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 $1V$, 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)

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