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A revived interest in the Sagnac effect has recently resulted from the development of the self-oscillating laser version of the original Sagnac interferometer. The Sagnac interferometer or ring laser is an example of an electromagnetic sensor of absolute rotation, so historical and theoretical background information is useful in evaluating the possibilities of electromagnetic sensing of absolute rotation. A critical literature study of the many experimental ramifications and the older kinematical theory of the effect is presented. This geometric optical theory is then complemented and compared with more recent work that is based on a physical optical analysis using a complete electromagnetic description of the phenomenon.
E. J. Post (Sat,) studied this question.