538 JAMES CLERK MAXWELL. 



particles of electricity in this wire will be acted on in the 

 same way as those in the dielectric, but meeting with very 

 little resistance to their motion along the wire, they find it 

 easier to move through the wire than at once to transmit 

 the vortex motion to the elastic bodies on the other side of 

 them. But when a driver and follower are connected by a 

 differential wheel, if the follower be retarded only by its 

 own inertia, however small a resistance the differential wheel 

 may experience to its motion of translation it will at 

 length cause the follower to turn at the same rate as the 

 driver, and will itself cease to move. Hence the resistance 

 of the conductor at length brings the electric particles to rest, 

 and causes them to communicate the vortex motion to cells 

 beyond them. Thus when a current is started in a wire 

 transitory currents in the opposite direction will be induced 

 in neighbouring conductors, while electric stress will be pro- 

 duced in the dielectric, the elastic cells whose motion con- 

 stitutes the molecular vortices being at first deformed by the 

 tangential stress of the electric particles, but both the 

 induced currents and the stress will have entirely ceased 

 as soon as all the molecular vortices are in full swing. 



Before a current can be maintained in a primary wire, the 

 molecular vortices in the surrounding field must be properly 

 started, and this requires the expenditure of work in con- 

 sequence of the mass of the bodies which constitute the 

 vortices. It is therefore impossible for a finite electro- 

 motive force to start a finite current in an indefinitely short 

 time, in the same way as it is impossible for a finite force 

 to produce instantaneously a finite velocity in a material 

 body, and, just as in dynamics we sometimes speak of the 

 reaction of a body against acceleration as though it were a 

 force opposing the force applied, so we sometimes speak of 

 the corresponding action in the case of the current as though 

 it were a force opposing the battery or other electro-motor, 

 and speak of it as the electro-motive force of self-induction. 

 As, however, this depends not on the current in the wire 

 simply, but on the molecular vortices in the surrounding 

 medium, it is clear that the self-induction of a wire will 



