Explain sensitivity of moving coil galvanometer. Show with the help of a circuit diagram that how a moving coil galvanometer can be converted into an ammeter of given range. Write necessary mathematical relation. What is the resistance of ideal ammeter?

Explanation

A moving-coil galvanometer can be converted into an ammeter by reducing its effective resistance by connecting a parallel low resistance S across the coil. Such a parallel low resistance is called a shunt since it shunts a part of the current around the coil. That makes it possible to protect the meter and increases the range of the meter.

Let `I` be the maximum current to be measured and `I_(g)` be the current for which the galvanometer of resistance G shows a full-scale deflection. Then, the shunt resistances S should be such that the remaining current `I-I_(g)=I_(s)` passes through it.
In the parallel combination,
p.d. across the galvanometer = p.d. across the shunt
`therefore I_(g)G=I_(s)S=(I-I_(g))S`
`therefore S=((I_(g))/(I-I_(g)))G " " `...(1)
This gives the required resistance of the shunt. The scale of the galvanometer is then calibrated so as to read the current in ampere directly.