# How does it know the potential difference between the two objects?

1. Aug 4, 2006

### quasar987

Suppose you have two charged conductors. Then you "use" a voltmeter on them. How does it know the potential difference between the two objects? There's no current, so it cannot use V=RI.

thx

2. Aug 4, 2006

### chroot

Staff Emeritus
The potential difference is essentially applied across a transistor, which does indeed draw a very small (non-zero) current. The transistor permits a current to flow which is proportional to the applied voltage, and the voltmeter uses this for its indication.

- Warren

3. Aug 4, 2006

voltage is not dependent on current...

V=IR is just the energy per charge u lose over resistance...

and there is current if it is a voltmeter(not a static one) that u use...
the voltmeter has a very high resistance within it, though current still applies, a very low one, so by this current, and the known resistance, it may measure the voltage...

though there are better devices which do not allow current at all, therefor do not change the charge on the bodies in time, and can measure very high voltages... and they are simple to understand, cuz its based on simple mechanics, therefor cooler..

4. Aug 5, 2006

### quasar987

Is this the way a voltmeter works?

The voltmeter is actually just a huge resistance and a device capable of detecting the magnitude of the current passing through the resistance. By pluging the voltmeter on both charged conductor, I am allowing charges to flow from one conductor to the other. Since R is very large, the current is very tiny and so the charges on both sphere after measurement is esentially the same.

5. Aug 5, 2006

yep.

though coltmeters like i said are not too good for high static voltages... industrious voltmeter can measure like.. 1 kv max... plus the current changes the voltage in time.

btw, ane analog version of voltmeters, is a niddle moved by magnetic forces made by the current.

6. Aug 5, 2006

### quasar987

What do you mean by "analogue"? Are you talking about the instrument that measures the magnetic field?

And continuing on the same line of inquiries, what exactly is this "device" in voltmeters that measures the magnitude of currents? How does it work?

7. Aug 5, 2006

### Gokul43201

Staff Emeritus
He said "analog", which means that the device does not use digital circuitry. The device he was describign is known as a Moving Coil Galvanometer, and it is used to measure currents (or voltages). Most present day analog voltmeters run on the same basic principle of the MCG - that a current carrying coil in a magnetic field experiences a torque proportional to the current. If the coil is attached to a helical spring and has a pointer mounted on it, the deflection of the pointer tells you the value of the current.

8. Aug 6, 2006

and btw, the most common multitesters are digital, the principle is unknown to me.

though the galvanometer is much more accurate i hear... but more incumbering...

9. Aug 7, 2006

### lalbatros

Hello,

An electroscope (see on wiki) is also a kind of voltmeter and it does not rely on the current but on the forces between electrical charges.

I find it always impressive that the whole story of electromagnetism started with such kind of devices. It is impressive because these early experiments where really fundamental. Today we learn electostatics but we mainly measure voltages with Ohm-law based devices and operational amplifiers: our measurement devices are very far from the fundamental physics.

Measuring the current is only one way for measuring a potential and it is not always applicable of course.

Note further that all measurements -even classically- have a pertubating effect on the measured phenomena. QM mechanics says that it cannot be reduce indefinitively. Classical physics assumes it can be reduced as much as necessary but you have to engineer you measuring device in more and more sophisticated ways. For example: if you want to measure an electrostatic potential without drawing a perturbing current, you will have to adjust your technology. Maybe you go back to an electroscope!

Michel

Postscriptum
I would be interrested to know how very high voltages are routinely measured, on distribution line for example or in high voltage electrostatics laboratories. Ohm again?

Last edited: Aug 7, 2006