THERMIONIC VACUUM TUBES 



35 



stant temperature, the life would be somewhat longer; furthermore, 

 the larger the wire the longer the life for a given temperature. The 

 activity of thoriated filaments tends to diminish with use but may 



f%~~ WATTS x 10 '° 

 POWER-EMISSION CHART 



Fig. 3 



be re-activated by heating to a temperature higher than the normal 

 operating temperature. The useful life of these filaments is, there- 

 fore, somewhat indefinite but taking the possibility of re-activation 

 into account, it is probably well in excess of 2,000 hours. 



In using Figs. 1, 2, 3, it should be borne in mind that the emission 

 values given represent saturation currents and that in general the 

 normal operating space current in a tube must, for reasons which will 



