In an empty apartment room,my multimeter shows 3.6 mV voltage when I put 2 probes with 2 meters distance.
I am curious,what may cause voltage between 2 distant probes of multimeter in apartment room?
Thanks for any hint.
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2$\begingroup$ If you touch the ends , is it zero ? Try measuring an AA battery DC, volts, low scale. Does it get it right? $\endgroup$– anna vCommented Sep 5, 2022 at 5:51
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$\begingroup$ @annav,when I touch the probe,the number is not zero.Test AA battery,multimeter get correct number. $\endgroup$– kittygirlCommented Sep 5, 2022 at 14:36
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$\begingroup$ By touching the ends I meant to each other. It should register zero when the two ends of the probe are touching, because it is supposed to be measuring the AC between the two points, (as with the dc and battery.) So something is blown inside it. Modern stuff have digital memory that could be affected. , I would try resetting it taking its batteries off and then on . $\endgroup$– anna vCommented Sep 5, 2022 at 17:31
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2 Answers
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Small 'ghost' or 'phantom' voltages may be seen on modern digital multimeters (DMMs) when set to a low voltage range, especially inside building with AC wiring in walls, under floors, etc. DMMS tend to have high input impedance. This is something that users of older style meters with a moving pointer did not experience.
The cause is usually capacitive coupling between the meter leads and household AC wiring in the walls, under floors, etc. You may notice a change in the reading if you move the wires about, short the probes together, touch one of the probes, etc.
This 'feature' of modern DMMs can confuse people when testing conductors in AC power wiring to see if they are alive or dead, for example a wire disconnected at both ends, but running parallel to energised circuits carrying power may give quite a big reading to ground (e.g. 75 volts in a 110 volt situation). The very high input impedance of a DMM does not load the conductor and so the DMM picks up the induced voltage. An old moving-coil meter would present a tiny load but still enough to collapse the induced voltage.
It also means you need to be careful and take steps in lab work if measuring very small AC voltages.
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$\begingroup$ Does old moving-coil meter avoid to get 'ghost' voltage when measure small voltage like mV? $\endgroup$ Commented Sep 5, 2022 at 14:45
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$\begingroup$ @kittygirl - yes, usually. $\endgroup$ Commented Sep 5, 2022 at 17:21
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I note that the meter is set at $\rm mV$ ac.
You have a made a "dipole" antenna and are picking up electromagnetic waves including those at mains frequency.
Does changing the orientation and/or length of the wires, you holding one wire, make a difference to the reading?
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$\begingroup$ When I change the orientation,holding one wire, multimeter reading number changes. $\endgroup$ Commented Sep 5, 2022 at 14:43
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$\begingroup$ Yes, 2 probes wire actually is dipole antenna.Further,any cheap solution to measure EM waves frequency? $\endgroup$ Commented Sep 5, 2022 at 14:50