Question

The resistance of a galvanometer is $10\Omega$. It gives full-scale deflection when 1 mA current is passed. The resistance connected in series for converting it into a voltmeter of 2.5 V will be

• A. $24.9\Omega$
• B. $249\Omega$
• C. $2490\Omega$
• D. $24900\Omega$

Answer 1

The correct option is C $2490\Omega$

Let $G$ be the resistance of a galvanometer and is equal to $10\Omega$.
$I_g$ be the current through the galvanometer for full scale deflection and is 1 mA.
Now, to convert galvanometer into an voltmeter a large resistance R should be connected in series with galvanometer.
Hence, current through the circuit is
$i_g=\frac{V}{G+R}$
${10}^{-3}=\frac{2.5}{10+R}$
$R=2490\Omega$