A general method for the absolute measurement of microwave power

In an earlier paper it has been shown possible to use the phenomenon of radiation pressure in a method of measuring microwave power. The purpose of the present paper is to provide a generalization of the method which does not require a knowledge of the distribution of electromagnetic energy in the apparatus, or of the geometrical form of the movable element. Assuming the theorem of adiabatic invariance of action, proved for a loss-free resonator by W. R. MacLean, it is shown that any loss-free cavity with input and output waveguides, containing a movable element, can be used to measure the power flowing through it in terms of the force on the movable element. The constant of proportionality between force and power can be determined by certain subsidiary measurements using a standing-wave indicator and a movable piston. In some simple cases the constant can be calculated in terms of known parameters of the system, and some examples are given. The particular advantages of the method of power measurement described here are, iirst, that a negligible amount of power is absorbed by the measuring apparatus, and secondly, that the power is determined absolutely in terms of measurements of mass, length and time. More over, so far as the author is aware, it is the only existing alternative to methods based on a healing effect.