( 240 ) 



This observation throws a new liglit on the signification of the 

 well-known experiments of' K.\rF:\r.\.\x (Blcherkr, Hupka). Tiiese 

 experiments arc cai-ried out wilh a [)nrpose to investigalc wliether 

 Ihc electrons conlracl when nioxiiig. We here see however that, even 

 if' the accuracy of' formula (2) is perfectly confirmed by experiments 

 of this kind, this by no means pi-oves the existence of the contraction. 

 What really can i)e decided by these experiments is whether we 

 have I'ightly attributed mass to the energy. 



In order to deduce e(iuation (2j we have assumed that the increment 



1 



of the mass is equal to -^ the increment of the energy. We are 



C' 



1 

 now inclined to ask whether also m„ =z - 6„ {^^ = the energy of 



the body when its velocity is zero). Specially we will put this 

 question for electrons with surface charge. For the electromagnetic 

 energy and the electromagnetic momentum we find respectively : 



1 



c^ H 1^" 



3 , ^ ^., 4 i> 

 f „ and (^ = 



ly' 



V- ^ , r / i^- 



C K c' 



e'o representing the electrostatic energy of the stationary electi-on. 

 These values do not agree with the formulae: 



f = — and 0) = BV . 



1 



but it does not follow tiiat these formulae would not be satisfied if 

 we had taken the total energy and the total momentum instead of 

 t' and &. It is namely known that an electron has besides its electro- 

 magnetic energy, still energy of another kind ^j (elastic energy), in 

 consequence of which its mass and its momentum must be augmented 

 by a positive term. But there is another reason why &' must be 

 diminished by a certain amount in order to find the total momentum. 

 For inside the electron is an amount of momentum whose direction 

 is opposite to the direction of the motion of the electron. To prove 

 this we will investigate the ^■ector of Poynttng when the electron 

 moves in the direction of the positive lA'-axis. At the half of the 

 electron turned towards the positive A'-axis this vector is directed 

 inward, at the half directed towards — A" it is directed outward; 



') Goinp. i. a. H. A. Lorentz. The tlieory of electrons p. 113 and 114, where 

 also the remarks of Poinc.\ré and Abraham referrini;,- to this are discussed. 



