UNDER THE INFLUENCE OF CHEMICAL ACTION. 
11 
voltage is at present an entirely arbitrary matter, in default of any knowledge of the 
actual contact difference of potential Iv occurring in any single experiment. The 
displacements have, however, been so chosen as to bring the point A (fig. 4), formed by 
the intersection of a horizontal line through the standard saturation value unity and 
the dotted extension of the straight or slightly concave part FB of the characteristic, 
over the zero on the volt scale. 
That the point A will not be far from that which corresponds to the condition of zero 
electric field between the two electrodes, i.e., V + K = 0, might be anticipated on the 
following grounds. The data in fig. 3, and still more in fig. 4, indicate that there is no 
considerable direct effect of the applied field in helping the electron emission. In fact, 
I have frequently made explicit tests to see if the part DE of the curve slopes upwards, 
as appears to be suggested by the points in fig. 4, but I have never been able to convince 
myself that it does. Some experiments have given a small increase, some a small 
decrease, and others no change with rising voltage. This is a point which deserves 
further examination, but if there is any effect * of this kind it is certainly small. It 
is presumed, of course, that the voltages used in such tests are not such as to generate 
appreciable ionization in the surrounding gas. Inasmuch as the direct effect of the 
electric field is negligible, the curving of the part BCD of the characteristic is to be 
attributed to such secondary factors as the mutual repulsion of the electrons, electron 
reflection at the copper electrode, the return to the drop of some of the emitted electrons 
owing to collisions with gas molecules, and the effects of the holes in the copper sphere. 
Inasmuch as under ideal conditions in which the electron-emission or saturation current 
is very small, the vacuum is so high that gas collisions are negligible, there is no 
electron reflection at the receiving electrode, and the receiving electrode is a complete 
sphere, in the absence of electric field across the gap every electron once emitted from 
the drop will reach the copper sphere, because there is nothing to turn it back or to 
enable it to go elsewhere. Thus under such ideally perfect conditions the characteristic 
will consist of a flat saturation part AE intersecting with a part such as AFG, in which 
the emitted electrons are returned to the source by the opposition of the retarding field. 
This argument does not establish the coincidence of the point A with the zero potential 
difference, inasmuch as there is nothing a priori which enables us to foretell the shape 
of the part AFG. I have, however, succeeded in making independent experiments 
which determine, within limits, the contact difference of potential K and the charac¬ 
teristic curve for the chemical emission simultaneously. These experiments, which 
will be described later, fix the position of true zero volts somewhat to the left of the 
point A. 
Fig. 4 shows the results of displacing the data along the voltage axis in the manner 
I have explained. The magnitudes of the displacement, and certain details which may 
* For a case in which there is a definite indication of diminishing currents with rising voltage on this 
part of the curve, see p. 25 infra (fig. 12). 
f 1 5! 
W LJ 
