154 Mr. 0. Heaviside on Electromagnetic Waves, and the 



that the master is on the wrong track, the result must be, as Prof. 

 Fitzgerald observed (in effect) disastrous to progress. Now Max- 

 well's theory and methods have stood the test of time, and showed 

 themselves to be eminently rational and developable. 



It is not, however, with the general question that we are here 

 concerned, but with the different kinds of " velocity of electricity." 

 As Sir "W. Thomson points out, bis electrostatic tbeory, by ignoring 

 electromagnetic induction, leads to infinite speed of electricity 

 through the wire. Interpreted in terms of Maxwell's theory, this 

 speed is not that of electricity through the wire at all, but of the 

 waves through the dielectric, guided by the wire. It results, then, 

 from the assumption /x = 0, destroying inertia (not of the electric 

 current, but of the magnetic field), and leaving only forces of 

 elasticity and resistance. 



But he also points out another way of getting an infinite speed, 

 when we, in the case of a suspended wire, not of great length, 

 igrore the static charge. This is illustrated by the pushing of 

 incompressible water through an unyielding pipe, constraining the 

 current to be the same in all parts of the circuit. This, in Max- 

 well's theory, amounts to stopping the elastic displacement in the 

 dielectric, and so making the speed of the wave through it in- 

 finite. As, however, the physical actions must be the same, 

 whether a wire be long or short, the assumption being only war- 

 rantable for purposes of calculation, I have explained the matter 

 thus. The .electromagnetic waves are sent to and fro with such 

 great frequency (owing to the shortness of the line) that only the 

 mean value of the oscillatory V at any part can be perceived, and 

 this is the final value ; at the same time, by reason of current in 

 the negative waves being of the same sign as in the positive, the 

 current C mounts up by little jumps, which are, however, packed 

 so closely together as to make a practically continuous rise of 

 current in a smooth curve, which is that given by the electro- 

 magnetic theory. This curve is of course practically the same all 

 over the circuit, because of the little jumps being imperceptible. 



But in any case this speed is not the speed of electricity through 

 the wire, but through the dielectric outside it. Maxwell remarked 

 that we know nothing of the speed of electricity in a wire sup- 

 porting current; it may be an inch in an hour, or immensely 

 great. This is on the assumption, apparently, that the electric 

 current in a wire really consists in the transfer of electricity 

 through the wire. I have been forced,(to make Maxwell's scheme 

 intelligible to myselfJ)to go further, and add that the electricity may be 

 standing still, whicfl is as much as to say that there is no current, 

 in a literal sense, inside a conductor. [The slipping of electrification 

 over the surface of a wire is quite another thing. That is merely 

 the movement of the wave through the dielectric, guided by the 

 wire. It occurs in a non-distortional circuit, owing to the absence 

 of tailing, in the most plainly evident manner.] In other words, 

 take Maxwell's definition of eiectric current in terms of magnetic 



