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



are liberated, and their expulsion involves the recoil which has been measured in the 

 present experimental investigation. 



Under these conditions the mechanical effect would be least likely to be disturbed 

 by electric forces within the arc, because the oxygen atom approaching with two 

 positive charges would contribute to the attractive force upon the pole an amount 

 not very different from the repulsive force occasioned by the two receding electronic 

 charges. 



In a normal arc the effects at the anode are very complicated, there is electronic 

 projection due to thermionic and photoelectric action, and probably access of electrons 

 and negatively charged atoms which carry the current to it. Nevertheless, the 

 values of e/m obtained by the method already described is of the right order of 

 magnitude, though three times higher than it should be, if the recoil is in this case 

 also to i>e explained by the projection and impact of electrons and if they bear half 

 the current. If we could accept the view that the momentum of the electron derived 

 from the cathode is handed on through the vapour from atom to atom until it 

 reaches the anode, the discrepancy would be reduced. Elsewhere we have shown 

 that it is possible to reduce the carbon consumed by the anode to almost negligible 

 quantities, it would be interesting to determine the changes in the anode recoil under 

 these circumstances, but the experiments would be of very great difficulty. 



The writer tenders the above account of the mechanism of the arc with due 

 appreciation of the assumptions underlying it. As far as the details are concerned, 

 a great deal depends upon the accuracy of DUDDELL'S results, but any reasonable 

 assumption regarding the magnitude of the potential drop across the cathode pole 

 face would lead to a value for e/m which is of the order of magnitude of that of the 

 electron and far removed from that associated with atoms. If instead of assuming 

 random projection, we assumed normal projection from a small area on the cathode, 

 the values of c/m woiild be four times those given in Table X., and still in accord 

 with their electronic rather than their atomic nature. 



The view I have taken of the mechanism of the arc attributes the fall of 

 potential across the negative pole face to electronic projection there, contrary to the 

 theory which regards the electric force as responsible for the extraction of the 

 electron. POLLOCK,* assuming electronic projection, took the same view, and from 

 DUDDELL'S work calculated the velocities in different parts of the arc in an important 

 contribution to this subject. The discharge of electrons has frequently been assumed, 

 but I do not think that there has hitherto been any mechanical evidence in its 

 favour. 



Such action, photoelectric or thermionic, as occasions in the arc a discharge of 

 negative electrons from the poles is probably assisted by the chemical interactions 

 between the poles and the surrounding gas. This point has already been discussed 

 in the paper by the writer, to which reference has been made. 



* POLLOCK, 'Phil. Mag.,' vol. XVIL, p. 361, 1909. 



