Why do I see 60Hz mains when connecting long wire to probe?

[pwnell]

Another newbie question - if I connect a single 10ft wire that goes nowhere to the positive terminal of my 10x passive probe, and the ground is left disconnected, why do I see a 60Hz semi sinusoidal wave of 5Vpp on the scope? From where does it get induced and why such a large magnitude?

[ataradov]

It is a capacitive coupling. Wiring in the walls and your piece of wire form a capacitor, which passes AC current.

You don't even need the wire, you can just touch the tip of the probe with your finger. Your body is full of water and salt, so it is a decent conductor. Again, good enough to couple some AC signal.

[radiolistener]

It come from mains, your wire works as antenna. Input impedance is 1 MOhm, so it is very sensitive.

[Doctorandus_P]

A simple way to test if your scope still works is to touch the probe with a finger.
It's not unusual to measure 70Vac on your fingertip, especially when you hold your other hand near anything mains voltage related.

[Gyro]

It come from mains, your wire works as antenna. Input impedance is 1 MOhm, so it is very sensitive.

A small correction, he's using a x10 probe, so 10MOhm - even more sensitive.

[pwnell]

Yup I verified that - touching the tip of the probe and then with the other hand wrapping it around the (insulated of course!) power cord of my oscilloscope causes a nice 10Vpp 60Hz (distorted) sine wave to appear.  Very cool stuff - thanks for explaining.