Magnetics 6 Question 3
3. A moving coil galvanometer allows a full scale current of $10^{-4} A$. A series resistance of $2 M \Omega$ is required to convert the above galvanometer into a voltmeter of range $0-5 V$. Therefore, the value of shunt resistance required to convert the above galvanometer into an ammeter of range $0.10 mA$ is
(2019 Main, 10 April I)
(a) $100 \Omega$
(b) $500 \Omega$
(c) $200 \Omega$
(d) $10 \Omega$
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Solution:
- Given data,
$$ \begin{aligned} I & =10^{-4} A, \\ R _S & =2 m \Omega=2 \times 10^{6} \Omega \\ V _{\max } & =5 V \end{aligned} $$
Let internal resistance of galvanometer is $R _G$.
$$ \begin{array}{lc} \text { Then, } & I \times R _S+I \times R _G=V _{\max } \\ \Rightarrow & 2 \times 10^{6} \times 10^{-4}+10^{-4} \times R _G=5 \\ \Rightarrow & 10^{-4} R _G=5-200=-195 \\ \text { or } & R _G=-195 \times 10^{4} \Omega \end{array} $$
Resistance cannot be negative.
$\therefore$ No option is correct
