When I touch the positive end of the oscilloscope probe without touching the ground, I see the following waveform on the screen. The Vpp is around 45 volts as shown in the picture below:
When I touch the ground end of the probe while touching the positive tip, the Vpp drops to 3.6V as shown in the image below.
What is going on?
Your ungrounded body is picking up power line noise, which is shown in the first trace.
When you touch the scope ground at the same time, you’re grounding your body, which conducts away most of the coupling. The remainder that isn’t coupled away is what’s shown in the second trace. The closer you place the probe tip to where you touch the ground, the smaller that residual voltage will be.
Notice the period of the signal in the first example. it is more or less 20ms which translated to frequency is 50Hz. Your ungrounded body has become "an antenna" for mains voltage. When you ground yourself it becomes less noticeable.
