236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



attention was paid to the behavior of the individual electrons in the 

 tube than is customarily expended on the motion of the individual 

 steam molecules in the valve. The English designation "electron 

 valve" is singularly fitting for this period. 



In the succeeding period, taking up much of the twenties and 

 thirties, the directed, rather than random, character of electron motion 

 in vacuum was applied in the cathode-ray tube and for improving the 

 efficiency of amplifier and oscillator tubes by proper shaping and align- 

 ing of electrodes. The electrostatic secondary-emission multiplier 

 phototube, in which secondary electrons are guided from one electrode 

 to the next by a careful adjustment of electrode shapes, is a typical 

 product of this epoch. 



The third period, beginning some time before the Second World 

 War, further subdivided the beams of electrons into groups and dealt 

 primarily with the latter. 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 ; the electron microscope and the image tube are 

 typical representatives of this group. A somewhat different form of 

 group selection, on the basis of time and place of injection, occurs in 

 the betatron. 



The fourth period in the development of electronics, in which we 

 find ourselves at present, is concerned with the control of electrons 

 within solids. Here, with crystal diodes, transistors, photoconductive 

 pick-up tubes, etc., we have barely made a beginning. However, in 

 large part with the aid of the typical products of the preceding periods, 

 which are at the same time undergoing continous further develop- 

 ment, this period should prove at least as productive as any of the 

 earlier ones. 



Let us visualize some of the trends in the evolution of the electronic 

 art in the near future. We may distinguish here between the develop- 

 ment of new devices and the extension of their practical application. 

 In the field of amplifying tubes and oscillators we can safely predict 

 that the stress will continue to be placed more and more on types suit- 

 able for the very-short-wavelength region of the radio spectrum, where 

 the time of travel of the electrons can no longer be neglected in com- 

 parison with a period of oscillation. In oscillators and narrow-band 

 amplifiers the tube geometry will, as now, for example, in magnetrons 

 and klystrons, be determined in large part by the frequency of the 

 oscillation generated or amplified. At the same time, wide-band 

 amplifying tubes will increasingly be provided by devices in which 

 energy is exchanged between electron streams and traveling wave 

 fields, as in the traveling-wave tube, the double-stream tube, and 

 others. Similar techniques of energy interchange, in reverse direction, 

 serve to provide high-energy electron beams in linear accelerators. 



