THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 133 



molecules which they replace), the value of e/m appropriate to these conditions is 

 TGxlO 7 E.M.U. which approximates closely to the value for the electron derived 

 from other methods, namely, 177 x 10 7 E.M.U. 



The corresponding velocity of projection is 1'4 x 10 8 cm. per second, which is of the 

 order of magnitude to be expected if the emission is due to photoelectric action, 

 though higher than the velocities measured by LENARD from carbon plates. But in 

 the arc the proximity of the luminous vapour to the poles enables light of very 

 short wave-length to reach them, so a correspondingly high electronic velocity is to 

 be expected. Moreover, the condition of the pole, its high temperature, boiling and 

 intense incandescence, are favourable for the liberation of the corpuscles with the 

 minimum loss of energy, indeed, it may be that it is the undiminished momentum 

 of the electron as it leaves the atom which has been measured. If the arguments 

 are sound the experiments constitute the measurement of quanta by g, direct 

 mechanical method. 



The kinetic energy of the electron as it leaves the cathode is given by ^mv 2 , which, 

 from the data of the present set of experiments, amounts to 8'6xl()~ 12 erg. 

 Assuming that this is due to photoelectric action, and taking the radiation 

 constant h as 6'55x 10~ 27 erg seconds, we find X, the mean wave-length of the light 

 emitted by carbon vapour, which may be regarded as effective in promoting the 

 emission, to be l'22xlO~ 6 cm.; this is a reasonable result as it is smaller than the 

 threshold wave-length for soot given by HUGHES as 2'6xlO~ 5 cm. We note that 

 the electronic energy is less on emission than the amount 5'5xlO~ n ergs, which is 

 the minimum required to produce ionization (RUTHERFORD), but in the arc the further 

 fall of potential beyond the negative pole face rapidly increases the velocity and 

 therefore the kinetic energy of the corpuscle. 



The Mechanism of' the Arc. 



It is clear that the balance of evidence favours the conclusion that the particles 

 responsible for the recoil are electrons. It is doubtful if we can press our results 

 much further than this in view of the very small forces to be measured and the 

 complex conditions under which experiments of this nature must be conducted, but 

 the view of the mechanism of the arc which is most favoured by this research (indeed 

 the agreement with it is remarkable, though it may be accidental) is that an oxygen 

 atom arrives at the cathode with two positive charges of electronic magnitude, and 

 that uncharged CO is formed which removes two of the four electrons, which we 

 have already shown to be associated with the departure of each carbon atom from 

 this pole,* and which are derived ultimately from the source of current supply. The 

 oxygen atoms on arrival arid departure contribute no more to the pressure than do the 

 air molecules on the other side of the suspended pole. The remaining two electrons 



* DUFFIELD, loc. nt. 

 VOL. COXX. A. U 



