132 DR. W. G. DTJFFIELD, MESSRS. THOS. H. BURNHAM AND A. H. DAVIS ON 



Assuming in the first instance that each particle carries a single electronic charge, 

 e, and that electrons alone are responsible for carrying the current between the poles, 

 the current C in absolute units is given by 



C = ne ............. (2) 



The potential drop, V, across the pole face is, on this assumption, due to the 

 projection of these particles, whence their kinetic energy is equal to that derived 

 from the source of current supply, and we have 



VC = %mnv 2 ............ (3) 



From (1) and (3) 



VC 



v = - > 



P 



whence from (l) and (2) 



= G!' = 



m 2p p 



DUDDELL* has found the values of V at the anode and cathode of an arc 6 mm. 

 long carrying a current of 10 amperes to be 1G7 and 6'1 volts respectively, both 

 being electromotive forces acting towards the poles. These are electromotive forces 

 across the pole faces and are distinct from those within the vapour in the arc-gap. 



We have already set forth in Table X. the values of p for a similar arc employed 

 in this series of experiments, and, by substitution in the above formulae, we derive 

 the values of e/m and of v, which are recorded in the final column of the same table ; 

 for the cathode the mean value of e/m is 6'4x 10 7 E.M.U. The values of e/m for 

 electrons and for hydrogen atoms are 177 xlO 7 and 9'58 x 10 3 E.M.U. respectively, 

 and, if carbon is quadrivalent, c/m for that element is 3'2xl0 3 . Even without 

 further refinement of our assumptions, the experimental evidence is overwhelmingly 

 in favour of the projection responsible for the reaction being electronic rather than 

 molecular. 



We expect to find electronic projection from the poles of an arc, because the 

 intense heat may occasion thermionic action, and the richness of the arc light in 

 waves of short length is favourable for photoelectric action. 



Electronic emission is thus in accord with expectation, but it is at first sight 

 surprising that it should be capable of producing a measurable recoil. 



It has been assumed in the foregoing that electrons carry the whole of the current, 

 but the case for an atomic drift of positively charged atoms on to the cathode has 

 already been considered by the writer (see " The Consumption of Carbon in the 

 Electric Arc No. 1," ' Eoy. Soc. Proc.,' A, vol. 92, p. 122 (1915)). If we assume 

 that half the current in the neighbourhood of the cathode is carried by such atoms 

 (which may be supposed to contribute no more to the pressure than do the gas 



* DUDDELL, ' Phil. Trans. Roy. Soc.,' A, vol. 203, p. 305, 1904. 



