Question

A galvanometer gives full scale deflection with $0.006\text{A}$ current. By connecting it to a $4990\Omega$ resistance, it can be converted into a voltmeter of range $0-30\text{V}$. If connected to a $\frac{2n}{249}\Omega$ resistance, it becomes an ammeter of range $0-1.5\text{A}$. The value of $n$ is

Answer 1

$i_g=0.006\text{Amp}$
From voltmeter circuit, we get
$V=I{R}_{eq}$
$\Rightarrow 30=\frac{6}{1000}[4990+R_g]$
$\Rightarrow 4990+R_g=5000$
$\Rightarrow R_g=10\Omega$
Now, from the ammeter circuit,
Let resistance be $S=\frac{2n}{249}\Omega$
Therefore, $i_g{R}_g=i_{S}S$
$\Rightarrow \frac{S}{10}=\frac{0.006}{1.494}=\frac{6}{1494}$
$\Rightarrow S=\frac{60}{1494}=\frac{10}{249}\Omega$
$\Rightarrow n=5$