32 BELL SYSTEM TECHNICAL JOURNAL 



w represents the energy lost by each electron as a result of becom- 

 ing free, X is a number which does not differ much from unity, and k 

 is the gas constant per molecule. Experiments show that the value 

 of X for a wide range of substances is about unity, but its exact value 

 is of little practical importance, since the variation of I s with T is 

 almost entirely controlled by the term in which Tenters as an exponent. 



The quantity w which expresses the electron affinity of the emitting 

 substance is Usually called the internal work of evaporation. In 

 Equation 1, it is in terms of ergs per electron. Calling v the value of 

 w when expressed in equivalent volts, w = 1.59 X10 -12 v . 



The term v is of great importance when considering the economy 

 with which a substance acts as a thermionic emitter. Assuming 

 that the emission of an electron occurs when its velocity acquires a 

 value sufficiently high to overcome the potential drop v , it is apparent 

 that the smaller v , the more copious will be the thermionic emission 

 at any given temperature. For the substances thus far examined, 

 v ranges between about two volts and five volts. 3 The two sub- 

 stances whose thermionic properties we shall consider particularly are 

 platinum, coated with a mixture of barium and strontium oxides, and 

 tungsten. For tungsten the value of v is approximately 5 volts, 

 and for coated platinum it varies between 1.67 and 2.05 volts. The 

 value of (v ) A — {v ) B for two substances A and B is equal (except for 

 a small term expressing the Peltier coefficient) to their contact dif- 

 ference of potential. 4 



2. Thermionic Properties of Filaments. In designing electron tubes 

 with predetermined characteristics knowledge of the thermionic 

 emissivity of the proposed filament is necessary. This property may 

 be conveniently represented by curves of the type shown in Figs. 1, 

 2 and 3. The coordinates have been so proportioned 5 that, provided 

 the electronic emission varies with the temperature as indicated by 

 Equation 1 and the thermal radiation from the filament varies as the 

 fourth power of the temperature, then the relation between the 

 thermionic emission and the heating power supplied to the filament 

 is a straight line. 



Fig. 1 gives data for tungsten and for coated platinum filaments, 

 Fig. 2 compares thoriated tungsten filament with pure tungsten and 

 Fig. 3 gives data relating to a special coated filament, the core of which 

 consists mainly of platinum alloyed with about 5% of nickel. Since 



3 For a Table of Values of v for the materials commonly used see Van der Bijl — 

 Thermionic Vacuum Tube, page 29; also Langmuir, Trans. Am. Electro-cherm 

 Soc. 29,166, 1916. 



4 Richardson, Electron Theory of Matter, 1916, p. 455. 



5 See Van der Bijl, The Thermionic Vacuum Tube, p. 82. 



