SAGNAC EFFECT - A CENTURY OF EARTH-ROTATED INTERFEROMETERS

The earliest prediction of the Sagnac effect, and of the possibility o f detecting the Earth's rotation with an interferometer of square kilo meter area, is by Lodge (1893, 1897). We illustrate the extraordinary range of theoretical motivations for the experimental study of the Sag nac effect, starting with previously unpublished correspondence betwee n Lodge and Larmor, and ending with present (and planned) ring interfe rometer experiments whose sensitivity to the Earth's rotation is of th e order of parts per million (billion, respectively).