A galvanometer having a resistance of $20 \Omega$ and 30 division on both sides has figure of merit 0.005…
A galvanometer having a resistance of $20 \Omega$ and 30 division on both sides has figure of merit 0.005 ampere/ division. The resistance that should be connected in series such that it can be used as a voltmeter upto 15 volt, is:
$100 \Omega$
$120 \Omega$
$80 \Omega$
$125 \Omega$
Solution
Deflection current
$=\operatorname{Ig}_{\max }=n x k=0.005 \times 30$
Where, $\mathrm{n}=$ Number of divisions $=30$ and $\mathrm{k}=0.005 \mathrm{amp} /$
division
$\begin{array}{l}
=15 \times 10^{-2}=0.15 \\
\mathrm{v}=\mathrm{I}_{\mathrm{g}}[20+\mathrm{R}] \\
15=0.15[20+\mathrm{R}] \\
100=20+\mathrm{R} \\
\mathrm{R}=80 \Omega
\end{array}$