Question

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 :

• A. $10^3$
• B. $10^5$
• C. $99995$
• D. $9995$

Answer 1

Answer: (D)

$9995$

Answer 2

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 $R.$ $\Rightarrow V=I_{g}\left(R+G\right)$ $\therefore 150=15\times10^{-3}\left(R+5\right)$ $\Rightarrow 10^{4}=R+5$ $\Rightarrow R =10000-5 = 9995$