PHYSICS: D. L. WEBSTER 
163 
unpublished work by Goodwin and Wilson of the Massachusetts Institute of 
Technology. In figure 2 the solid hne shows the experimentally determined 
variation with the pressure of the overvoltage of nickel. Similar curves 
were found for mercury and lead. The dotted line shows the variation as 
calculated by Equation 2, using the overvoltage at one atmosphere as a 
basis for computing the values for the other pressures. The difference be- 
tween these two curves may be explained by an increase of stirring at the 
lower pressures, since many more bubbles are produced per mol of gas. 
It appears quite probable, then, that the factor that determines the overvolt- 
age of an electrode at any one pressure is the size of the gaseous nuclei that 
can cling to it. A number of observers have called attention to the fact that 
electrodes with low overvoltages are those that have large adsorptive powers. 
This adsorptive power is undoubtedly related to the attraction of an electrode 
for a gaseous nucleus. 
AN APPROXIMATE LAW OF ENERGY DISTRIBUTION IN THE 
GENERAL X-RAY SPECTRUM 
By David L. Webster 
Department of Physics, Massachusetts Institute of Technology 
Communicated by E. H. Hall, April 9, 1919 
In the spectra of X-rays as ordinarily determined there are factors of ab- 
sorption in the anticathode, the glass of the tube, the reflecting crystal and 
the ionized gas, and of efficiency of reflection that are all functions of the fre- 
quency. Fortunately, except at the discontinuities of any of these absorptions, 
the unknown factors vary continuously with frequency, so that the measured 
intensities in the spectrum represent the energy distribution qualitatively, but 
by no means quantitatively. The problem of the present paper is to com- 
bine other available data in such a way as to find an approximate law of 
energy distribution, not involving unknown absorption factors, and avoiding 
also any a priori assumptions about the emitting mechanism. The data are 
incomplete and this work is merely a first approximation. 
For data we have (a) some graphs of intensity against potential at con- 
stant frequency (where the unknown factors are all constant in each graph), and 
(b) the total energy measurements by Beatty,^ who made the absorption negli- 
gible by using a thin window and no crystal. Some of the intensity-potential 
graphs were obtained in the course of experiments for another purpose with a 
rhodium target by the author,^ and with platinum by the author and Dr. H. 
Clark,^ and others were obtained by taking points at the same wave length 
from intensity-wave length graphs drawn for tungsten by A. W. Hull,^ Hull 
and Rice^ and Ulrey,^ and for molybdenum by Hull. 
In the experiments on rhodium and platinum, the spectrometer was kept 
at a fixed wave length and the potential was changed between readings. 
