468 ON PHYSICAL LINES OF FORCE. 



of lines of force about magnets and currents?" These questions are certainly 

 of a higher order of difficulty than either of the former ; and I wish to separate 

 the suggestions I may offer by way of provisional answer to them, from the 

 mechanical deductions which resolved the first question, and the hypothesis of 

 vortices which gave a probable answer to the second. 



We have, in fact, now come to inquire into the physical connexion of these 

 vortices with electric currents, while we are still in doubt as to the nature of 

 electricity, whether it is one substance, two substances, or not a substance at 

 all, or in what way it differs from matter, and how it is connected with it. 



We know that the lines of force are affected by electric currents, and we 

 know the distribution of those lines about a current ; so that from the force 

 we can determine the amount of the current. Assuming that our explanation 

 of the lines of force by molecular vortices is correct, why does a particular 

 distribution of vortices indicate an electric current ? A satisfactory answer to 

 this question would lead us a long way towards that of a very important one, 

 "What is an electric current?" 



I have found great difficulty in conceiving of the existence of vortices in a 

 medium, side by side, revolving in the same direction about parallel axes. The 

 contiguous portions of consecutive vortices must be moving in opposite directions ; 

 and it is difficult to understand how the motion of one part of the medium 

 can coexist with, and even produce, an opposite motion of a part in contact 

 with it. 



The only conception which has at all aided me in conceiving of this kind of 

 motion is that of the vortices being separated by a layer of particles, revolving 

 each on its own axis in the opposite direction to that of the vortices, so that 

 the contiguous surfaces of the particles and of the vortices have the same 

 motion. 



In mechanism, when two wheels are intended to revolve in the same direc- 

 tion, a wheel is placed between them so as to be in gear with both, and this 

 wheel is called an "idle wheel." The hypothesis about the vortices which I 

 have to suggest is that a layer of particles, acting as idle wheels, is interposed 

 between each vortex and the next, so that each vortex has a tendency to make 

 the neighbouring vortices revolve in the same direction with itself. 



In mechanism, the idle wheel is generally made to rotate about a fixed 

 axle ; but in epicyclic trains and other contrivances, as, for instance, in Siemens's 



