An inclosure, including two or more electrodes, in which a flow of electrons may occur through a vacuum or a contained gas. the electronic conduction takes place either by means of electrons emitted from a cathode or by ionization of the contained gas. includes phototubes (photoemissive type only), geiger-mueller tubes, traveling wave tubes, cathode ray tubes and microwave tubes (klystron, magnetron, or the like). excludes items whose primary purpose is to produce light energy. also excludes photoelectric cell; and semiconductor device (as modified).