212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



accuracy of the electron charge value which had been used in the earlier 

 X-ray estimates. The difference was not large, only about 1 part in 

 200, but it meant either that experiments had not been as accurate as 

 believed or that there was some unrecognized complicating factor. 



So Millikan's work has been repeated in various laboratories with 

 refinements, such as the use of a remarkably nonvolatile oil for the 

 drop. But the chief error was found to lie in the measurements of the 

 viscosity of air. During the past year Kelletrop, of Uppsala, has thus 

 published a revised "oil-drop" determination of electronic charge as 

 4.800 X 10-1" E. S. U., which is in excellent agreement with the "X- 

 ray" determinations. Bcarden has just presented his own confirma- 

 tion of this agreement before the American Physical Society. 



It is an interesting couicidenco that this best value of the charge 

 of the electron is exactly the same as the figure given by Rutherford 

 30 years ago, though then determined with so much less precision that 

 not much confidence was placed in it, except as to order of magnitude. 

 It was then known that the alpha rays from rndium arc helium atoms 

 wliich have lost two electrons and are therefore doubly positively 

 charged. Rutherford caught a lot of these alpha rays in a metal 

 trap, measuring their aggregate electric charge with an electroscope, 

 and counting them by the scintillations which they produced on 

 striking a fiuorcscent screen or otherwise. Dividing the total charge 

 by the number gave him double the electronic charge, which he thus 

 calculated to bo 4.8 X 10"'° E. S. U. Already knowing the ratio of 

 charge to mass with high precision, this value of the charge enables 

 us to fix the electron's mass as 9.051 X 10"^^ grams. 



ELECTROMAGNETIC MASS 



Wlien we speak of the mass of an electron, however, wo enter a 

 whole new field of ideas. Some years before the discovery of electrons, 

 J. J. Thomson had pointed out that an electrified particle will possess 

 inertia, that is, mass, simply in virtue of its charge alone, irrespective 

 of whether or not it has any mass of the gravitational typo which we 

 have been accustomed to tliink of. This "electromagnetic" mass 

 comes about from the fact that any mechanical energy which is 

 expended in accelerating an electric charge is transformed into the 

 energy of the magnetic field surrounding the electrified particle in 

 virtue of its motion. In fact, the kinetic energy of a moving electric 

 charge is found to be simply the energy of its magnetic field and 

 depends only on the square of the velocity of the charge, the amount of 

 charge and the geometrical shape of the charge. 



Maldng the simplest possible assumptions about the shape of an 

 electron, such as a solid sphere or a hollow spherical shell of electricity, 

 and assuming all its mass to be of electromagnetic origin, the diameter 

 of an electron was calculated to be of the order of 10"*^ cm. It must 



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