L. Page — A Century's Progress in Physics. 337 



lines of force spread out radially and uniformly in all 

 directions. In fig. 2 the electron is supposed to have a 

 velocity v horizontally to the right of an amount smaller 

 than, though comparable with, the velocity of light c. 

 It is seen that the lines of electric force still diverge 

 radially from the charge, but are crowded in the equato- 

 rial plane and spread apart in the polar regions. The 

 dissymmetry grows as the velocity increases until if the 

 velocity of light should be reached the field would be 

 entirely concentrated in a plane at right angles to the 

 direction of motion. Now it may be shown that fig. 2 is 

 obtainable from fig. 1 by reducing dimensions in the 

 direction of motion in the ratio of 



i/l — . P* : *> where P = I • 



For a uniformly convected electric field differs from an 

 electrostatic field only in that the dimensions in the direc- 

 tion of motion are contracted in this particular ratio. 

 Fig. 3 represents the electric field of a charged particle 

 which has a uniform acceleration to the right. Consider 

 Faraday's analogy between lines of force and stretched 

 elastic bands. The symmetry of the first two figures 

 shows that in neither of these cases would there be a 

 resultant force on the charged particle. But in the third 

 figure it is obvious that a force to the left is exerted on 

 the charge by its own field. Calculation shows this force 

 to be proportional in magnitude to the acceleration. Let 

 it be postulated that the resultant force on a charged 

 particle is always zero. Then if F is the applied force, 

 the force on the particle due to the reaction of its field 

 will be — m f, where / stands for the acceleration and m 

 is a positive constant, and we have the fundamental 

 equation of dynamics 



Hence, instead of admitting Kelvin's contention that all 

 physical phenomena must be given a mechanical explana- 

 tion, it - would seem more logical to assert that electro- 

 dynamics actually underlies mechanics. 



Calculation shows the electromagnetic mass m to vary 

 inversely with the radius of the charged particle. Now 

 Thomson's experiments made it possible to calculate the 

 mass of an electron. Hence its radius can be computed, 

 and is found to be about 2(10)~ 13 part of a centimeter, or 



