Question

In the following circuit if key $K$ is pressed then the galvanometer reading becomes half. The resistance of galvanometer is

• A. $20\Omega$
• B. $30\Omega$
• C. $40\Omega$
• D. $50\Omega$

Answer 1

The correct option is C $40\Omega$

Consider the circuit shown in figure. Let $I$ is the initial current through circuit and $R$ is the resistance of galvanometer.
When key $K$ is pressed the shunt will be connected to the galvanometer in parallel.
Since, shunt and galvanometer are in parallel, voltage across them is same.
Hence,
$\frac{I}{2}R=\frac{I}{2}40$ ................................$($since$,V=IR)$
$\Rightarrow R=40\Omega$