696 



Physics. — "Experimental proof of the existeace of Ainpère's 

 molecular curretity." By Prof. A. Einstkin and Dr. W. J. dk Haas. 

 (Communicated by Prof. H. A. Lobentz), 



(Communicated in the meeting of April 23, 1915). 



When it had been disco\ered by Oersted that magnetic actions 

 are exerted not only by permanent mae;nefs, bnt also by electric 

 currents, thei-e seemed to be two entirely ditferent ways in which 

 a magnetic field can be produced. This conception, however, could 

 hardly be considered as satisfactory and physicists soon tried to 

 refer the two actions to one and the same cause. Ampère succeeded 

 in doing so by his celebrated hypothesis of currents circulating 

 around the molecules withont encountering any resistance. 



The same assumption is made in the theory of electrons in the 

 form e.g. in whicli it has been developed by H. A. Lorentz, the 

 only difference being that, like electric currents in general, the 

 molecular currents are now i-egarded as a circulation of elementary 

 charges or electrons. 



It cannot be denied that these views call forth some objections. 

 One of these is even more serious than it was in Ampere's days; 

 it is difficult to conceive a circulation of electricity free from all 

 resistance and therefore continuing for ever. Indeed, according to 

 Maxwell's equations circulating electrons must lose their energy 

 by radiation ; the molecules of a magnetic body would therefore 

 gradually lose their magnetic moment. Nothing of the kind having 

 ever been observed, the hypothesis seems irreconcilable with a 

 general validity of the fundamental laws of electromagnetism. 



Again, the law of Cürie-Langevin requires that the magnetic 

 moment of a molecule shall be independent of the temperature, and 

 shall still exist at the absolute zero. The energy of the revolving 

 electrons would therefore be a true zero point energy. In the 

 opinion of many physicists however, the existence of an energy of 

 this kind is very improbable. 



It appears by these remarks that after all as much mtiy be said 

 in favour of Ampere's hypothesis as against it and that the question 

 concerns important physical principles. We have therefore made 

 the experiments here to be described, by which we have been able 

 to show that the magnetic moment of an iron molecule is really 

 due to a circulation of electrons. 



The possibility of an experimental proof lies in the fact that every 

 negative electi'on circulating in a closed path has a moment of 



