Absolute power measurement at microwave frequencies

The radiation pressure of electromagnetic waves on a reflecting surface may be used as the basis of a method of power measurement at microwave frequencies. The method is absolute, depending only on measurements of mass, length and time; no secondary electrical standards are involved in the experimental procedure. Moreover, no appreciable power is absorbed by the apparatus. This method has been compared with a balanced calorimeter wattmeter, and agreement to within 1¿2 watts has been obtained in the range 10¿50 watts. The major difficulty encountered in obtaining greater accuracy with the radiation-pressure apparatus described is the disturbing effect of convection currents of air, caused by the heating of the reflector by the small fraction of the incident microwave power which is not reflected.