422 Messrs. Stead and Gosslino; on Relative Ionization 



& 



considered in the previous paragraph, where the electron 

 emission is one-tenth of that corresponding to the central 

 part of the filament, the temperature would be considerably 

 lower than 2500° K., and the correction would be reduced 

 from 044 volt to about 040 volt. This correction must be 

 added to the measured voltage, and it is seen to be almost 

 compensated for by the amount (0*3 volt) to be subtracted 

 on account of the non-uniformity of potential along the 

 filament. It is to be observed that the velocities of the 

 electrons emitted from a hot filament are distributed 

 according to Maxwell's law, and the correction of 0'4 volt 

 applies only to those electrons which are emitted with the 

 average velocity corresponding to the given filament tem- 

 perature. According to Maxwell's law of distribution an 

 appreciable number of electrons, some 10 per cent, in fact, 

 must come off with a velocity corresponding to at least one volt. 

 It is clear that both this effect and also the want of uniformity 

 of the potential of the filament must tend to make the position 

 of the critical point in the current-voltage curves less sharply 

 defined, and diminish the accuracy with which the ionization 

 potential can be determined. Goucher*has overcome one of 

 these difficulties by using as a source of electrons a platinum 

 thimble heated to the required temperature by the radiation 

 from a spiral of tungsten wire inside it, the tungsten wire 

 being itself heated by an electric current. Such a source 

 would not be satisfactory for the experiments here described 

 because only a comparatively low temperature of the platinum 

 is possible, and hence the electron emission is very small 

 compared with that here employed. 



The whole question of the contact difference of potential 

 between a hot filament and a cold anode in a moderate or 

 high vacuum appears to be obscure at the present time, and 

 much work remains to be done before the correction to be 

 applied to ionization potentials on this account can be com- 

 puted with any degree of confidence. It seems quite likely, 

 however, that a correction of the order of 0*5 volt may be 

 necessary f. 



(5) Discussion of Results. — All these experiments on 

 ionization potentials were carried out in May, June, and 

 July, 1917, with the exception of those on helium, which 

 were not undertaken until September 1917. Some of the 



* Physical Review, viii. p. 561 (1916). 



t Richardson, ' Emission of Electricity from Hot Bodies ' ; Stoekle, 

 Phys. Rev., viii. p. 534 (1916). 



