Conversion of Galvanometer to Ammeter

[Ilikecereal]

This is kind of a stupid question but...to measure high currents, a galvanometer is connected parallel to a shunt resistance so that most of the current passes through the shunt resistance but only a small amount of current passes through the galvanometer. But wouldn't this defeat the purpose of measuring a large current? Since only a small current is being measured. :|

 

[Delphi51]

Think of it as measuring a known portion of the current. Anyway, you can easily figure out what the large current is from the measurement. When measuring the current in a power line, it is very inconvenient and costly to flow the entire current (at a high voltage) through your meter.

Pretty much all Ammeters are actually galvanometers with a shunt. Or galvanometers connected to coils to measure the magnetic field caused by the current.

 

[Ilikecereal]

How would you figure out the larger current? Won't the values of the two be different?

Sorry, I'm a little confused here

 

[Delphi51]

Say you have a galvanometer with a resistance of 1 ohm. You make your shunt 1 ohm as well. Then half of the current goes each way. Your meter measures half the current, so you multiply it by 2 to get the whole current. Usually the shunt resistance is made smaller so only a tenth, hundredth or thousandth of the current goes through the meter.

 

[Ilikecereal]

Oh, I get it now. Thanks!

 

[Delphi51]

Most welcome! I forgot to mention convenience. When you need a meter to measure current, you don't need to order one specifically for the range required. Any meter will work with the correct shunt. A meter with multiple current ranges just switches in different shunts. Very handy!

 

[rohans]

can the range of an ammeter be decreased ?
@Delphi51 in the above comment you said that we don't need ammeter of a particular range . i comprehend that as by connecting a shunt the range can be increased . but for a given purticular ammeter is there any way that the range might be decreased ... i referred a book which says that the range can never be decreased but one of my mentors says that the range can be increased . HELP