196 JAMES CLKIIK MAXWELL 



consists of a tension along tho lines of force and an 

 equal pressure at right angles to them, the values 

 of tho tension and the pressure being each equal 

 to that of the magnetic energy per unit of volume, 

 or p. H 2 /8 TT. 



In a case in which both electric and magnetic 

 force exists, these t\vo states of stress are super- 

 posed. The total energy per unit of volume is 

 K H'/$TT 4- pH'/tiTT ; the total stress is made up 

 of tensions KR7 cS<7r an< ^ A A H~A* 7r nloiig the lines 

 of electric and magnetic force respectively, and equal 

 pressures at right angles to these lines. 



We see, then, from Maxwell's theory, that electric? 

 force produced at any given point in space is trans- 

 mitted from that point by the action of the ether. 

 The question suggests itself, Does the transmission 

 take time, and if so, does it proceed with a definite 

 velocity depending on the nature of the medium 

 through which the change is proceeding? 



According t< the molecular- vortex theory, wi 

 have seen that waves of electric force are transmitted 

 with a definite velocity. The more general theory 

 developed in the " Electricity and Magnetism" leads 

 to the same result. Electric force produced at any 

 point travels outwards from that point with a velocity 

 given by 1/v/K/i. At a distant point the force is 

 zero, until the disturbance reaches it. If the dis- 

 turbance last only for a limited interval, its etVects 

 will at any future time be confined to the space 

 within a spherical shell of constant thickness depend- 

 ing on the interval ; the radii of this shell increase 

 with uniform speed l/\/ K p.. 



