130 DR. W. G. DUFFIELD, MESSRS. THOS. H. BURNHAM AND A. H. DAVIS ON 



PART II. THE ORIGIN OF THE PRESSURE. 

 By Prof. W. G. DUFFIELD. 



In the foregoing pages experimental evidence has been adduced demonstrating 

 (1) the existence of a pressure upon the poles, (2) its variation with arc length, 

 (3) its variation with current strength. 



There is reasonably good agreement between the results obtained by different 

 methods in the various series of observations both for direct and alternating current. 

 In view of the delicate nature of the observations, it is satisfactory to find that the 

 rigorous examination given in the section upon the separation of the electromagnetic 

 and pressure effects confirms both qualitatively and quantitatively the existence of 

 the pressure. 



Assuming that the corrections for convexion currents have been made with approxi- 

 mate accuracy it remains to attempt to account for the reaction upon the poles. 



Radiometer Action and Evaporation. 



Before giving the evidence in favour of the reaction being occasioned by electronic 

 projection from the poles, it is necessary to consider whether it is explicable either by 

 radiometer action, produced by the departure with increased velocity of air molecules 

 after impact upon the hot poles, or by the evaporation of carbon atoms. 



Against the effect being due to either of these causes we have the experimental 

 evidence that, whereas for a constant current the reaction upon the cathode remains 

 constant or diminishes with increasing arc-length (figs. 10 and ll), the amount of 

 carbon lost from the poles increases rapidly with the arc-length* over the same range. 

 As the amount of carbon consumed depends upon the rate of evaporation or (and) 

 upon the access of air molecules to the poles, it would be expected that, if the 

 reaction depended upon either of these factors, it would increase with the consumption 

 of carbon, which is not the case. 



Furthermore, radiometer action is not usually appreciable at atmospheric pressure, 

 though, when the object has been of very small dimensions, it has been observed at 

 about 3 atmosphere. It may be argued that radiometer action is to be expected 

 because the pole face is curved and the intensity of the reaction against the air 

 molecules is not necessarily equal to the reaction upon the poles ; against this we 

 have experimental evidence that, starting with a flat pole face, the reaction upon the 

 pole became perceptibly less as it burnt to the usual curved form. 



Experiment 3, p. 125, shows that the hot pole is subject to a negative pressure when 

 the current is off, indicating that, if any radiometer effect exists, it is very small and 

 is masked by convexion effects. 



Again, if we assume that a pressure can arise from the expulsion of carbon atoms 



* " Consumption of Carbon in the Electric Arc," DUFFIELD, ' Roy. Soc. Proc.,' A, vol. 92, p. 122, 1915. 

 See Diagrams 1, 2, 4, 5. 



