What is the principle of connecting the negative pole of the multimeter to the negative pole of the power supply to make the reading jump?

Nubility

What is the principle of connecting the negative pole of the multimeter to the negative pole of the power supply to make the reading jump? [Copy link]

I discovered a strange phenomenon today: When the negative pole of the multimeter (DC gear) is used to touch the negative pole of the DC head of the power adapter, and the other parts are not connected, the reading of the multimeter may change between 0.4 and 0.13V. The multimeter is powered by an ordinary battery, and the power adapter is a 220V AC to 5V DC converter. Why does connecting two completely unrelated grounds change the reading of the multimeter?

maychang

[Use the negative pole of the multimeter (DC gear) to touch the negative pole of the DC head of the power adapter. Do not connect other parts. The reading of the multimeter may vary between 0.4 and 0.13V.]

The output end of your power adapter (regardless of the positive or negative pole) has a certain voltage to the AC mains, and of course to the earth. This voltage may be the power frequency or the power frequency superimposed on the high frequency. This voltage is applied to the multimeter through the distributed capacitance between your multimeter and the positive probe and the body or the probe and the earth, resulting in a tiny current in the multimeter.

maychang

If you take a several-meter-long insulated wire and hang it at a high place, use the voltage range of a multimeter, and touch the wire with one probe, the multimeter may also have a tiny non-zero reading.

okhxyyo

Because they are not connected to the earth, the voltage between them may not be equal. If there is a change when they are directly connected, it means there is a difference.