Question

A galvanometer having a coil resistance $100\Omega$ gives a full scale deflection when a current of $1\text{mA}$ 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 $10\text{V}$?

• A. $10\text{k}\Omega$
• B. $8.9\text{k}\Omega$
• C. $7.9\text{k}\Omega$
• D. $9.9\text{k}\Omega$

Answer 1

The correct option is D $9.9\text{k}\Omega$

Given,

$\text{Resistance of galvanometer},G=100\Omega$
$\text{current},i_g=1mA$

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

$\text{Total resistance of the combination}=G+R$

$\text{According to Ohm's law,}V=i_g(G+R)$

$\therefore 10=1\times {10}^{-3}(100+R_0)$

$\Rightarrow 10000-100=9900\Omega =R_0$

$\Rightarrow R_0=9.9\text{k}\Omega$