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
..............


"Arfa Daily" wrote in message
...

Yes, a speaker lead passes straight through the middle. In this use of the
device, there is no 'primary sense turns' issue. The wire passing through
the middle represents a single turn primary. With the ratio being 2000:1,
that provides a more than adequate output level for really very small
power levels - just a few watts in fact. I dare say that if you were
bothered, you could rectify the voltage appearing across the secondary,
and hang a suitable meter and calibration pot across the output, and get a
fair indication of relative, if not absolute power levels. The transformer
is rated to 20A, which I believe is the point at which the core starts to
saturate, so the output becomes non-linear, rather than where anything
nasty starts to happen. 20A allows a pretty powerful amp to be run through
it.

You can try a solenoid. I'm sure that you will get something out, but I'm
not sure how much. CTs are usually torroidal in construction - at least
the small ones that I've seen - and I guess there's a reason for that. I
mean, at the end of the day, you can drink coffee out of a shoe, but a cup
works better ... :-)

Arfa