JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VIII JANUARY 4, 1918 No. 1 



PHYSICS. — The size and shape of the electron. Arthur H. 

 CoMPTON, Research Laboratory, Westinghouse Lamp Com- 

 pany. (Communicated by G. K. Burgess.) 



The radius of the electron is usually deduced from the energy 

 of the electron in motion, assuming its magnetic energy to be 

 identical with its kinetic energy. If the electron is a sphere, its 

 radius must be, according to this assumption, about 1 X 10 ~^' 

 cm. It is thus sufficiently small to act as a point charge of 

 electricity even with the shortest 7-rays. 



Calculating on the basis of such an electron, J. J. Thomson^ 

 has shown that the fraction of the energy of an electromagnetic 

 wave incident upon an electron which is scattered by it is given 

 by the expression 



Stt e^ 



This corresponds to a mass absorption coefficient due to a 

 scattering of the primary beam equal to 



where N is the number of electrons which contribute to the 

 scattering in a gram of the absorbing medium, C is the velocity 

 of light, and e and m have their usual significance. As Barkla 

 has pointed out, there may be absorption due to other causes, 



1 Thomson, J. J. Conduction of Electricity through Gases, 2d ed., p. 321. 



1 



