504 Mr. Norman Campbell on the 



laws, will bold for all quiet systems wbich are not accelerated 

 relatively to that quiet system *. In particular tbe theory 

 expressed by the fundamental equations of the electron theory 

 has proved perfectly satisfactory so long as it is applied to a 

 quiet system. So long, that is, as all distances are measured 

 relatively to axes fixed in the earth, all times are measured 

 on clocks fixed in the earth, and the phenomona considered 

 are those of charged bodies fixed in the earth in magnetic 

 fields produced by instruments fixed in the earth, or those of 

 sources of light fixed in the earth observed by instruments 

 fixed in the earth, no conclusions have ever been obtained 

 which are not consistent with that theory. The Principle 

 asserts that, it' the whole system of axes, scales, clocks, 

 charges, magnets, light-sources, telescopes, and observers 

 were placed on a ship moving uniformly relative to the earth 

 and the experiments repeated, exactly the same relation 

 would be found between the quantities measured. 



This proposition is known as the First Postulate of 

 Relativity. The justification for it is that it is in accordance 

 with all known experimental facts : it is, moreover, directly 

 implicated in the usual formulation of the theory of 

 dynamics. 



Now it is the main object of the Principle of Relativity to 

 establish a connexion between the laws of a quiet system and 

 tho*e of a disturbed system. In order to establish such a 

 connexion Einstein shows that it is only necessary to introduce 

 a small number of additional fundamental propositions into 

 his theory, and he shows also — it is this which makes his 

 work so brilliantly ingenious — that the additional propositions 

 which must be introduced do not concern the laws of any 

 quiet system. Consequently, if the theory is true, it will tell 



* k. difficulty arises from the fact that no system where anything 

 u happens " can possibly be quiet, for almost all the changes which 

 physics investigates involve relative motion of the parts of the system 

 investigated. The logical questions raised by this difficulty will be 

 discussed more fully in another place : for the present we may regard 

 " laws for a quiet system " as propositions toward which the laws of 

 actual systems tend as the relative velocity of the parts of those systems 

 tends to zero. Thus the fundamental equations of the electron theorv, 

 involving the terms pv, cannot be applied to quiet systems, since they 

 consider the motion of an electron (which is part of the system) relative 

 to the rest of the system : they may be regarded as the limiting form of 

 the equations valid for disturbed systems, as the velocity v tends to zero. 

 There is no practical difficulty about this extrapolation, for the form of 

 laws is not found to change with the relative velocity of the parts of 

 disturbed systems over a large range limited, on the one hand, by a 

 relative velocity of about 10 6 cm. /sec, and, on the other, by the smallest 

 velocity which can be detected experimentally. The laws for a quiet 

 system are, then, the laws which hold within this range of velocities. 



