350 BELL SYSTEM TECHNICAL JOURNAL 



the abscissa axis. The hne for P = appears on the lower curve as the 

 dotted hne s. Here the ordinate is the total velocity squared, normalized 

 with respect to the value without oscillation. Efficiencies can therefore be 

 put on the plot directly as shown by the right-hand scale in per cent. The 

 line 5 is therefore a plot of the maximum possible efficiency. This refers 

 to what we might call the electronic efficiency since no account is taken of 

 circuit losses. Now in any physical device there are some circuit losses and 

 hence a lower value of electronic efficiency for which sustained oscillations 

 are not possible. The dotted line p is Posthumus' experimental value for 

 this lower limit. Between the lines p and s, then, oscillations are possible 

 at frequencies given by the abscissae and with field values shown on the 

 solid lines. Actual data for an experimental tube are shown on the plot, 

 oscillations occurring at the wavelengths indicated and over the ranges in 

 field shown by the lines terminating in arrows. 



One additional line t is shown on the plot connecting points on the different 

 Z lines for which the efficiency is a maximum. The optimum design would 

 be one based on the intersection of this line with the p line. Still other facts 

 will appear from a detailed study of these results but we shall not be able 

 to devote any more time to this interesting subject. 



Conclusion 



In concluding a talk of this sort and particularly in concluding a series of 

 talks, it is usually appropriate to look ahead to the future and predict the 

 trend of aflfairs, or perhaps to point out certain fruitful fields of research. 

 I find this a singularly difficult thing to do. However, it is not revealing 

 any military secrets to say that much of the progress of the last few years 

 has been in the direction of making things work and not toward getting a 

 clearer understanding of the underlying theory. If, for example, an il- 

 luminating approach could be devised which would make the problems 

 associated with transverse fields, both electric and magnetic, appear as 

 simple and straightforward as do longitudinal-electric-field problems, as a 

 result of the velocity-modulation concept, then I believe even more striking 

 advances could be made in the ultra-high-frequency field than those which 

 the war years have brought forth. 



Selected Bibliography 

 A. Papers of a Historical or Review Nature 



H. Backhausen and K. Kurz, "The Shortest Waves Obtained with Vacuum Tubes", Phys. 



Zeits, vol. 21, p. 1 (1920). 

 E. W. B. Gill and G. H. Morrell, "Short Electric Waves Obtained by Valves", Phil. Mag., 



vol. 44, p. 161 (1922). 

 A. W. Hull, "The Effect of a Uniform Magnetic Field on the Motion of Electrons between 



Coaxial Cylinders", Phys. Rev., vol. 18, p. 31 (1921). 

 E. Habann, "A New Vacuum Tube Generator", Zeits f. Hochfreq., vol. 24, p. 115 (1924). 



