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A simple gas-discharge microwave power coupler consists of a cavity supporting two resonant modes at the same frequency, a gas discharge symmetrically located in the cavity with respect to each mode, and a magnetic field arranged to be perpendicular mutually to the electric fields of both modes in the region of the discharge. Power is coupled between modes by the interaction of the electrons with the electric fields of the modes in the presence of a magnetic field. If power is coupled into one mode with a suitably oriented probe, power can be coupled out of the other mode to a load by another probe. The degree of coupling is a direct function of the magnetic field and the electron density in the discharge, which may be controlled by means of an external circuit governing the current in the discharge. A large gyromagnetic resonance effect is observed when BÂ¿Â¿m/e and this resonance broadens for increasing electron density. When either the magnetic field is zero or the discharge is extinguished, no power is transferred. The mechanism has possible microwave applications for controlled or automatic power switching, amplitude modulation of power, power division, and sensing of either electron density or magnetic-field strength for control or measurement purposes. Devices having externally controlled discharges are limited to low power of the order of milliwatts. Higher-power devices may utilize self-excited discharges. Experimental results and a mathematical theory of the coupler operation are presented after a discussion of the mechanism.