THERMIONIC VACUUM TUBES 



43 



9. Grid Current. For certain purposes, a consideration of the grid 

 current I g , is necessary. Fig. 12 represents a characteristic relation 

 between I g and E g for various E p . Note that E g in excess of about 

 10 volts results in secondary emission of electrons from the grid. 

 These secondary electrons flow to the plate and, as shown, their num- 

 ber may actually exceed the total number of primary electrons striking 

 the grid. The character of the grid surface plays an important part 



?5 30 35 ao 



Fig. 12 



in determining the amount of secondary emission. The secondary 

 emission from the grid of a tube containing a pure tungsten filament 

 is, in general, less than that from the grid of a tube with an oxide- 

 coated filament. At high temperature a coated filament appears 

 to evaporate a minute amount of its coating, some of which is de- 

 posited upon the grid presumably augmenting the secondary activity. 23 



10. Vacuum Tube Constants. The two most important constants 

 of the three electrode tube are /jl and its internal resistance r p . The 

 determination of u and r P from the characteristic curves (Figs. 10, 11) 

 is obvious. For general design purposes these curves give the best 

 insight into the behavior of a tube and furnish the most instructive 

 means of determining ^ and r p . 



11. Dynamic Methods of Measuring Vacuum Tube Constants. How- 

 ever, in cases where many tubes, all practically alike, have to be 

 tested, certain "dynamic" methods are timesavers. Several such 

 methods have been devised, but all are modifications of a scheme 

 first published by Miller. 24 



23 A. W. Hull has designed two types of tube known as the dynatron and plio- 

 dynatron which utilize the negative resistance characteristic (AB of Fig. 11) resulting 

 from secondary emission. Procd. Inst. Radio Engrs., Vol. 6, p. 5, 1918. 



24 J. M. Miller, Proc. I. R. E., Vol. 6, p. 141, 1918. For variations of Miller's 

 dynamic method the reader is referred to Van der Bijl, 1. c, p. 198, Method of G. H. 

 Stevenson. 



