638 



BELL SYSTEM TECHNICAL JOURNAL 



Equation (5) shows that as long as space charge is too small to 

 affect the flow of current, that is when i is equal to j, the mean square 

 shot voltage is directly proportional to the space current. As emission 

 is increased, however, space charge begins to control and finally limits 

 the space current so that the value bi dijdj approaches zero. Thus 

 the shot voltage increases less rapidly as space charge becomes effective 

 and then finally decreases rapidly toward zero as complete space 

 charge control is reached. 



Experimental curves showing the effect of space charge on tube 

 noise are shown in Fig. 1 where abscissae represent space current in 



1 2 3 4 5 6 7 



SPACE CURRENT IN MILLIAMPERES 



Fig. 1 — The effect of space charge on fluctuation noise. Three tubes_having 

 filaments composed of tungste-Xi, thoriated tungsten, and barium oxide. E''- is the 

 mean square noise voltage across the output measuring device expressed in arbitrary 

 units. The variation in space current was obtained by changing the cathode tem- 

 perature, the plate voltage remaining constant. 



