Question

A moving coil galvanometer has 150 equal divisions.Its current sensitivity is 10 divisions per milli ampere and voltage sensitivity is 2 divisions per milli volt. In order that each division reads 1 volt, the resistance in ohm needed to be connected in series, with the coil will be

• A. $1000$
• B. $100000$
• C. $99995$
• D. $9995$

Answer 1

The correct option is D $9995$

It is given that a moving coil galvanometer has 150 equal divisions.

Also current sensitivity $=10div/mA$ and voltage sensitivity $=2div/mV$

Now the maximum current $I_g$ that can flow through the galvanometer for full scale deflection is

$I_g=\frac{150}{10}=15mA$

Also the maximum voltage$V_g$ that can flow through the galvanometer for full scale deflection is

$V_g=\frac{150}{2}=75mV$

Hence resistance of galvanometer G, is

$G=\frac{75mV}{15mA}=5$ ohm

Thus the resistance needed in series with the galvanometer to convert it into

voltmeter of range $V=150\times 1=150V$, is

$R=\left(\frac{V}{I_g}\right)-G$

$R=\left(\frac{150}{(15\times {10}^{-3})}\right)-5$

$R=10000-5=9995$ ohm