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.
$\Rightarrow 5I_g\times 5=I_g\times G$
$\Rightarrow G=25\Omega$
If further $1\Omega$ resistance is connected then the situation would be as shown in adjacent figure.

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