“In the second period,” beginning in the 1920s, Zworykin continued, “the directed, rather than random, character of electron motion in vacuum was applied in the cathode-ray tube.” In the third period, beginning in the 1930s, beams of electrons were further subdivided into groups. “This subdivision was either on the basis of time, the electrons being bunched at certain phases of an applied high-frequency field as in the klystron or magnetron, or of space, as in image-forming devices,” Zworykin explained. “The electron microscope and the image tube are typical representatives of this group.”7
