Question

A galvanometer of resistor 10 ohms gives a full scale deflection when a current of 50amphere flow through it.

How can it be converted to an ammeter to measure a current of 5Amphere?

How can it be converted to a voltmeter to measure a voltage of 3volts?

Answer 1

(a) Ammeter

A galvanometer can function as an ammeter when it is placed in parallel with a small resistance $R$ .

$IR=I_Gr\to R=\frac{I_G}{I}r=\frac{0.000005}{5-0.000005}\cdot 10=1\cdot10^{-5} \ (\Omega)$

(b) Voltmeter

A galvanometer can function as a voltmeter when it is connected in series with a large resistance $R$ .

$R_{total}=R+r=V/I=3/(50\cdot10^{-6})=60000\ (\Omega)$

$R=R_{total}-r=60000-10=59990\ (\Omega)$