Question

To increase the current sensitivity of a moving coil galvanometer by 50% ,its resistance is increased so that the new resistance becomes twice its initial resistance. By what factor does the voltage sensitivity change ?

• A. decreased by 75%
• B. Increased by 75%
• C. decreased by 25%
• D. Increased by 25%

Answer 1

Answer: (C)

decreased by 25%

Answer 2

Let CS and VS be the original current sensitivity and voltage sensitivity of MCG Changed current sensitivity $Cs'=Cs +\frac{50}{100}Cs =\frac{3}{2}CS$ since $\,Vs =\frac{Cs}{R}$ changed voltage sensitivity, i.e., $VS'=\frac{Cs'}{2R}=\frac{(3/2)CS}{2R}$ $=\frac{3}{4}\left(\frac{CS}{R}\right)=0.75 VS =75$ Thus, voltage sensitivity decreases by 25%