191 1.] OF ELECTRICITY AND MATTER. 329 



There are times in tlie history of science when various contra- 

 dictions require that the process of building stop until the most 

 fundamental concepts are reexamined. This was true in the his- 

 tory of astronomy at the time of Copernicus. The prevailing con- 

 ception of the earth as a fixed center about which all the other 

 bodies revolve had been practically sufficient for a long time, but 

 gradually difficulty after difficulty arose until it was no longer 

 possible to patch up the old theory to meet the accumulation of 

 stubborn facts. Only by a change in the most fundamental con- 

 ception, namely, that of the earth as a fixed center, could harmony 

 be brought out of chaos and a new period of development com- 

 menced. We seem to be passing through a somewhat similar 

 period in physics, and the " Principle of Relativity " contains the 

 modified concepts. 



By way of transition, let me make one or two statements at this 

 point about electric and magnetic systems in general. It can readily 

 be shown to follow from known electromagnetic laws that two 

 electric charges of the same sign moving side by side with the 

 same velocity repel each other less than when the two are stationary. 

 This is due largely to the fact that, when moving, each charge is 

 surrounded by a magnetic field and this magnetic field introduces 

 new forces. 



This simple statement introduces a far more general one, for it 

 may be shown that a steady motion of any electromagnetic system 

 so changes the force between the various parts of the system that it 

 tends to take up a new position of equilibrium. The forces arc such 

 that the icholc system tends to contract along the line of motion. 

 If it be allowed so to contract until it reaches this new position of 

 equilibrium, then everything will be as before the system was set in 

 motion, with two important exceptions, if the system has any in- 

 ternal motions caused by electromagnetic force. First, two mo- 

 tions, say the oscillation of two charges, one in the front of the 

 system and the other behind, which in a stationary system take 

 place simultaneously, will, in the moving system, take place not 

 quite simultaneously; for the forward one will be somewhat behind 

 in time; and, second, all such motions will take place a little slower 

 than they did in the stationary system. The term electromagnetic 



