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# Voltmeters take current to operate?

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1. Voltmeters take current to operate?
In one of my books it says that ''a voltmeter must take some current in order to operate''? I though current isn't ''used up'' and remains constant throughout the circuit? Also, what exactly determines the brightness of a bulb? Is it the current ?

I'm having some trouble with electricity..
2. Re: Voltmeters take current to operate?
(Original post by sabre2th1)
In one of my books it says that ''a voltmeter must take some current in order to operate''? I though current isn't ''used up'' and remains constant throughout the circuit? Also, what exactly determines the brightness of a bulb? Is it the current ?

I'm having some trouble with electricity..
I don't know what you mean by 'used up' but an ideal voltmeter has infinite resistance which in turn means that it draws no current.

In reality a voltmeter is likely to have a very high but definitely not infinite resistance. Hence, it will draw a small current causing a potential drop across itself. This in turn means the displayed value on the voltmeter is LESS than the actual value.
3. Re: Voltmeters take current to operate?
Well, the brightness of a bulb, I believe, can be related to its electrical power.

So, P=VI

and

P=I^2*R

or

P=V^2/R

So I believe it can be related to the current and/or the potential applied across the lamp.
4. Re: Voltmeters take current to operate?
Yeah, if it's an analogue meter it works by the magnetic field created by a current going through a moving coil in a static field to move the needle against a spring.
Usually it's a very high resistance in comparison to the components you'll be looking at and has no measurable effect at the accuracy you're able to read the dial. DVMs and CROs too fwiw though they work a bit differently.

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