# Galvanometer to ammeter?

1. Jul 22, 2014

### Hardik Batra

To make ammeter,
Resistance of proper small value is joined in parallel to the coil of galvanometer.This resistance is called a shunt.

For smaller resistance value,
We can not use the smallest resistance wire that is directly wound on coil of galvanometer so the resistance will become minimum.?

2. Jul 22, 2014

### davenn

hi there Hardik

do you understand what the shunt resistance does ?
I suspect not

the coil resistance on its own may give a full scale reading of say 100mA
but what if you wanted to read 1A or 10A or 100A ?

so for example if you want to read 10A full scale, and the coil can only read 100mA full scale, then the shunt must pass the other 9.9 Amps (9900 mA)

Here's an excellent article on the use and calculating of shunt resistors .....

cheers
Dave

3. Jul 23, 2014

### Hardik Batra

In (fig. a) we want to measure current passing through the resistance R in a circuit shown in fig.

I=V/R

In (fig. b) galvanometer is joined in series with resistance R, as shown in fig.

In such connection resistance G of the galvanometer is added in the circuit

and as the total resistance of the circuit is changed the value of current to be measured itself is changed

I = V/(R+G)

Thus the true value of current is not obtained.

this fact indicates that the resistance of current meter should be as small as possible.

So we can't use the low resistance wire that directly wound on coil so that you will get directly,

I = V/R.

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4. Jul 23, 2014

### sophiecentaur

As with all Engineering, it's a matter of compromise, using the materials that are available.
The galvanometer needs to be sensitive enough to detect very small currents. The movement needs to be very light, so it responds quickly to a short pulse. That forces the use of thin wire which will burn out with higher currents. So you are stuck with a resistance that is not that low (you need sufficient turns on the armature to get a deflection).

For measuring higher values of current, the shunt dissipates most of the energy so the coil won't burn out.