A method and an apparatus for measuring the strength of an AC electric field or an equivalent voltage, or the strength of an AC magnetic field or an equivalent current, based on a light beam which is transmitted through and thus modulated by an optical sensing head due to the Pockel's and Faraday effects,...http://www.google.de/patents/US4933629?utm_source=gb-gplus-sharePatent US4933629 - Method and apparatus for optically measuring electric and magnetic quantities having an optical sensing head exhibiting the Pockel's and Faraday effects
