There is yet another kind of ammeter which employs the Faraday
*effect* and Ampere's law to measure AC or DC current in long
distance power transmission lines. I saw it at the BC Hydro
Research Laboratory about twenty years ago. They may have
invented it there.
One does not want to get too cozy with high voltage power
lines. Noninvasive measurement is desirable. The Faraday
effect produces a rotation of the plane of polarized light in
a transparent medium which is proportional to the strength of
the component of an applied magnetic field parallel to the
direction of propagation and to the optical path length. The
BC Hydro engineers encircle a wire with a long optical fiber.
They then shine plane polarized light into one end and monitor
the plane polarization on the output end. You will realize
that the rotation of the plane of polarization is proportional
to the integral of B.dl around the loop. The resulting signal
is unambiguous after calibration for AC, and it also works for
DC if one is careful. As many turns of optical fiber may be
used as are deemed necessary to obtain the desired sensitivity
since fibers have such low loss. For the same reason the loop
may be made as large as one wishes, and quite large in the
case of high voltage lines. The relatively cheap loop can be
built into transmission towers and left. The monitoring
equipment can be portable, like a portable megavolt
kiloammeter.
I haven't heard a thing about the use of this method since my
visit to that lab. Does anyone else know about it?