Recent Theories of Electricity. 213 



ruhende Korper modifizieren miisste, em Schritt, zu dem 

 sich wolil einstweilen schwer jemand wird entschliessen 

 wollen." 



Hupka *, by a different method, arrived at contrary 

 results. His conclusions are: — "Das wichtigste Resultat 

 der beschriebenen Versuche ist jedoch ein Eeitrag zur Ent- 

 scheidung der miteinander im Wettstreit liegenden Theorien 

 von Abraham und Lorentz-Einstein. Die mitgeteilten 

 Messungen sprechen zugunsten der letzteren. Zu demselben 

 Ergebnis ist bekanntlich auch Hr. Bucherer gelangt." 



It is not neces-ary to weigh their contradictory evidence, 

 as it has been done most exhaustively by W. Heil f . He 

 shows^ and I think conclusively, that it may be taken as 

 established that the apparent, inertia of an electron depends 

 on its velocity ; but in no case is the experimental accuracy 

 sufficiently great to decide the question of the nature of the 

 electron. 



Let us now turn to the mathematical side of the dis- 

 cussion and examine the expressions for the electromagnetic 

 mass : we shall find contradictions between the two general 

 ideas and further evidence that the founding of mechanics on 

 an electromagnetic base does not harmonize with the other 

 branches of physics. The values of U, T, and | G | all 

 become infinite when the velocity of the particle equals the 

 velocity of light ; yet several physicists have given this 

 velocity to the orbital motion of the electron, and others 

 have attempted to give the same velocity to electrified light- 

 particles. On the other hand, when the particle is at rest, 



T and | G | become zero and U =— p. It is proper 



that the magnetic energy and the electromagnetic momentum 

 should then vanish, as both of them are kinetic functions of 

 electricity ; but how are we to account for the finite values 

 of the electrical energy and of the two forms of electro- 

 magnetic mass? To do so would require us to assume that 

 a free electric charge at rest still possesses inertia, a con- 

 sequence difficult to reconcile with our experimental evidence 

 of static electricity. 



It is even more convenient to turn to an analysis of 

 Kaufmann's J equations. He has derived the electric and 

 magnetic deflexions of a charged particle moving through 

 an electric and a magnetic field. 



* Annal. der Physik, xxxi. p. 203 (1910). 



f Annal. der Physik, xxxi. p. 519 ; xxxiii. p. 403 (1910). 



} Loc. eit. 



