40 BELL SYSTEM TECHNICAL JOURNAL 



plications at the present time and a complete account of its proper- 

 ties is to be found in recent literature. 15 



No definite statement can be made as to the exact range of fre- 

 quencies over which the rectification of alternating currents can be 

 satisfactorily accomplished by means of thermionic tubes, but it is 

 apparent that this range is large. The degree of smoothness required 

 in the d.c. output is of primary importance in setting the lower limit 

 of the frequency range; on the other hand, the smaller the load re- 

 sistance, the higher the frequency which may be satisfactorily rectified 

 before the internal capacity of the tubes permits the flow of an objec- 

 tionable amount of alternating current. Whenever an output with a 

 minimum of ripple is required it is in general desirable to use as high 

 a frequency as is readily available. 



III. Three-Electrode Tubes 



In 1906 De Forest brought out the three-electrode tube 16 in which 

 a grid is interposed between filament and plate. Since the introduc- 

 tion of this tube, much study has been devoted to its properties and 

 many investigations have been made concerning the best substances 

 to use as thermionic emitters, the best metals for plates and grids, 

 the best varieties of glass for the containing bulbs, 17 and the best 

 methods of exhaustion, 18 so that today problems of design are well 

 understood. At present the three electrode tube finds use as a rec- 

 tifier, amplifier of small currents and voltages, detector of small a.c. 

 voltages, modulator of alternating currents, and generator of electric 

 oscillations. Tubes have been built which range in size from those 

 about one inch long with a space current of about a milliampere to 

 others which are water-cooled and have an individual output capacity 

 of 100 k.w. 19 Amplifiers with a capacity of 150 k.w. are in operation 

 (see footnote 13). 



7. Action of the Grid. The general theory of the grid action is 

 simple. As pointed out by Fry 19 the space charge creates a region of 



15 G. A. Campbell, Bell System Technical Journal, Nov., 1922; U. S. Patents 

 1,237,113 and 1,237,114, May 22, 1917; O. J. Zobel, Bell System Technical Journal, 

 Jan., 1923; Carson and Zobel, Bell System Technical Journal, July, 1923; G. W. Pierce, 

 Electric Oscillations and Electric Waves, p. 186, 1920; Karl W. Wagner, Arch. f. 

 Electr., Vol. 3, p. 315, 1915; Vol. 8, p. 61, 1919. 



16 Variously called the audion, vacuum tube, triode, pliotron, etc. 



17 Measurements of Gases Evolved from Glasses of Known Chemical Composi- 

 tion — Harris & Schumacher, Jour. Ind. & Eng. Chem., Feb., 1923; also Bell System 

 Technical Journal, Jan., 1923. 



18 For methods of exhaustion, see Dushman, Gen. Elect. Review, Vol. 23, p. 493, 

 1920, et seq. 



19 See W. Wilson, Bell System Technical Journal, July, 1922. 

 19 T. C. Fry, 1. c. 



