DETERMINATION OP THE CHARGE OP AN ELECTRON BY WILSON’S 
METHOD, USING RADIUM. 
BY L. BEGEMAN. 
The following discussion of the experimental determination of the charge 
e of an electron may prove interesting inasmuch as the conditions of the ex- 
periment were in many respects more favorable than those of previous attempts. 
As is generally known, determinations of e have been made by J. J. Thomson 
and H. A. Wilson at the Cavendish-laboratory, England. Their methods were 
quite different, although both obtained their data from observations of ionized 
clouds produced by the rapid expansion of supersaturated air in a fog chamber. 
A description of the apparatus employed by J. J. Thomson will be found 
in his work entitled “Conductivity of Electricity Thru Gases,” 1906 edition; 
also in the Phil. Mag., Vol. 5, page 346. To secure a constant source of radia- 
tion J. J. Thomson used radium instead of X-rays. The radium was suspended 
at a given distance above the fog chamber and maintained continuously in the 
same position during the observations. C. T. R. Wilson’s device for rapid ex- 
pansions was used to produce the clouds in the fog chamber. The clouds were 
produced between two plane electrodes maintained at a constant difference of 
potential. Now if n is the number of droplets in the cloud and e the charge 
on each, then the total charge of the cloud will be n e. Purthermore, if % is 
the mean velocity of the positive and negative ions in a given electric field, then 
the current thru unit area of the ionized gas in the field will he n e u. Hence, it 
is only necessary to measure the current u to determine the value of n e. Thom* 
son used the method of Zeleny and Rutherford to get the value ot i. e., the 
velocity of ions in a field of known strength. The method of C. T. R. Wilson 
was used to determine the number of particles of water vapor in a unit volume 
of the cloud. Knowing the current velocity and the number of particles it Is 
a simple calculation to determine e the charge of an electron. J. J. Thomson’s 
average determination gives the value of e as 3.4x10-10 E. S. units. 
In the light of more recent investigations of ionized cloud phenomena, J. J. 
Thomson’s determination is certainly open to serious criticism. For instance, 
he states that when the expansion is greater than 1.31, positive as well as nega- 
tive ions are caught, and that the number is about twice as many as is obtained 
at expansions varying from 1.27 to 1.29. He says also that when the expansion 
is greater than 1.33, the number of nuclei caught by the cloud does not depend 
upon the amount of the expansion. We understand that Thomson’s observations 
were all taken at an expansion of about 1.33, where he assumed that every 
droplet in the cloud carried but one ion either positive or negative. In this 
assumption we see the probable error of his work. 
According to Carl Barus in the May publication of the Carnegie Institute 
entitled “The Condensation of Vapor Induced by Nuclei and Ions,” the nuclea- 
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