ON PHYSICAL LINES OF FORCE. 485 



of the vortices will be altered from their state previous to the motion of the 

 wire. The change in the lines of force is shewn in fig. 5. The vortices in 

 front of the wire, instead of merely producing pressures, actually increase in 

 velocity, while those behind have their velocity diminished, and those at the 

 sides of the wire have the direction of their axes altered; so that the final 

 effect is to produce a force acting on the wire as a resistance to its motion. 

 We may now recapitulate the assumptions we have made, and the results we 

 have obtained. 



(1) Magneto-electric phenomena are due to the existence of matter under 

 certain conditions of motion or of pressure in every part of the magnetic field, 

 and not to direct action at a distance between the magnets or currents. The 

 substance producing these effects may be a certain part of ordinary matter, or 

 it may be an aether associated with matter. Its density is greatest in iron, 

 and least in diamagnetic substances ; but it must be in all cases, except that of 

 iron, very rare, since no other substance has a large ratio of magnetic capacity 

 to what we call a vacuum. 



(2) The condition of any part of the field, through which lines of magnetic 

 force pass, is one of unequal pressure in different directions, the direction of 

 the lines of force being that of least pressure, so that the lines of force may 

 be considered lines of tension. 



(3) This inequality of pressure is produced by the existence in the medium 

 of vortices or eddies, having their axes in the direction of the lines of force, 

 and having their direction of rotation determined by that of the lines of force. 



We have supposed that the direction was that, of a watch to a spectator 

 looking from south to north. We might with equal propriety have chosen the 

 reverse direction, as far as known facts are concerned, by supposing resinous elec- 

 tricity instead of vitreous to be positive. The effect of these vortices depends 

 on their density, and on their velocity at the circumference, and is independent 

 of their diameter. The density must be proportional to the capacity of the 

 substance for magnetic induction, that of the vortices in air being 1. The 

 velocity must be very great, in order to produce so powerful effects in so rare 

 a medium. 



The size of the vortices is indeterminate, but is probably very small as 

 compared with that of a complete molecule of ordinary matter*. 



* The angular momentum of the system of vortices depends on their average diameter ; so that if the 

 diameter were sensible, we might expect that a magnet would behave as if it contained a revolving body 



