A galvanometer having a resistance of $20\,\Omega $ and $30\, divisions$ 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 ........... $\Omega$
JEE MAIN 2019, Diffcult
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$\mathrm{R}_{9}=20 \,\Omega$
$\mathrm{N}_{\mathrm{L}}=\mathrm{N}_{\mathrm{g}}=\mathrm{N}=30$
$\mathrm{FOM}=\frac{1}{\phi}=0.005 \,\mathrm{A/Div}$
Current sentivity $=\mathrm{CS}=\left(\frac{1}{0.005}\right)=\frac{\phi}{1}$
$I{g_{\max }} = 0.005 \times 30$
$=15 \times 10^{-2}=0.15$
$15=0.15[20+\mathrm{R}]$
$100=20+R$
$R=80$.
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