Question

# Full scale deflection current for a galvanometer is $$1mA$$. The value of shunt resistance (approximately) so that galvanometer shows half scale deflection is

A
1.5 mΩ
B
3 mΩ
C
10 mΩ
D
15 mΩ

Solution

## The correct option is A $$1.5$$ m$$\Omega$$In the given part of circuit,the full scale deflection current through galvanometer is$$I_{fsd} = 10^{-3} A$$When shunt resistance is introduced in the circuit galvanometer shows half scale deflection current$$I_{hsd} = \dfrac {1}{2} 10^{-3} A$$Therefore,$$\dfrac{1}{2}10^{-3}(30)=S[10-\dfrac{1}{2}10^{-3}]$$$$S = \dfrac {30\times 10^{-3}}{20 - 10^{-3}}$$Hence,$$S=\dfrac{30}{20000-1}\cong \dfrac{30}{20000}\cong1.5$$m$$\Omega .$$Physics

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