Question

When a galvanometer is shunted by $5 \Omega$ resistance, the deflection is reduced to $1 / 6^{th}$ of the original value. If galvanometer is further shunted with a resistance of $1 \Omega$ the deflection (as measured from original value) will be

• A. reduced by $\frac{1}{20}$
• B. reduced by $\frac{1}{30}$
• C. reduced by $\frac{1}{31}$
• D. reduced by $\frac{6}{31}$

Answer 1

Answer: (C)

When deflection is reduced 'by 1 / n, it means range of the ammeter becomes $n$ times the full scale deflection current of galvanometer.
$\Rightarrow 5 I_{g} \times 5=I_{g} \times G$
$ \Rightarrow G=25 \,\Omega$
If further $1\, \Omega$ resistance is connected then the situation would be as shown in adjacent figure.

$\Rightarrow I' \times 1 =25\,I_{g}$
$\Rightarrow I' =25I_{g}$
$\therefore I=I_{g}[1+5+25]=31 I_{g}$
So, deflection gets reduced to $ \frac{1}{31} $ of its initial value