Oscilloscope shows 60V on lamp ground

[nbritton]

So I was just playing with my new scope when I hooked up my (ungrounded) probe the chassis of a desk lamp and to my surprise the scope measured a 60Hz sine wave of 60Vpp (+/-30V). I know induction can pickup some stray voltage, but 60V? Furthermore, the lamp and oscilloscope are plugged into the same power strip, so they should have the same ground reference and phase.

What's going on here?

[tautech]

Mis-match between probe attenuation and scope input attenuation?

[alsetalokin4017]

What do you see when you just hold the (ungrounded) probe tip between thumb and forefinger?

[Gyro]

Possibly most likely is that the lamp has an ungrounded chassis (2 wire cord) and what you are seeing is leakage.

[alsetalokin4017]

Sigh.

Here's what I get from holding the (ungrounded) probe tip between my thumb and forefinger.

I assure you that I am not plugged into anything, myself.   

[tec5c]

Sigh.

Here's what I get from holding the (ungrounded) probe tip between my thumb and forefinger.

I assure you that I am not plugged into anything, myself.   

Right, and this is what you'd expect. Due to the high impedance of the probe it acts as an antenna. So you'd still pick up the 60Hz (50Hz some countries) ambient noise even without pinching the probe with your fingers, it will just be a weaker signal, perhaps not noticeable on certain scopes. When you make contact with the probe your body is then acting as an antenna (and a better one than empty space).

This is due to a capacitive divider effect. One capacitor is within the scope probe and essentially is connected from the tip to the scope ground. The other much smaller capacitance is across empty space: from the probe tip to any distant mains wires in the walls. The empty space around the probe is the dielectric of this capacitor.