172 JAMES CLERK MAXWELL 



it is touched by tlio adjacent cells ; it will therefore 

 be set in rotation, but there will be no force tending 

 to drive it onwards ; it does not matter whether the 

 axis on which it rotates is free to move or fixed, in 

 either case the idle wheel simply rotates. But suppose 

 now the adjacent cells are not rotating at the same 

 rate. In addition to its rotation the idle wheel will bo 

 urged onward with a velocity which depends on the 

 ditlercnce between the rotations, and, if it can move 

 freely, it will move on from between the two cells. 

 Imagine now that the interstices between the cells 

 are lilted with a string of idle wheels. So long as tho 

 adjacent cells move with ditVercnt velocity there will 

 be a continual stream of rolling particles or idle wheels 

 between them. Maxwell in the paper considered 

 these rolling particles to be particles of electricity. 

 Their motion constitutes an electric current. In a 

 uniform magnetic field there is no electric current ; 

 if the strength of the lield varies, the idle wheels are 

 set in motion and there may be a current. 



These particles arc very small compare 1 with tho 

 magnetic vortices. The mass of all the particles is in- 

 appreciable compared with the mass of the vortices, 

 and a great many vortices with their surrounding 

 particles are contained in a molecule of ihe medium ; 

 the particles roll on tho vortices without touching 

 each other, so that so long as they remain within the 

 same molecule there is no loss of energy by resistance. 

 When, however, there is a current or general trans- 

 ference of particles in one direction they must pass 

 from one molecule to another, and in doing so may 

 experience resistance and generate heat, 



