APR K.] CHARGE IN RAPID MOTION 223 



great acceleration that has ever been practically dealt 

 with as, for instance, the case of a cannon-ball stopped by 

 armour plate is any sort of unusual effect to be expected ; 

 even on the hypothesis that matter is entirely electrically 

 composed. Nevertheless, now that among free corpuscles 

 in a vacuum tube, or among those expelled from radium, 

 it is becoming practically possible to attain these high 

 speeds, and even to begin to base crucial determinations 

 upon them, it becomes necessary to consider the matter 

 more carefully. In a publication at Gottingen in January, 

 1902, Dr. Abraham has thus discriminated what he calls 

 " longitudinal " from what he calls " transverse " inertia ; 

 making inertia depend not only on the speed but on the 

 direction of acceleration; each direction having a different 

 inertia of its own. 



And all these results are still further complicated by a 

 consideration of the effect of acceleration itself, which, 

 whenever it is violent, gives rise to some perceptible 

 radiation, involving dissipation of energy; and this radia- 

 tion loss of energy, though it will be primarily represented 

 in the motion as a resistance or velocity term, may second- 

 arily have an effect on inertia; probably, however, quite 

 a small and subordinate effect in all practical cases, and 

 no effect at all so long as motion occurs with uniform 

 speed in a straight line: for then there is no radiation. 

 But then, of course, under those conditions it is not possible 

 to test or measure the inertia of a body ; it is only when 

 the motion is either curved or changed in some way that 

 inertia becomes prominent, and then there is necessarily 

 some, though usually very small, radiation too. 



When magnetic deflexion of a charged body is being 

 observed at ultra-high speeds it may be asked whether 

 it is possible for the ordinary expression for the force 

 exerted on a current by a moving field to be departed from. 



