CONTRIBUTIONS TO SCIENCE. 



535 



Hence electro-magnetic induction, which is the name given 

 to the action we are now discussing, will be propagated 

 through space with a finite velocity, but of this we must say 

 more hereafter. 



From what has been said it appears that when a steady 

 (i.e. constant) current is flowing in a wire, molecular vortices 

 will be set up in the surrounding dielectric, the axis of 

 rotation of each vortex being perpendicular to the plane 

 passing through the wire and the vortex. The axes about 

 which the vortices turn will therefore form circles surround- 

 ing the wire, while the vortices themselves will constitute 

 vortex rings, spinning with very great velocity in the same 

 manner as the indiarubber ring above referred to, or the 

 rings of smoke which are sometimes seen to emerge from a 

 tobacco pipe. But the lines about which the molecular 

 vortices rotate are magnetic lines of force, there being a 

 tension in the medium along these lines, and a pressure 

 everywhere at right angles to them. Hence a straight line 

 carrying an electric current will be surrounded with magnetic 

 lines of force, forming circles with their centres on the axis 

 of the wire, and since the direction of the magnetic force is 

 that in which a right-handed screw would advance if rotat- 

 ing with the vortices, it follows that the direction of the 

 magnetic force around the wire will be that in which a right- 



handed screw would rotate if advancing with the current. 

 The medium will be subject to tension in circles around the 



