Potentials of Gases observed in Thermionic Valves. 421 



many valves which nominally contain helium, or nitrogen, 

 give ionization voltages of about 15 after they have been in 

 use for a short time. This indicates the presence of electrode 

 gas, consisting probably of a mixture of carbon compounds 

 and hydrogen, and shows that the bombardment of the anode 

 and grid during the evacuation has not been carried far 

 enough. 



(4) Corrections. — The values of the ionization potentials 

 obtained by the method here described require some cor- 

 rection, owing to the fact that the filament of a valve is not 

 all at the same potential, nor at the same temperature, whilst 

 correction is also necessary for the initial velocities of the 

 emitted electrons, and for any contact potential difference 

 which may exist between the filament and the collecting- 

 electrodes. As already remarked, the potential differences 

 recorded are between the anode and the negative end of the 

 filament, i. e., they are the maximum potential difference in 

 the valve. As a matter of fact the number of electrons which 

 fall through this maximum difference of potential is entirely 

 nei>"lio-ible, owing to the cooling- of the ends of the filament 

 by the leads. The magnitude of the cooling effect due to the 

 leads has been fully discussed by one of the writers in a 

 previous paper*; and it appears that if a point is taken on 

 the negarive limb of the filament such that the electron 

 emission from it (per cm.) is one-tenth of that from the 

 hottest part of the filament, then the potential difference 

 between this point and the negative end is about 0'3 volt for 

 filaments of the diameter used. The fraction 1/10 is of course 

 chosen arbitrarily, bat it probably gives the order of the 

 quantity to be subtracted from the measured voltage to allow 

 for the coolino- due to the leads. 



As regards the correction for the initial velocity of the 

 electrons, it has been shown f that the average kinetic energy 

 of the emitted electrons is equal to 2kT, where T is the 

 absolute temperature of the filament and k is Boltzmann's 

 constant. Now the average kinetic energy of the electrons 

 i vit /tin the filament is the same as that of a gas molecule at 

 the same temperature, viz. §&T. In the experiments here 

 considered the value of T for the central part of the filament 

 was about 2500° K., and at this temperature the value of 

 | /:T, expressed in equivalent volts, is about 0*33. Hence 2/tT 

 is equivalent to about 0'44 volt. At the point on the filament, 



* Stead, Journal of Inst. Elect. Engineers, vol. lviii. Jan. 1920, p. 107. 

 t O. W. Richardson, ' Emission of Electricity from Hot Bodies ' 

 pp. 140, 141. 



