Question

The deflection in a galvanometer, decreases to 1/4 th of the original deflection when it is shunted by 15 ohm. The galvanometer resistance is---

• A. 40 ohm.
• B. 45 ohm..
• C. 60 ohm.
• D. 75 ohm..

Answer 1

The correct option is B 45 ohm..

Given: S=Shunt resistance=15$\Omega$ ,

Initial difflection =$\theta$ Final difflection =$\frac{1}{4}\theta$

Solution :

Difflection $\theta \propto I$

$\frac{I_g}{I}=\frac{1}{4}$ this meanse $I_g=\frac{1}{4}$of total current I

Both Resistors are in parellalso both voltage will be same

applying ohm's law we get

iG=iS

$\frac{IG}{4}=\frac{3IS}{4}$

$\frac{G}{4}=\frac{3\times 15\Omega }{4}$

$G=45\Omega$

The correct Opt=B