CH. i.] ACCELERATED CHARGE 9 



of induction the generation of an induced E.M.F., 

 of value at any point equal to the rate of change 

 of the lines of magnetic force there. This is what 

 happens whenever an electric charge is accelerated ; 

 for then its magnetic field which, as we have seen, 

 depends upon the velocity necessarily changes in 

 strength, and so an E.M.F. is induced. There being 

 no conductor, this E.M.F. will propel no current, but it 

 will represent an electric force which was not there 

 before, and the new force will be in a new direc- 

 tion ; the direction of an induced electric force is 

 perpendicular to the direction in which the grow- 

 ing magnetic lines are moving, which in the 

 present case is outwards from the charge. Con- 

 sequently the new or induced E.M.F. points in the 

 direction of motion, though in the sense opposed 

 to any change in it ; and the effect of the super- 

 position of this new E.M.F. upon the already existing 

 magnetic field is to cause a certain small transmission . 

 of energy in a radial direction out and away from 

 the accelerated charge. Some energy therefore flashes 

 away with the speed of light ; and although in 

 ordinary cases it may be an exceedingly small amount 

 which is thus radiated into space, yet it is the only 

 mode of generating radiation with which we are 

 acquainted. 



It is from an electric charge during its epochs of 

 acceleration or retardation that we get the pheno- 

 menon called radiation ; it is this and this alone 

 which excites ethereal waves, and gives us the 

 different varieties of light. 



The energy radiated per second has been shown 

 by Larmor to be 



