Question

A galvanometer with a coil of resistance $120ohm$ shows full scale deflection for a current of $2.5mA.$ How will you convert the galvanometer into an ammeter of range $0$ to $7.5A?$ Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more then the actual current in the original circuit? Justify your answer.

Answer 1

$R_g=120\Omega I_g=2.5mA=0.0025A{R}_{auge},I=0-7.5A$

Galvanometer can be converted into ammeter by putting a shunt in parallel,

$R_5=\frac{I_g}{I-I_g}\times R_g=\frac{0.0025}{7.5-0.0025}\times 120=0.04\Omega$

$\therefore$ By connecting a shunt of $0.04\Omega$ across the given galvanometer, we get an ammeter of range ($0$ to $7.5A$)

$\therefore$ Net resistance of ammeter

$=\frac{120\times 0.04}{120+0.04}=0.03998\Omega$

Due to a very small resistance, an ammeter reads slightly less than the actual current.