534 JAMES CLERK MAXWELL. 



tides are unable to pass from molecule to molecule of the 

 body, but in a conductor they can do so, the passage, how- 

 ever, being opposed by friction, so that heat is generated 

 and energy dissipated in the transfer. 



Now suppose that we have a current of electricity 

 flowing through a conducting wire. Let us confine our 

 attention at first to the central line of particles. These, as 

 they flow, will cause all the cells they touch to rotate about 

 axes perpendicular to the line of flow, so that the stream of 

 particles will be surrounded by rings of vortices. Each 

 ring of vortices will behave like an indiarubber umbrella 

 ring when we pass it over the finger or the stick of an 

 umbrella. Instead of sliding into its place it proceeds by a 

 rolling motion, continually turning itself inside out, as it 

 were, each circular section of the ring or tore rotating about 

 its own centre. Now this motion of the vortices would 

 tend to cause the layer of electric particles outside them to 

 move in the opposite direction to the central stream, and 

 this tendency, to which we shall again refer when we speak 

 of induction, can only be overcome by causing the next ring 

 of cells to rotate in the same direction as the inner ring, 

 when the particles may simply roll round between the 

 coaxial rings of vortices without moving backwards or for- 

 wards. But if the layer of particles be compelled to move 

 forwards like the inner stream the layer of vortices surround- 

 ing it must rotate more rapidly that the layer within it, and 

 so on, each successive shell of vortices rotating more rapidly 

 until we reach the extreme layer contained within the 

 conducting wire. The shell of vortices which bounds the 

 conductor must by the same mechanism set up molecular 

 vortices in the dielectric, the motion being communicated in 

 ever-widening circles to an unlimited distance. It does not 

 follow that this communication of motion is instantaneous. 

 The cells may consist of elastic material which does not 

 assume its final state of motion as soon as the tangential 

 action of the electric particles is exerted upon it, but begins 

 at first to undergo a deformation, the time taken to set up a 

 given rotation in each depending on its density and elasticity. 



