Conversion of a Galvanometer to an Ammeter

[zorro]

I had this doubt for over 1 year! I thought of asking about it here.

In the conversion of a galvanometer to an ammeter by connecting a shunt in parallel to the galvanometer, maximum amount of current flows through the shunt because it has a very very low resistance. In other words, very less amount of current flows through the galvanometer branch. How does it measure the correct current flowing through the circuit?

If we connect a normal ammeter in series, all the current flows through it and hence it measure the correct current flowing.

 

[NobodySpecial]

An ammeter normally has a low resistance and is used in series so all the current flows through it. The lower you can make it's resistance the more sensitive it is - since less current is needed to show a reading.

But suppose you had an ammeter which went to full scale with only 10mA flowing through it and you wanted to measure a current of 10amps? You need to make it so only 1/1000 of the current goes through the meter - so you put a resistor of 1/1000 the resistance of the ammeter in series with it.

 

[zorro]

The lower you can make it's resistance the more sensitive it is - since less current is needed to show a reading.

For an ammeter (converted) to show a reading of say 10mA, all this current should flow through the galvanometer branch. If only a small part of it say 0.01mA flows through it, then the galvanometer measures 0.01mA as the current flowing. How will it measure 10mA?

 

[Integral]

For an ammeter (converted) to show a reading of say 10mA, all this current should flow through the galvanometer branch. If only a small part of it say 0.01mA flows through it, then the galvanometer measures 0.01mA as the current flowing. How will it measure 10mA?

That is why you have a calibrated scale behind the needle. The shunt resistor used determines which scale you read. So the same current through the galvo coil will represent a different current through the shunt.

 

[zorro]

That is why you have a calibrated scale behind the needle. The shunt resistor used determines which scale you read. So the same current through the galvo coil will represent a different current through the shunt.

What happens if we make the shunt resistance very large compared to the galvanometer so that all the current flows through the galvanometer. Can't we calibrate the galvanometer this way?

 

[rl.bhat]

If you see a galvanometer, there is a rating. 20 μA/div. If the galvanometer has 30 divisions, it gives full scale deflection for 600 μA. You can measure the current up to 600 μA with the accuracy of 20 μA. If you want to increase the range of the ammeter from 0 to 10 A, constructed by the above galvanometer, you have to connect a low resistance shunt across the galvanometer such that when 10 A current is flowing in the circuit, only 600 μA flows through the galvanometer. Rest of the current flows through the shut to the main circuit.

 

[Studiot]

I suggest the best way to think of a galvanometer is as a very sensitive ammeter - a microammeter in fact.

The term galvanometer was introduced long before dedicated voltmeters, ammeters or modern convenient multimeters were introduced.

You can perform the same shunt ( or parallel) resistance trick with any ammeter to anhance its range.

Similarly you can convert it into a voltmeter by adding a series resistance.

 

[zorro]

If you see a galvanometer, there is a rating. 20 μA/div. If the galvanometer has 30 divisions, it gives full scale deflection for 600 μA. You can measure the current up to 600 μA with the accuracy of 20 μA. If you want to increase the range of the ammeter from 0 to 10 A, constructed by the above galvanometer, you have to connect a low resistance shunt across the galvanometer such that when 10 A current is flowing in the circuit, only 600 μA flows through the galvanometer. Rest of the current flows through the shut to the main circuit.

Thanks! I understood it with your example.