A CENTURY'S PROGRESS IN PHYSICS 375 



tion was forthcoming. It has been pointed out that a 

 uniformly convected electric field is derivable from an 

 electrostatic field by contracting dimensions in the direc- 

 tion of motion in the ratio 



Fitzgerald and Lorentz showed independently that if 

 moving matter is distorted in this same way the result 

 obtained by Michelson would be just that to be expected. 

 For then the distance of the mirror c from m would be 



instead of I, and the path of the ray moving parallel to 

 the earth's orbit 



which is just that of the other ray. Of course when the 

 apparatus is rotated through 90, the distance of this 

 mirror from m assumes its normal value again, and the 

 distance of the other mirror becomes shortened. As all 

 measurement consists in comparing the object to be 

 measured with a standard this contraction could never 

 be detected by experimental methods, for the measuring 

 rod would contract in exactly the same ratio as the body 

 to be measured. 



In computing its electromagnetic mass Abraham had 

 assumed the electron to be a uniformly charged rigid 

 sphere which keeps its spherical form no matter how 

 great a velocity it may be given. He found that the mass 

 increases with the speed at very high velocities, becom- 

 ing infinite as the velocity of light is approached, and 

 that its value depends upon the direction of the applied 

 force. After the Fitzgerald-Lorentz contraction was 

 seen to be necessary in order to explain Michelson 's 

 result, Lorentz calculated the electromagnetic mass of a 

 charged sphere which is deformed into an oblate spheroid 

 when set in motion. For this type of electron too, the 

 mass approaches infinity for velocities as great as that of 

 light, and is different for different directions. If a 

 force is applied in the direction of motion the inertia to 



