A Resonant Cavity Method of Measuring the Velocity of Light

A method is proposed to determine the velocity of propagation of electromagnetic waves by means of a resonant cavity. A circular cylindrical cavity is used operating in the TE011 mode. It is shown that for a particular length-to- diameter ratio, the resonance frequency is only a function of the cavity volume and the velocity of electromagnetic waves. The latter can be calculated, when resonance frequency and cavity volume are determined experimentally. The main advantage of this method is, that the volume has to be measured only to one-third the accuracy which is desired for the propagation velocity. Linear dimensions, on the other hand, have to be determined to the same accuracy. Furthermore, the volume method requires only reasonable tolerances in the construction of the cavity. The effects of various cavity imperfections on the resonance frequency were analysed. The frequency shifts due to, the finite conductivity of the walls, the deformations in the boundary surface, grooves, and coupling irises, were calculated. The problem of a thick iris was treated numerically.