38 BELL SYSTEM TECHNICAL JOURNAL 



4. Temperature Saturation and Voltage Saturation. When a vacuum 

 tube operates at such filament temperature and E p that an increase in 

 temperature produces no increase in I p the tube is said to show temper- 

 ature saturation. On the other hand when temperature and E p are 

 such that an increase in E p does not increase I p , the tube shows 

 voltage saturation. Referring to Fig. 2 the curve T\ shows temperature 

 saturation between and A and approaches voltage saturation beyond 

 the point A. Similarly the curve T 2 shows temperature saturation 

 up to the point B and approaches voltage saturation beyond. 



Fig. 6 



5. Effect of Gas. In thermionic tubes as usually pumped the gas 

 pressure is between 10~ 5 and 10~ 6 mm. At this pressure the gas 

 generally does not manifest its presence in the operation of the tube. 

 However, at higher pressures, and particularly above 10~ 4 mm, it 

 produces certain objectionable disturbances. Thus many gases 

 seriously reduce the filament activity; also for E p greater than about 

 20 volts, ionization occurs and the resulting discharge differs in im- 

 portant respects u from the pure electron discharge of Fig. 4. 



II. Two-Electrode Tubes 



The two-electrode tube, which was first due to Edison, found an 

 early practical application when Fleming used it to detect wireless 

 telegraph signals. 



However, since the advent of the three-electrode tube of De Forest, 

 the earlier device has been almost entirely superseded as a detector 

 and finds its principal application as a rectifier of a.c. voltages. Its 

 range of applicability in this field is extremely large. With properly 

 designed tubes, Hull 12 has succeeded in rectifying 5 k.w. at a potential 

 of 100,000 volts, and in its transatlantic radio telephone experiments, 



11 See Van der Bijl: The Thermionic Vacuum Tube, p. 86. 



12 A. W. Hull, General Electric Review, Vol. 1 ( ), p. 177, 1916. A nor her good source 

 of information is Van der Bijl's "The Thermionic Vacuum Tube." 



