104 Prbf.W. Siemens on Telephony. 



The same holds good in the case of disturbances produced by 

 induction, if the two insulated conductors forming a telephone- 

 circuit are united into a separate cable encased with iron wires. 

 If, on the contrary, as is usually done for the sake of saving 

 expense, a greater number of insulated conductors are com- 

 bined in one cable, voltaic as well as static induction makes 

 its appearance in augmented measure on account of the 

 slightness of the distance between them, and act very dis- 

 turbingly on the telephonic correspondence. This secondary 

 electrostatic induction occurs perturbingly even in long cables 

 for telegraphic correspondence, with which very sensitive ap- 

 paratus must be employed. Hence I have proposed, for 

 avoiding it, to provide the individual conductors which are 

 combined in a cable containing several wires with a conduc- 

 ting metallic sheath in conducting connexion with the outer 

 iron spinning or with the earth. Even encasing the insulated 

 individual conductors with a thin layer of tin foil gets rid of 

 secondary electrostatic induction completely. Any one can 

 easily convince himself of this by experiment if he places one 

 upon the other two mica or thin gutta-percha plates, each 

 lined on both sides with tin foil. If the inner linings be in- 

 sulated and the charge between the outer ones be tested by 

 the deflection of a galvanometer by connecting the free pole 

 of a battery led away to earth with one of the outside sheets 

 of tin foil, while the second is connected through the galva- 

 nometer-wire with the earth, or in a similar manner by aid of 

 the commutator, as great a charge is obtained as if the sheets 

 in the middle were absent. But if the latter are connected 

 with the earth, no trace is obtained of a secondary charge in 

 the tin foil connected with the galvanometer. 



We get the same negative result when the individual insu- 

 lated conductors of a cable consisting of several such have 

 been tightly wrapped round with tin foil or strips of thin plate 

 of any metal. The metallic conductive casing, though very 

 thin, completely prevents any secondary electrostatic induc- 

 tion or charge of one conductor by the charge of another. 

 On the other hand, however, the electrodynamic induction 

 exerted by the wires upon one another is not thereby removed, 

 as Foucault asserted *. 



This can easily be convincingly shown by a simple experi- 

 ment. If two wires, insulated with gutta percha or caout- 

 chouc, be wound together upon a roller, powerful charge as well 



* Foucault, on the 2nd of July, 1869, took out a patent in England for 

 encasing the individual conductors with tin foil or other conducting sub- 

 stances, with the expressed purpose of compensating electrodynamic in- 

 duction by the countercurrents arising in the tin casing. 



