A galvanometer having a coil resistance


A galvanometer having a coil resistance $100 \Omega$ gives a full scale deflection when a current of $1 \mathrm{~mA}$ is passed through it. What is the value of the resistance which can convert this galvanometer into a voltmeter giving full scale deflection for a potential difference of $10 \mathrm{~V}$ ?

  1. (1) $10 \mathrm{k} \Omega$

  2. (2) $8.9 \mathrm{k} \Omega$

  3. (3) $7.9 \mathrm{k} \Omega$

  4. (4) $9.9 \mathrm{k} \Omega$

Correct Option: 4,




Resistance of galvanometer, $G=100 \Omega$

Current, $i_{\mathrm{g}}=1 \mathrm{~mA}$

A galvanometer can be converted into voltmeter by connecting a large resistance $R$ in series with it.

Total resistance of the combination $=G+R$

According to Ohm's law, $V=i_{\mathrm{g}}(G+R)$

$\therefore 10=1 \times 10^{-3}\left(100+R_{0}\right)$

$\Rightarrow 10000-100=9900 \Omega=\mathrm{R}_{0}$

$\Rightarrow \mathrm{R}_{0}=9.9 \mathrm{k} \Omega$

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