Closing Years of the Nineteenth Century. 437 



In the general case, it is only necessary to replace u by the 

 component of velocity of the electric charge in the direction of 

 the radius vector from the point at which the potential is to be 

 computed. This component may be written v cos (v . r), where r 

 is measured positively from the point in question to the charge, 

 and v denotes the velocity of the charge. Thus 



cde' = {c + v cos (v . r) } ~p dx'dy'dz', 

 and therefore 



f de' 

 }cr + (r.v)' 



where the integration is extended over all the charges in the 

 field, and the bars over the letters imply that the position of 

 the charge considered is that which it occupied at the instant 

 t - r/c. In the same way the vector- potential may be shown to 

 have the value 



r vde' 

 J cr + (r . v 



Meanwhile the unsettled problem of the relative motion of 

 earth and aether was provoking a fresh series of experimental 

 investigations. The most interesting of these was due to 

 Fitz Gerald,* who shortly before his death in February, 1901, 

 commenced to examine the phenomena manifested by a 

 charged electrical condenser, as it is carried through space in 

 consequence of the terrestrial motion. On the assumption 

 that a moving charge develops a magnetic field, there will be 

 associated with the condenser a magnetic force at right angles 

 to the lines of electric force and to the direction of the 

 motion: magnetic energy must therefore be stored in the 

 medium, when the plane of the condenser includes the direc- 

 tion of the drift; but when the plane of the condenser is at 

 right angles to the terrestrial motion, the effects of the 

 opposite charges neutralize each other. Fitz Gerald's original 

 idea was that, in order to supply the magnetic energy, there 

 must be a mechanical drag on the condenser at the moment of 



* Fitz Gerald's Scientific Writings, p. 557. 



