107 



can be made visible to an audience by means of a divided scale 

 on the wall of the lecture-room. 



The dependence of the direction of the torsion on that of the 

 current and the magnetic field (N == north, S = south pole), is 

 shown in fig. ioi\ this drawing needs no further comment. 



Evidently we have to deal with the superposition of two causes 

 having the symmetries C and Cg, with their axes of isotropy 

 parallel to each other, giving as effect 

 a torsion of the symmetry D^ or 

 C^ . In accordance with the fact 

 that these relations between the three 

 groups are reciprocal, a magnetised 

 steel-wire in its turn, if twisted by a 

 force at both its ends, will show a 

 difference of potential (current) produ- 

 ced in it, the presence of which can 

 be easily demonstrated by the induction- 

 current it produces in an encircling 

 solenoid. Here also the superposition of 

 the magnetic field (Cg ) and the torsion 

 (DOO ) or CQJ, , will have a result of the 

 symmetry C^ , and this group C^ being 

 a sub-group of Cg, the occurrence of 

 an electric current having the latter 

 symmetry is compatible with the 

 superposition of both causes. 



In the same way a soft iron rod will 

 become a magnet if an electric current 

 passes through it, while the rod is twisted 

 by a force applied to one of its ends; 

 the other end is held fast. This pheno- 

 menon can also be easily demonstrated 

 by means of the induction-current which the magnet produced 

 will start in an encircling solenoid. It is a fact worthy of 

 attention, that the free electrons of the iron rod moved by the 

 electric force, are not the same as the electrons which are attached 

 to the iron-atoms themselves and whose motion is the cause 

 of Ampere's "molecular currents". The kinetic energy of both 

 kinds of electrons in the metal must, therefore, be interchanged to 

 and fro in some way or other, because in experiments like these, 



Fig. 100. 



