Interferometric Method of Velocity of Light Measurement

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Abstract
An association of interferometric methods in the 3.2-cm microwave and visible-light optical domains is now being used for the measurement of the velocity of light in terms of primary atomic constants, an atomic clock for the frequency and a light source (Hg 198, Kr 86, optical maser) for the length. The microwave interferometry is carried out by hydraulically and continuously moving a piston in a perfect, silver-plated, optically polished, fused-quartz, cylindrical resonant cavity. The optical and mechanical systems involve several novel features extending the interferometric servo-control method using photoelectric detection successfully applied by the author to the ruling of high-resolution gratings. With precisions as good or better than those obtained in any previous measurement of c, an increase in accuracy by one to three orders of magnitude results from the use of primary standards for the frequency and length rather than the unreliable secondary standards used heretofore. This fact applies to other velocity-of-light measurements as well and should help in reducing the rather disturbing dispersion in the values of c, which heretofore considerably exceeded the precisions obtained in the measurements.