# How does a ammeter and voltmeter works?advanced thanks.

1. Aug 19, 2010

### amaresh92

how does a ammeter and voltmeter works?

2. Aug 19, 2010

### schip666!

3. Aug 19, 2010

### dlgoff

Re: ammeters

You may also want to check out http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/ammet.html" [Broken].
And here's their http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/galvan.html#c1".

Last edited by a moderator: May 4, 2017
4. Aug 20, 2010

### amaresh92

Re: ammeters

thanks .
but how a resistance added parallel to galvanometer can detect that potential difference in circuit & similarly resistance in series detect the current? though galvanometer shows deflection on respective magnetic field.

5. Aug 20, 2010

### dlgoff

Re: ammeters

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html" [Broken]

Last edited by a moderator: May 4, 2017
6. Aug 20, 2010

### vk6kro

Re: ammeters

The meter in a voltmeter or ammeter is too sensitive for most normal measurements.
Typically, it will need 50 micro-amps to read full scale or it may need 0.1 volt across it to make it read full scale.

So a meter like this will have a resistance of 2000 ohms. (Because of OHM's Law. 0.1 volt / 0.00005 amps = 2000 ohms)

So, to measure 10 volts on this meter full scale, the 10 volts has to be dropped to 0.1 volts with a series resistor so that there is no more than 0.1 volts across the meter.
It will have 9.9 volts across it and be conducting 0.00005 amps, so it should be 198000 ohms.
(Ohms's Law again. 9.9 volts / 0.00005 amps = 198000 ohms.)

Now, if you wanted to use this meter as a current meter, it would usually be too sensitive.

Suppose you wanted it to read 10 mA full scale. You would have to put a resistor in parallel with the meter so that 9.95 mA flows through the parallel resistor and 50 µA flows in the meter (making a total of 10 mA). This resistor will have 0.1 volts across it and be conducting 9.95 mA so it should be 10.05 ohms. (Ohms Law again. 0.1 volts / 0.00995 Amps = 10.05 ohms)

Work through this carefully and you should see what is happening. It is all about making the meter less sensitive than it normally is, but doing it in ways that make the meter more useful.

7. Aug 20, 2010

### amaresh92

Re: ammeters

voltage and current are different quantities.then how galvanometer detect both of them with the help of magnetic field created inside it?
where the magnetic field created depend on the current only.

Last edited: Aug 20, 2010
8. Aug 20, 2010

### sophiecentaur

Re: ammeters

I must point out that this is the wrong way round.
A galvo will measure a very small current (a few micro amps, possibly) with only a small voltage across it.
You measure the small current through a large SERIES resistor to tell you the overall Volts (i.e. a Voltmeter) and you measure the small proportion of current going through the galvo' in PARALLEL with a very low resistor to tell you the total current (i.e. an Ammeter).

9. Aug 20, 2010

### vk6kro

Re: ammeters

The deflection of a sensitive meter like a galvanometer depends on the current passing through it. There are magnets inside the meter, but these are a fixed part of the design. Only the current in the meter changes the reading on the meter.

So, if the meter reading only depends on current, we can measure current by taking a small sample of the current flowing and passing it through the meter.

But we can also measure voltage. If you put a large resistor in series with the meter, a small current will flow in the resistor and through the meter.
The meter is still measuring current, but we can work out what voltage caused that current to flow.

[PLAIN]http://dl.dropbox.com/u/4222062/meters.PNG [Broken]

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10. Aug 20, 2010

### dlgoff

Re: ammeters

So a VOM (volt, ohm, milliamp) meter would look something like this:

Just to show how one would also measure resistance with a moving coil meter.

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11. Aug 21, 2010

### amaresh92

Re: ammeters

as per "vk6kro" diagram if i join a voltmeter across a 9.9v then the deflection will be zero?
but the current will be same 50micro ampere ,so both the deflection on ammeter and voltmeter will be same in this case.But how it can be possible to measure current and voltage like this?i am not able to follow how it measure?

12. Aug 21, 2010

### vk6kro

Re: ammeters

as per "vk6kro" diagram if i join a voltmeter across a 9.9v then the deflection will be zero?
but the current will be same 50micro ampere ,so both the deflection on ammeter and voltmeter will be same in this case.But how it can be possible to measure current and voltage like this?i am not able to follow how it measure?

No, if you put this voltmeter across 9.9 volts, it will indicate 9.9 volts.
If you put it across 9 volts, it will indicate 9 volts. That is what voltmeters do.

If you put the meter and series resistor across 9 volts, the current that flows will be (9 volts divided by the total series resistance which is 200000 ohms). So the current will be 45 µAmps.
(9 / 200000).
So the meter will read 90% of full scale. (45/50 = 0.9). This is calibrated to read 9 volts.

This diagram also shows what happens if you put the meter and resistor across 3 volts:

[PLAIN]http://dl.dropbox.com/u/4222062/meters2.PNG [Broken]

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13. Aug 21, 2010

### amaresh92

Re: ammeters

thank you very much "vk6kro".