"Wild_Bill" wrote in message
...
Basically, a coil of wire will sense electrical activity on any conductor
that passes thru the coil.
A coil also senses fields near it without having to wrap around a
conductor.

A coil on a ferrous core will generally be more sensitive, but as little
as a few turns of wire will also work.
Coils with ferrous cores are used in amp-clamp-type meters and inductive
pickups, and some current probes.

Sencore marketed a product they named the Snoop Loop, which was a length
of coaxial cable with a few turns of wire at the end.. which could be
attached to a scope or other test equipment to sample signals in circuit
components such as tiny board-mounted inductors.

A suitable DIY accessory can be a few turns of wire connected to a female
BNC connector (panel type), so the loop can be easily be attached to a
common scope probe by inserting the probe tip into the connector.
When placed around a small inter-stage transformer the signal becomes
loosely coupled to the loop.

Several turns of wire placed around RF transmission cables is often
sensitive enough to get frequency or modulation readings. I used this
method and capacitive coupling (T connector) when bench testing CB radios
years ago.

--
Cheers,
WB
.............

All that you say is of course true, but the key to the whole thing is
sensitivity. For sure, you can sense RF fields with just a few turns of
wire, but sensing small currents at audio frequencies is a whole different
ball game. Before settling on the CT that I used in my dummy load / power
meter, I tried all sorts of DIY solutions such as messing about with ready
wound solenoids as Nigel is proposing, and wrapping turns around a piece of
wire and so on. None of those trials produced anything like a sufficiency of
output as to be useful for much. The CT that I used is small, neat, readily
available, cheap, and very sensitive with its 2000:1 ratio. Basically, it
does the job it's designed to do, much like that coffee cup that I
mentioned, and I don't really see the point in trying to bodge up some other
device to serve the same purpose ...

Arfa