ELECTRICAL CONFERENCE AT PHILADELPHIA 633 



The assumption of a medium allows us to solve in some cases that 

 problem so long under discussion by electricians namely, the true ve- 

 locity of an electric current. We now know that the term velocity 

 hardly applies to this case, and that the current arrives at different 

 points so gradually that we know not when to say it has arrived. But 

 there is certainly a minimum time when even an infinitesimal current 

 can reach a distant point. Suppose two wires stretched in space with 

 their ends near together at one end and a Leyden jar be discharged from 

 one to the other at the near end. The minimum possible time of obtain- 

 ing a spark at the distant end will evidently be the time required by 

 light to pass from the Leyden jar to the distant point, not around the 

 wire, but in a straight line. In this case the greatest maximum velocity 

 is thus twice that of light reckoned around the wire, and may be any 

 amount greater when we bend the wire. For all ordinary distances this 

 velocity may be considered infinite, and the retardation to depend 

 only on the electrostatic capacity and magnetic self-induction of the 

 wire. Treated in this way, we have Thomson's mathematical theory 

 of the propagation of an electric wave along a telegraph wire or cable, 

 a theory of great practical use in telegraphy and telephony. But until 

 the action in the external medium is also taken into account, it can only 

 be considered an approximation. For we can never move a magnet, 

 discharge a Leyden jar, or complete the circuit of a battery, without 

 causing a wave of electro-magnetic disturbance in the ether, and every 

 signal which is sent along a telegraph line is accompanied by a wave in 

 the ether, which travels outward into space with the velocity of light. 

 Truly the idea of a medium is to-day the keystone of electrical theory, 

 but we can hardly suppose that it has even yet attained a fraction of 

 the importance to which it is destined to rise. 



Let me now call your attention to one of the most wonderful facts 

 connected with electrical science. When we are dealing with the elec- 

 trostatic action of electricity, we find that it is the so-called electric fluid 

 which attracts the opposite. Not only do we observe the attraction of 

 bodies oppositely charged, but the electricity itself on the two bodies is 

 displaced by its mutual action. But when we come to investigate the 

 mutual attraction or repulsion of electric currents on each other, we find 

 an entirely different law. In this case the conductors carrying the cur- 

 rents attract or repel each other, but the currents within those con- 

 ductors have no influence of attraction or repulsion to displace them- 

 selves within the body of the conductor. In other words, the current 

 is not displaced by the action of a neighboring magnet, but flows on 

 calmly as if it were not present. 



