INTERACTIONS OF MOVING ELECTRONS 203 



of loops which are free to rotate about individual 

 axes but have no individual freedom of translation. 

 Suppose also that each loop is closed on itself instead 

 of being connected in series with the others and that 

 all are carrying equal currents. What would be the 

 effect if such a group of loops were placed along the 

 axis of a solenoid which is also carrying a current ? 

 Each loop will turn so as to reduce, as much as is 

 possible, the potential energy of those similar systems 

 which it forms with the loops of the solenoid, and then 

 a further reduction will be occasioned by a transla- 

 tion of the group as a whole toward the solenoid. 



The fact that a current is flowing in each loop means 

 merely that electrons are rotating about its center. 

 In the particular case under discussion such a rotation 

 is accomplished by the motion, in the circumferential 

 path formed by the metallic conductor, of its free 

 electrons. The molecules of the metal do not move, 

 and so far as concerns the effect in which we are in- 

 terested the only function the metal serves is to keep 

 the electrons in the path. Suppose, therefore, that a 

 positive nucleus was located at the center of the loop. 

 It would serve the same purpose. If, then, there are 

 any substances the atoms of which are formed by 

 electrons rotating in a plane about the positive nucleus, 

 such atoms ought to act just like the single loops 

 which we have been discussing. 



What would be the test ? Obviously if we bring long 

 cylindrical pieces of different substances near to a 

 solenoid which is carrying a current then those whose 

 atoms are of this description will tractate toward it. 

 In the case of iron and its compounds and also of 



