412 BELL SYSTEM TECIISICAL JOIRXAL 



solid particle, and the amount of its charge determined. This amount 

 is 4.774.10"'° in electrostatic units, according to Millikan. It is 

 designated by the symbol e. When a magnetic field is applied to a 

 stream of electrons all moving with the same speed, the electrons are 

 deflected all to the same degree, which shows that they all have the 

 same mass. This mass is practically equal to 9.10 "* in grammes, 

 unless the electron is moving at a very uncommonly high speed, in 

 which case it is appreciably greater. 



These facts of experience are about all that is definitely known 

 or needs to be known about the electron, in order to appreciate its 

 role in modern atomic theory. There is no good way of determining 

 its size, although the length of its mean free path in certain gases 

 indicates, perhaps definitely pro\es, that it is much smaller than 

 an atom. If the electron is a spherule of negative electricity uni- 

 formly dense, then its radius cannot be less than 2.10"" cm, for if 

 it were, the electromagnetic mass of the spherule would exceed the 

 observed mass of the electron.' This size is much smaller than the 

 one which it is expedient to attribute to the atom, happily for us, 

 since otherwise it would be difficult to conceive of atoms containing 

 electrons. 



Since electrons can be coaxed or forced out of substances of every 

 kind — elements and compounds, metals and non-metals, liquids and 

 solids and gases — the atoms are supposed to contain one or more 

 electrons apiece. This argument was formerly fortified !)>■ the fact 

 that the light emitted from glowing gases is in many respects such as 

 oscillating electrons would emit. This second argument is for the 

 present under a cloud. ^ 



' This is a short way of saying that, if the electron were a particle of smaller radius 

 than 2.10~" cm., more energy would have to be supplied to it to increase its speed 

 than is actually required. For, in order to set an electrified particle into motion, 

 energy must l)e supplied to build up the magnetic field which surrounds a moving 

 electric charge; this energy V is additional to the kinetic energy \mv- recjuired to 

 set the mass m associated with the charge into motion with s|>ecd f, and it may be 

 regarded as the kinetic energy associated with an extra "electromagnetic" mass 

 2U/v' which the particle possesses by virtue of its charge. This (piantity 21' v'' 

 can Ik; calculated, for a given size and shaiie of the electron; if we make the electron 

 too small, 21' v' conies out larger than its observed mass, which is a reduclio ad 

 ahsurdiim. This illustrates the rather surprising fact that we are not permitted 

 to imagine the electron as an infinitely small particle, a mere geometrical point 

 loaded with an infinitely concentrated charge and mass. Speculations about its 

 size and sha|K- and the distribution of charge within it are not nccessiirily trivial; 

 some may even lie verifiable. W'c also meet with this dilemma: how dt)es the elec- 

 tron, a piece of negative electricity of which each part should repel every other, 

 keep from exphxling.'' 



* I'erhaps I ought to mention that K. Khrenhaft of Vienna has been ardently 

 contending for about fifteen years that there is no such thing as an electron. He 

 maintains that he can demonstrate negative charges much smaller in amount than 



