How does a voltmeter work?

Well, they cannot, however, a modern voltmeter with a very high input impedance (10 megomhs or so)does not drain the AC line to any detectable degree. However, if you are measuring the vertical field gradient in dry air you will find you cannot use a 10 megohm, as it shorts out the emf. In that case you need special high impedance equipment.

Volt meters do effect the current when placed in across a load. Any time a load is placed in parallel to another load the current though each load will change. In the case of a voltmeter, the better the quality of the meter the less effect it will have on the load current. The ohms/volt rating of the volt meter is the key, the higher the rating the better. A quality meter has minimal negligible effect on load current.

Hmmm just so happens I'm designing a small electrometer at the moment!!

Forget 10 Mohms the input impedance of this thing is in the 10^11 ohms!!

John.

Hello Electroman,

What instrument you have used to measure input impedance?

As others have mentioned, meters do affect the circuit. But for "everyday use" the induced error is insignificant. Ignoring field affects and other oddities, it's only at the extremes, very very high or very very low voltages, currents and resistances where it's a concern. For these there are specialized devices and techniques.

The web sites of Fluke and others have some good info if you dig for it.

They can't.

In truth they introduce a very very small amount of distortion in to the signal being measured. In some cases threw field effect dampening, or by adding some amount of resistance. Voltmeters are typically parasitic in nature, generally threw field effect coupling. But more important, is to keep in mind ,that the size/amount of the distortion is extremely small. For most applications it's simply ignored, or considered as part of the signal background noise.

In the case where high end Digital Signal Processors (DSP) and Analog to Digital (AD) converters are used, for example in research or military applications. Software filters can be created to compensate for the estimated amount of error they introduce. Keep in mind that some radio telescopes can detect a signal as small as one trillionth of one watt, pretty small signal level.

For typical applications the amount of signal error introduced by the act of measurement is just considered to be part of the background noise. Consider an electric toaster, it really would not care if the voltage varied a couple of microvolts either way, it would still produce pretty good toast.

It's true that in physics, you don't get something for nothing. But you need to keep in mind the amount of distortion sometimes is meaningless. After all, it's all relative.

They do have an effect. The old standby, Simpson, multimeters had impedances ranging from 10,000 ohms per volt to 100,000 ohms per volt, depending on which range of voltage was being measured.

I think Heisenberg may have been the physicist who said you can't observe something without affecting it.

The answer is somewhat different for older electromechanical meters than for modern electronic meters. I'll start with the electromechanical ones and then briefly describe the electronic ones. An electromechanical meter has a coil of wire that pivots in a nearly friction-free bearing and has a needle attached to it. This coil also has a spring attached to it and that spring tends to restore the coil and needle to their zero orientation. Because the spring opposes any rotation of the coil and needle, the orientation of the needle depends on any other torque (twist) experienced by the coil of wire--the more torque the spring-loaded coil experiences, the farther the coil and needle will turn away from the zero orientation. The needle's angle of deflection is proportional to the extra torque on the coil. The extra torque exerted on the spring-load coil comes from magnetic forces. There is a permanent magnet surrounding the coil, so that when current flows through the coil it experiences a torque. Because a current-carrying coil is magnetic, the coil's magnetic poles and the permanent magnet's magnetic poles exert forces on one another and the coil experiences a torque. This magnetic torque is exactly proportional to the current flowing through the coil. Because the torque on the coil is proportional to the current and the needle's angle of deflection is proportional to this torque, the needle's angle of deflection is exactly proportional to the current in the wire. To use such a meter as a current meter (an ammeter), you must allow the current flowing through your circuit to pass through the meter. You must open the circuit and insert this ammeter in series with the rest of the circuit. That way, the current flowing through the circuit will also flow through the meter and its needle will move to indicate how much current is flowing. To use such a meter as a voltage meter (a voltmeter), some current is divert from the circuit to the meter through an electric resistor and then returned to the circuit. The amount of current that follows this bypass and flows through the electric resistor is proportional to the voltage difference across that resistor (a natural phenomenon described by Ohm's law). The voltmeter system thus diverts from the circuit an amount of current that is exactly proportional to the voltage difference between the place at which current enters the voltmeter and where it returns to the circuit. The needle's movement thus reflects this voltage difference. In an electronic voltmeter, sensitive electronic components directly measure the voltage difference between two wires. Virtually no current flows between those two wires, so that the meter simply makes a measurement of the charge differences on the two wires. An electron ammeter uses an electronic voltmeter to measure the tiny voltage difference across a wire that is carrying the current. Since the wire also obeys Ohm's law, this voltage difference is proportional to the current passing through the wire.

thanks for giving me so many interesting knowledge. thanks once again.....

dear All,

is there voltmeter for Medium Voltage (For grid). for example for 20 KV AC

warm regards