490 Mr. 0. Heaviside on Resistance 



together with the various reflected waves produced by change 

 of media and other causes. At the first moment of starting 

 an impressed force the only disturbance is at the vortex lines, 

 which are the first lines of magnetic induction. 



(a) Thus a uniform field of force suddenly started over all 

 space can produce no effect. For, either there are no vortex 

 lines at all, or they are at an infinite distance, so that an 

 infinite time must elapse to produce any effect at a finite 

 distance from the origin. 



(b) Copper and zinc put in contact. Whether the Volta- 

 force be at the contact or over the air-surfaces away from 

 and terminating at the contact (if perfectly metallic), the 

 vortex-line is the common meeting-place of air, zinc, and 

 copper ; the first line of magnetic force is there, and from it 

 the disturbance proceeds into the metals and out into the air, 

 which ends in the steady electric field*. 



Since the vortex-lines or tubes are closed, we need only 

 consider one at present — say, that due to a simple shell of 

 impressed force. If it be wholly within a conductor, the 

 initial wave emanating from it is so rapidly attenuated by the 

 conductivity (the process being akin to repeated internal 

 reflexions, say reflexion of 9 parts and transmission of 

 1 part, repeated at short intervals) that the transmission 

 to a distance through the conductor (if good) becomes a very 

 slow process, that of diffusion. Consequently, when the 

 impressed force is rapidly alternated, there is no sensible 

 disturbance except at and near the vortex-line. 



But if there be a dielectric outside the conductor, the 

 moment disturbances reach it, and therefore instantly if 

 the vortex-line be on the boundary, waves travel through 

 the dielectric at the speed of light unimpeded, and without 

 the attenuating process within the conductor, which therefore 

 becomes exposed to electric force all over its boundary in a 

 very short time ; hence diffusion inward from the boundary. 

 The electric telegraph would be impossible without the di- 

 electric. It would take ages if the wire itself had to be the 

 seat of transfer of energy. 



(c) In the electromagnetic theory of the rise of current in 

 a wire we have, at first sight, an exception to the law that at 

 the first moment there is no disturbance except at the vortex- 

 lines of impressed force. But it is that theory which is 

 incorrect, in assuming that there is no displacement. This 

 is equivalent to making the speed of propagation through 

 the dielectric infinitely great ; so that we have results mathe- 



* "Some Remarks on the Volta Force," Journal S. T. E. &R, 1885. 



