question_answer

The deflection in moving coil galvanometer falls from 80 division to 40 division when a shunt of \[20\Omega \]is connected across it. Determine the galvanometer resistance.

A)  \[40\,\Omega \]                             

B)  \[30\,\Omega \]

C)  \[20\,\Omega \]                                             

D)  \[15\,\Omega \]

Correct Answer: C

Solution :

Let current sensitivity of galvanometer is CS then, \[{{I}_{1}}=\frac{{{\theta }_{1}}}{CS}=\frac{80}{CS}\]unit where \[{{I}_{1}}\]is current through galvanometer coil when it is isolated. Also \[{{I}_{2}}=\frac{{{\theta }_{2}}}{CS}=\frac{40}{CS}\]unit where \[{{I}_{2}}\] is current through galvanometer coil when its has been shunted. So,       \[{{I}_{2}}\times G=({{I}_{1}}-{{I}_{2}})\times 20\] \[\frac{(40\times G)}{CS}=\frac{(40\times 20)}{CS}\] \[\therefore \]                  \[G=20\,\Omega \]