697 



momentum in a direction opposite to the vector that represents its 

 magnetic moment, the ratio between the two moments iiaving a 

 definite value which is independent of the geometric dimensions 

 and of' the time of circulation. Tiie magnetic molecule behaves as 

 a gyroscope whose axis coincides wich f he direcfion of (he magneti- 

 sation. Every change of magnefic stale involves an alteration of 

 the orientation of tiie gyroscopes and of the moment of momentum 

 of the magnetic elements. In virtue of the law of conservation of 

 moment of momentum tiie change of "magnetic" moment of momen- 

 tum must be compensated by an equal and opposite one in the 

 moment of momentum of ponderable matter. The magnetisation of a 

 body must therefore give rise to a couple, which makes the body 

 rotate. ') 



^ 1. Magnetic moment and moment of momentum of the molecule. 



The magnetic moment of a current of intensity i flowing along 

 a circle of area F is given by the formula 



in ^ IF, 



or if the current consists in an electron circulating n times per 

 second by 



111 = neF (1) 



It may be represented by a vector perpendicular to the plane 

 of the circle, the jiositive direction of this vector corresponding in 

 the well-known way to the positive direction of the current. 



The moment of momentum is 



s:)]^ = 2mnF, (2) 



if we let coincide its positive direction with that of the magnetic 

 moment. 



Hence : 



•2m 

 33Ï = — in (3) 



e 



For a body in which a certain number of electrons are circulating, 

 this becomes 



2 m 



2:m = - ^in, 

 or if we denote the magnetisation 2m by 1 



1) This paper had gone to press when we learned that 0. W. Richardson 

 (Pliys. Rev. Vol. 26, 1908 p. 248; had sought ah'eady for the effect in question, without 

 however obtaining a positive result. 



45* 



