36 



BELL SYSTEM TECHNICAL JOURNAL 



appear later, be appreciably less than the saturation current. The dif- 

 ference between the saturation current and the maximum operating 

 space current varies with the duty to which the tube is assigned. In 

 the case of high voltage rectifiers, the space current may at certain 

 points in the cycle reach the saturation value, while in a tube which is 

 used as an amplifier it is often desirable, in order to avoid distortion, to 

 have the total emission two to three times as great as the maximum 

 working space current. 



3. Space Current-Voltage Characteristic. Experiment shows that 

 in a vacuum tube containing an emitting electrode and a conveniently 

 placed anode, the space current I p , varies with the temperature of 

 the emitter and the anode potential E p , as in Fig. 4. The three curves 

 shown are for three temperatures such that Ti<T2<T 3 . It will be 

 observed that between points and A the three curves coincide; 



Anode Potential 

 Fig. 4 



between and B the curves for the two higher temperatures coincide. 

 The saturation values of the filament emission at the various temper- 

 atures are shown by I s . 



For values of T P ranging from zero to somewhat less than I s , the 

 relation between I P and E P may be expressed with a fair degree of 

 accuracy by I p = kEJ in which the exponent v does not differ greatly 

 from 3/2. This relation, therefore, is frequently known as the 3/2 

 power law. It has been deduced theoretically by Child 7 and Lang- 

 muir 8 and has been studied lately in greater detail by Fry. 9 Fry's 

 analysis takes account of the initial velocities of emission of electrons, 



7 Phys. Rev., Vol. 32, p. 498, 1911. 



*Phys. Rev. (2), Vol. 2, p. 450, 1913. 



•T. C. Fry, Phys. Rev., Vol. 17, p. 441, 1921. 



