A Resonant-Cavity Torque-Operated Wattmeter for Microwave Power

A sensitive method of microwave power measurement is described which makes use of the mechanical force exerted by the electromagnetic field on a small vane in a resonant cavity. It is shown that the force on the vane is a simple function of the Q-factor of the cavity, the power absorbed in it and the perturbation of its resonant frequency caused by the vane. The results of a comparison between an experimental wattmeter based on this principle and a water calorimeter are given, and the requirements of a practical instrument are discussed.