CH. L] CHARGE IN MOTION 3 



both ideas and equations. Whenever attention to 

 them is unnecessary, or their location immaterial, this 

 specification is avoided by treating them practically 

 as if they were at infinity, that is by ignoring them. 

 Every now and then this policy of ignoration must 

 be suspended, but for a multiplicity of purposes it 

 serves. 



Charge in uniform motion. 



Now consider how far this field of force belongs to 

 the body, and how far it belongs to space, that is to 

 the ether surrounding the body. The body is the 

 nucleus whence the lines radiate, but the lines them- 

 selves, the state of tension and other properties which 

 they represent and map out, do not belong to the 

 body at all ; at each point of space there is a peculiar 

 etherial condition called an electric potential, and this 

 potential represents something "occurring in the ether 

 and in the ether alone, though it is originated and 

 maintained by the body. 



Picture in the mind's eye such a charged body, say 

 a charged sphere, and let it change its position ; how 

 are we to regard the effect of the displacement on its 

 field of force ? Few things in physics are more certain 

 than this, that when a body moves along, the ether 

 in its neighbourhood is not dragged with it, as if it 

 were in the slightest degree viscous.* The ether, in 

 fact, as a whole i.e. when unmodified or in its 

 normal condition is stationary : it is susceptible to 

 strain, but not to motion ; it is the receptacle of 

 potential, not of locomotive kinetic energy. The only 

 generated motion to which it is possibly susceptible is 

 of what is called ' irrotational ' character in other 

 words it behaves as a perfect fluid. It may possess 



* Lodge, Phil. Trans. 1893, p. 727, and especially 1897, p. 149. 



