136 DR. DUFFIELD, MESSRS. BURNHAM AND DAVIS ON THE PRESSURE, ETC. 
loss of an atom of carbon from the cathode was associated with the transfer of four 
electronic charges between the poles, this favours the ejection of electrons from the 
atom itself rather than from the pole face considered as a whole, but the writer does 
not wish to rule the possibility of thermionic emission out of account. 
One point which emerges from the present research deserves mention. After the 
discovery of the recoil, and during the endeavour to find a means of disentangling it 
from the electromagnetic effect due to the rest of the circuit, it was suggested that 
the two might be identical, that is to say, that the mutual interactions between 
various parts of a circuit were occasioned by the mechanical effect of the flow of 
electrons through it. It seemed possible to find a plausible explanation of the motion 
of a movable wire in the plane of a circuit on this basis. The experiments described 
on pp. 123 and 124 showed that the two exist simultaneously, and that the electro¬ 
magnetic effect under the conditions of the experiment was about twice that observed 
for the recoil. Moreover, the rates of increase of the two with the current strength 
were different, a fact which effectively disposed of this idea. 
The writer has observed a similar recoil upon the suspended cathode within a 
highly exhausted vacuum tube, but the mechanical effect has not yet been 
measured. 
The experiments were conducted in the Physics Laboratory of University College, 
Reading, and valuable assistance was given by Mr. J. S. Burgess. Mr. Davis was 
in receipt of a Research Grant from the Committee of the Privy Council for Scientific 
and Industrial Research, to whom the thanks of the authors are accorded. 
2 7 DEC.1919 
