Why when we touch osciloscope probe it shows 50Hz noise?

It depends on the degree to which your body is capacitatively coupled to the mains. (and the degree to which it is earthed, and to a lesser extent the magnetic flux and the area of the loop formed by the circuit formed by touchiong the probe.

try touching the probe to a large piece of aluminium foil.

So magnetic field that is produced when we draw current from mains can be converted back to electrical current in our body and produces voltage. for example when large currents are drawn e.g. welding it causes to some tiny currents produced in our body?

the capacitance of our body should directly depends on our distance to wirings inside the wall, isn't that? why oscilloscope amplitude is about a few volts rather than 220V? this peak to peak voltage depends on what ? can we calculate it?

You need to pack your head with aluminium foil.

I said only reply if you understand the phenomenon. surely I don't need advice from people who don't have anything in their head.

No. Capacitive coupling is entirely an electrostatic phenomenon; when you stand in an electrical field and get that mains-frequency voltage impressed on you there is practically no current flow.

Magnetic fields would induce currents, yes, but that's a different issue.

The capacitance of our bodies to the wiring depends on that distance, yes, but there are a whole bunch of variables -- just assume that you're immersed in an electric field at the mains frequency.

I've seen tens of volts, depending on the environment (florescent lighting seems to enhance the coupling).

The placement and size of the wiring, the existence of other conductors in the electric field, whether those conductors are grounded, etc.

In theory, yes. In practice it works best if you are perfectly spherical, and floating in the center of a perfectly spherical room with all classes of conductors (mains wiring, telephone, metal studs, etc.) all arranged in a spherically symmetric way in the walls, and close enough together so that you can treat them as being uniformly distributed within the walls.

I think my main mistake was that I was assuming a magnetic field rather than electrical field produces this phenomenon. regards,

Because there's a lot of it about !

Graham

Graham

-- due to the hugely increased level of spam please make the obvious adjustment to my email address

Quick diode tester:

Grab scope probe tip with your left hand. Observe 50/60 Hz cruddy sine wave on scope.

Grab one end of diode with right hand, and touch free end to scope ground.

John

be.

end

i never thought of that..... thanks

I did this with a transistor once - I think it might have been a darlington - but I did see gain. :-)

Cheers! Rich

This didn't work for me.

What did work is:

1. Grab one end of diode with fingers
2. Touch (or clip) scope probe to the other end of the diode.

Rectified waveform shows on scope.

Thanks for the great tip.