682 Sir J. J. Thomson on 



make the measurement of the mass of an electron, for 

 example, depend upon the position of our measuring instru- 

 ments. We may illustrate this point in the following way. 

 Suppose we have a region A in which all the atoms and 

 electrons were initially at rest relatively to each other. 

 Now suppose that under electrostatic attraction an electron 

 gets set in motion. From our point of view this means that 

 some of the mass particles which initially were remote from 

 the electron have come much closer to it; this will produce 

 an increase in its mass, and from the equations of electro- 

 dynamics we can calculate the ratio of the increased mass 

 to the mass of the electron when it started from rest ; we can 

 also, even if every constituent atom or electron of the- 

 system gets set in motion under the electrostatic attraction 

 and the mass of each is in consequence increased, oalculate- 

 the ratio of the increased mass of each constituent to its- 

 original mass. 



Suppose, however, that the whole region A gets set in 

 motion as a rigid body by the action of an external system B ;. 

 while the velocity of A is increasing the mass particles will 

 be streaming into it, and while this is going on it is possible 

 that the relative masses of the constituents of A may be 

 affected. But when the velocity of A has become steady 

 and there is no longer any influx of mass particles into it 

 from the outside, the particles which have come into it while 

 this state was being reached w ill distribute themselves so 

 that the number of new particles in any region is proportional 

 to the number that were present before the influx. Thus 

 the relative masses of two constituents of A, say an electron 

 and an atom, will be unaltered. Thus an observer in A wili 

 be unable to detect any effect due to a uniform motion of 

 translation of this region, for though the mass of one of the 

 constituents, as measured by the number of mass particles 

 associated with it, may be altered, the mass of the unit by 

 which that of the constituents is measured will be altered in 

 the same proportion, so that the alteration will not be 

 detected. The argument is the same as that which applies 

 to any changes which the motion may produce in the shape 

 or size of the constituents of the region A ; these escape 

 detection by an observer in A because his units are altered 

 in the same proportion as the quantities measured. If, how- 

 ever, we had any method of counting the mass particles- 

 within the region A, an observer in this region ought to be 

 able to detect an effect due to changes in the velocity of 

 translation. 



Again, if an observer in a region which did not 



