472 



^ 7. Increase of the molecular magnetic moment vnth temperature. 

 In the disenssions of this and the former paper the molecular magnetic 

 moment has always been supposed to be constant and the magnetic 

 axis to be a fixed axis in the magnetic molecule. The simplest 

 hypothesis is that the electron or the electrons, whose motion (for 

 instance in a circular orbit) canses the magnetic moment, are bound 

 to a detinite place in the molecule. In this man/ier one is led to 

 the hypothesis, that a magnetic molecule owes its magnetic moment 

 to the fact, that it rotates round a definite axis, which then is the 

 magnetic axis, one or more electrons being situated at a distance 

 from it. As long as the rotational velocity is constant and the distan- 

 ces of these electrons from the axis remain the same, the magnetic 

 moment is constant. The question naturally arises, whether this 

 rotation (roimd the magnetic axis) also participates in the thermal 

 equilibrium. In the affirmative case the fact that the velocity of 

 rotation does not change appreciably from low temperatures upwards 

 over a large region of Temperatures would point to a large value 

 of the constant 6^ which corresponds to this rotation, and should 

 be attributed to a particularly small moment of inertia about the 

 corresponding axis. 



We may resume this as follows : a magnetic molecule would be 

 a molecule (or part of it) which has a small moment of inertia 

 round a definite axis, so that with zero-point energy it rotates with 

 great velocity round this axis, and contains at least one electron 

 which is bound to a place in the molecule situated at a distance 

 from that axis. 



According to this hypothesis about the [)cirticipation of the rotation 

 which we now consider in the thermal equilibrium, at higher tem- 

 peratures the magnetic moment will begin to increase with tempe- 

 rature, at first slowly, later on more rapidly. Weiss ^) has actually 

 been led by the observations above the CuRiE-point for magnetite, to 

 the hypothesis of a magnetic moment increasing with temperature. 

 A difference with Weiss's views lies in this, that Weiss adopts a 

 discontinuous increase of the molecular magnetic moment in rational 

 proportions, whereas from the above hypothesis one would rather 

 expect a continuous increase of the molecular magnetic moment 

 with temperature. A further investigation will have to show, whether 

 the measurements by Weiss and FoËx regarding this matter cannot 

 be interpreted in this way, in particular for instance with application 

 of the developments of § ^a to the rotational energy which here 

 comes into play. 



1) P. Weiss, Ar;h. sc. phys. et iiatur. (4) 31 (19il) p. 402, § 12. 



