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  4. A moving coil galvanometer has 150 equal divisions. Its current sensitivity is 10 divisions per milliampere and voltage sensitivity is 2 divisions per millivolt. In order that each division reads 1 V, the resistance in Ohm’s needed to be connected in series with the coil will be :

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Q. A moving coil galvanometer has $150$ equal divisions. Its current sensitivity is $10$ divisions per milliampere and voltage sensitivity is $2$ divisions per millivolt. In order that each division reads $1 V$, the resistance in Ohm’s needed to be connected in series with the coil will be :

AIEEEAIEEE 2005Moving Charges and Magnetism

Solution:

Full scale deflection current
$=\frac{150}{10}mA=15\,mA$
Full scale deflection voltage
$=\frac{150}{2} \,mV=72\,mV$
Galvanometer resistance $G=\frac{75mV}{15mA}=5\Omega$
Required full scale deflection voltage,
$V = 1\times150 = 150\times V$
Let resistance to be connected in series is $R.$
$\Rightarrow V=I_{g}\left(R+G\right)$
$\therefore 150=15\times10^{-3}\left(R+5\right)$
$\Rightarrow 10^{4}=R+5$
$\Rightarrow R =10000-5 = 9995$

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