Signals on a single wire

[Andrew Howard]

Could someone please explain how a signal can be transmitted on a single
wire? I have always been taught that a signal, or current, needs a circuit
to work. But then I see cases that when only one wire is used to carry a
signal.

eg. You touch the input on an audio amplifier, and you get a buzzing
noise, even when you only touch the positive wire.(the power supply used
doesn't have a grounding pin)

Is the second "wire" a path between you and the negative via your legs,
the ground, the table, then the casing or something? Or is it that there is
a path between negative and positive within the amp with some resistance,
which has a voltage drop, creatinge a potential difference between the
positive an negative? Am I even on the right track?

Another example, which probably has a different explanation, is antennas
on radio transmitters. How do they transmit anything, when there is only a
single wire?


Thanks,
Andrew Howard