242 TELEGRAPHY ACROSS SPACE. 



these apparently primitive pieces of apparatus, he then devoted himself 

 to the task of exploring the propagation of the waves. He found that, 

 like waves of light, they could be deflected by metallic surfaces, could 

 be refracted by prisms, concentrated by lenses, and even could be 

 polarized. He measured their wave length and velocity of propaga- 

 tion. He found that they could pass readily through walls of wood, 

 stone, or brick, which are opaque to ordinary light waves. Metals 

 and other conductors of electricity, on the contrary, absorbed them, 

 and were consequently opaque. 



In these researches of Hertz we meet, for the first time, 2 with the rec- 

 ognition of a true traveling wave. With this immense discovery, there 

 was opened out an entirely new field of possibilities. Hitherto, there 

 had been inductive actions known which might reach out from wire to 

 wire only to fall back again when their excitant cause dies away. But 

 now the electric wave, once started on its path, did not collapse back 

 into the wires when the spark 'ceased. On the contrary, it went trav- 

 eling on. And just as the javelin, which can travel on after the 

 impulse has ceased, can act at greater range than the sword, whose 

 thrust is limited by the length of arm and blade, so the true electric 

 wave, by the very fact that it is a true traveling wave, can carry sig- 

 nals to greater distances than the mere inductive influence that simply 

 extends outwards from a wire or from a coil. 



The work which Hertz had begun, was, after his death, carried on 

 by a whole army of investigators. Of these, and of their achievements, 

 the best account that has yet appeared is Professor Lodge's little book 

 on The Work of Hertz and his Successors. To that book inquiries 

 must be recommended for details. Suffice it here to say that much has 

 been done in perfecting both the oscillator and the detector. Notable 

 among these matters have been the forms of oscillator designed by 

 Lodge and by Bighi; the latter having the spark gap immersed in oil 

 or vaseline between two metal balls. Many forms of detector have 

 been proposed. Very early Lodge produced one under the name of 

 "coherer," consisting of a metallic point very lightly pressed against a 

 metal plate, and connected in circuit with a galvanometer and a local 

 cell. The light contact constitutes an imperfect joint, which is prac- 

 tically nonconductive until caused to cohere and conduct by the 

 impact of an electric wave; or, perhaps more accurately, by the stimu- 

 lus of the minute surging electric current which results from the impact 

 of an electric wave. Subsequently, taking a hint from M. Brauly, 

 Lodge substituted as a detector a new kind of coherer, consisting of a 



1 Many years before, Prof. Joseph Henry had transmitted induced electric sparks 

 from one circuit to another in different floors of a building. Doubtless these were 

 oscillatory; but it is impossible, at this time, to determine whether the arrange- 

 ments were such as to produce true traveling waves, or whether the action was (like 

 Lodge's later experiment of the two syntonic circuits) merely one of electro-magnetic 

 induction. 



