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  4. A galvanometer of resistance 20 W gives a full scale deflection when a current of 0.04 A is passed through it. It is desired to convert it into an ammeter of range 20 A. The only shunt available is 0.05 W. The resistance that must be connected in series with the coil of the galvanometer is

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Q. A galvanometer of resistance $20 W$ gives a full scale deflection when a current of $0.04 A$ is passed through it. It is desired to convert it into an ammeter of range $20 A$. The only shunt available is $0.05 W$. The resistance that must be connected in series with the coil of the galvanometer is

Moving Charges and Magnetism

Solution:

Let $R$ be resistance connected in series with the galvanometer.
From figure
image
$\frac{I_{g}}{I-I_{g}}=\frac{S}{G+R} $
$R=S\left(\frac{I}{I_{g}}-1\right)-G$
Substituting the given values, we get
$R=0.05\left(\frac{20}{0.04}-1\right)-20=4.95 \Omega$