Prof. W. Siemens on Telephony. 103 



lines attached to the same posts would soon, with increasing 

 length of the lines, be disturbed one by another, not only 

 through imperfect insulation permitting side currents to pass 

 over to neighbouring wires, but also through the production 

 of secondary currents in them by electrodynamic and electro- 

 static induction, generating confusing sounds. In telegraph- 

 lines electrodynamic induction can, as a rule, be entirely 

 neglected, because it does not increase with the length of the 

 line, if the resistance of the wire coils be left out of considera- 

 tion, and because the duration of the electrodynamically in- 

 duced currents is too short to affect the telegraphic instru- 

 ments ; but in telephonic apparatus the brief currents generated 

 by voltaic induction produce very audible sounds if the con- 

 ducting-lines run side by side for only a short distance. 



Further, secondary electrostatic induction, increasing as the 

 squares of the length of the conducting-line, will soon, as the 

 overland lines become longer, put a limit to the employment 

 of the telephone, even when the telephone-wires only are fixed 

 to the same posts. 



The circumstances are much more favourable in this respect 

 for the telephone when underground or submarine lines are 

 employed. Before I had ascertained that the intensity of the 

 currents which yet are capable of exciting the telephone to 

 the production of clearly intelligible speech-sounds is so ex- 

 tremely slight, I doubted the practicability of employing sub- 

 terranean wires for great distances, on account of the great 

 weakening which the current-waves called forth by rapidly 

 alternating electromotive forces in the conducting- wires 

 would undergo with the length of the conduction. The expe- 

 riments, however, which Postmaster-General Dr. Stephan (to 

 whom the German Empire owes the reintroduction of the 

 underground wires that had for a quarter of a century almost 

 fallen into oblivion) caused to be made with Bell telephones, 

 gave the surprising result that with them people can speak 

 with perfect distinctness and quite intelligibly at distances of 

 about sixty kilometres. Hence it is very probable that, with 

 telephones of more powerful action, adequate intelligibility 

 will be attained at twice or even three times that distance. 

 This, at all events, may be the extreme distance at which tele- 

 phonic correspondence is generally practicable. 



Unfortunately, even in underground conducting wires dis- 

 turbances by return currents from the earth, as well as by 

 electrodynamic and electrostatic induction, are not excluded. 

 The former could be pretty completely got rid of, as in lines above 

 ground, by the employment of entirely metallic conduction- 

 circuits, with the exclusion of the earth as return conductor. 



