question_answer

A galvanometer of resistance \[100\,\Omega \] gives full scale deflection for 20 mV. Find the resistance to be attached so that it gives full scale deflection of 5 V:

A)  \[24.9\times {{10}^{3}}\,\Omega \] in series

B)  \[24.9\times {{10}^{3}}\,\Omega \] in parallel

C)  \[44.9\times {{10}^{3}}\,\Omega \] in series

D)  \[44.9\times {{10}^{3}}\,\Omega \] in parallel

Correct Answer: B

Solution :

For full scale deflection for 20 mV. \[i=\frac{20mV}{100\,\Omega }=0.2\,mA\] Again for full scale deflector of 5 V. \[(R+100)\,i=5\,V\] or            \[R+100=\frac{5}{i}=\frac{5\,V}{0.2\,mA}\] \[=25\times {{10}^{3}}\Omega \] \[R=2500-100\] \[24900=24.9\times {{10}^{3}}\,\Omega \]