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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.
... 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....
Leigh