Question

A galvanometer coil has a resistance of $15\, \Omega$ and gives full scale deflection for a current of $4\, mA$. To convert it to an ammeter of range $0$ to $6\, A$

• A. $10\, m\Omega$ resistance is to be connected in parallel to the galvanometer
• B. $10\, m\Omega$ resistance is to be connected in series with the galvanometer
• C. $0.1\,\Omega$ resistance is to be connected in parallel to the galvanometer
• D. $0.1\, \Omega$ resistance is to be connected in series with the galvanometer

Answer 1

Answer: (A)

$10\, m\Omega$ resistance is to be connected in parallel to the galvanometer

Answer 2

To convert a galvanometer into an ammeter, a shunt resistance in parallel is connected to galvanometer. Since, galvanometer $G$ and shunt $S$ are in parallel, hence $i_{g} G=\left(i-i_{g}\right) S$ $\Rightarrow S=\frac{i_{g} G}{i -i_{g}}$ $=\frac{4 \times 10^{-3} \times 15}{6-4 \times 10^{-3}}$ $=\frac{60 \times 10^{-3}}{5.996}$ $=10 \times 10^{-3} \Omega$ $=10\, m\, \Omega$