shunt resistance required to convert the galvanometer into an ammeter of range $0-100\,mA$ is about

• A. $10 \, \Omega$
• B. $1 \, \Omega$
• C. $0.1 \, \Omega$
• D. $12.48 \, \Omega$

Answer 1

Answer: (C)

$0.1 \, \Omega$

Answer 2

Let shunt resistance be $S$
Voltage across shunt is
Now, $V=\left(I-I_{g}\right) .S$
and $I_{g} \cdot R_{G}=\left(I-I_{g}\right) .S$
$\Rightarrow S=\frac{I_{g} R_{G}}{\left(I-I_{g}\right)}$
$=\frac{10^{-3} \times 10}{\left(100 \times 10^{-3}-1 \times 10^{-3}\right)}$
$=\frac{10}{99}=0.1 \,\Omega$