A CENTURY'S PROGRESS IN PHYSICS 371 



obtainable from fig. 1 by reducing dimensions in the 

 direction of motion in the ratio of 



4/1 ft* : 1, where H -. 



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 



F m/ = 



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 

 one fifty-thousandth part of the radius of the atom. 

 Since numbers so small convey little meaning, consider 

 the following illustration, due, in part, to Kelvin. 

 Imagine a single drop of water to be magnified until it is 

 as large as the earth. The individual atoms would then 

 have the size of baseballs. Now magnify one of these 

 atoms until it is comparable in size with St. Peter's 

 cathedral at Rome. The electrons within the atom would 

 appear as a few grains of sand scattered about the nave. 



