THE PROBLEMS OF SOLUTION iii 



connection between the two modes of motion may 

 be illustrated by a familiar example. Suppose 

 that a long wooden rod is lying on the surface of 

 the ground, and that a push is given to one end of 

 it. The motion of the rod may be quite slow, an 

 inch an hour if we like. But, after moving one 

 end, the other end begins to move an extremely 

 minute fraction of a second after the starting of 

 the impulse. Perhaps it never has occurred to 

 us that any appreciable time elapses between the 

 starting of the two ends. Yet, if we think for a 

 moment, it is clear that the initial push must travel 

 as a wave of compression along the rod, and that 

 the far end can only begin to move when the wave 

 front reaches it. The bearing of the analogy is 

 now obvious. The slow movement of the rod as 

 a whole when once started corresponds with the 

 slow drift of the ions ; the almost instantaneous 

 passage of the wave of compression along the rod 

 corresponds with the velocity of electricity in the 

 electrolytic solution. 



A picture of the phenomena, more nearly 

 corresponding with the facts, is obtained by 

 considering that the rapid electric impulse travels 

 as an electric wave through the surrounding 

 insulating medium. On this view, due to Faraday 

 and Maxwell, and now universally accepted, the 

 electric forces always travel through the medium. 

 When thev act on electric charofes free to move, 

 as in metallic conductors, or on charges attached 

 to matter as in electrolytic solutions, they produce 

 a drift of the charges — a drift which constitues a 

 current. Along the line of the drift, that is, along 

 a conductor, energy is lost, and thus along that 

 line, and there alone, energy is constantly flowing. 



