"Phil Allison" wrote in message
...
"John Larkin"

Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.

** I just took a small mains toroidal ( 30VA) and with the primary
energised at 230 volts passed a one turn loop through the core and
measured
0.102 volts rms across the ends. The loop could be made as open as you
liked or tight wrapped as you liked with NO change in the measured
voltage.

The difference, of course, is that in a toroid, all the flux is constrained
to the core, so it'll work every time. As Mr. Larkin pointed out, since the
winding in question is on a bobbin, and would go in a cup core or pot core,
you would, in fact, lose leakage flux. So the problem does become kinda
non-trivial.

But I'm thinking some kind of temporary core, a la amprobe or some UI
core from the junk box, but then you're getting into Rube Golberg
stuff.

Cheers!
Rich