THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 
I 33 
molecules which they replace), the value of e/m appropriate to these conditions is 
1-6x10^ E.M.U. which approximates closely to the value for the electron derived 
from other methods, namely, 177 x 10^ E.M.U. 
The corresponding velocity of projection is l'4x 10* cm. per second, which is of the 
ordei 01 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 a direct 
mechanical method. 
The kinetic energy of the electron as it leaves the cathode is given by which, 
from the data of the present set of experiments, amounts to 8’6xl0~’^ erg. 
Assuming that this is due to photoelectric action, and taking the radiation 
constant h as 6'55 x 10“^^ erg seconds, we find X, the mean wave-leimth of the lin-ht 
emitted by carbon vapour, which may be regarded as effective in promoting the 
emission, to be r22 x 10“'^ cm.; this is a reasonable result as it is smaller than the 
threshold wave-length for soot given by Hughes as 2'6xl0“®cm. We note that 
the electronic energy is less on emission than the amount 5-5x10-“ 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 and departure contribute no more to tbe pressure than do the 
air molecules on the other side of the suspended pole. The remaining two electrons 
* Duffield, loc. cit. 
VOL. CCXX.-A. TT 
