Fleetie wrote:
The obvious choice, as Mr Cook says, is a crystal earpiece.

With one of those, the OP won't even need to make 2 connections;
just touching the tip of the jack plug on the conductor that's
oscillating at a few kHz and a few volts will be enough to make
it faintly audible with careful listening.


One of the most useful bit of kit in my toolbox is a device I made
myself some 35 or so years ago. It`s a crystal earpiece with one wire
terminated in a croc clip and the other in a probe made from an old
ballpoint pen and a brass nail, there`s a capacitor in series to provide
DC blocking.
With this simple device I can trace audio through a circuit- from the
output of a crystal pick-up to several hundreds watts, I can detect
digital pulses, hear hum on a DC supply, etc. etc.

Ron(UK)