Prof. W. Siemens on Telephony. 97 



has hitherto been scarcely any other means of testing than 

 the contractions of the leg of a frog. Also, in measuring re- 

 sistances by the bridge method, the telephone may often be 

 employed with advantage instead of the galvanometer in the 

 branch wire of the bridge ; but then it will be necessary to 

 employ as resistances only straight wires stretched at a greater 

 distance from each other, as otherwise perturbations would 

 arise through induction. 



This perfectly explains the extreme sensitiveness of the 

 telephone to electrical disturbances in the conductor, which, 

 indeed, almost entirely excludes its application to lines above 

 ground if the same posts support wires which are used for 

 telegraphic correspondence. Even when two neighbouring 

 conducting wares on the same posts are employed to form the 

 conduction-circuit, in which case the electrodynamic as well 

 as the electrostatic induction proceeding from the other more 

 distant wires is in great part compensated, still every current 

 that passes through these wires is heard in the telephone as a 

 loud cracking noise rendering the speech of the telephone 

 quite unintelligible if it is frequently repeated. 



Far worse still are these disturbances if the earth is used 

 for closing the circuit. Even when special earth-plates are 

 taken for the telephone-wire, or if gas- or water-pipes are 

 made use of for the same purpose, every current is distinctly 

 heard which is brought to earth through earth-plates in the 

 vicinity. Since, in the spreading of a current in the ground, 

 the electric potential diminishes with the cube of the distance 

 from the point at which the current enters the earth, this also 

 demonstrates the uncommon sensibility of the telephone to 

 feeble currents. 



For these reasons, with overland conducting-wires tele- 

 phones can only be employed if special posts are appropriated 

 to the support of the wires. Further, the earth's conduction 

 can only be used in places where there are no telegraph- 

 stations, or where the earth-plates used for telegraphing are 

 at a good distance from those which serve for the telephone- 

 conductions. 



Notwithstanding this sensitiveness of Bell's telephone, it 

 conveys but very imperfectly the sound-waves by which its 

 membrane is struck to the corresponding membrane and the 

 ear applied to it. When a loud-ticking watch was placed close 

 to the sound-aperture of a very sensitive telephone constructed 

 after Bell's plan, the ticking could not be heard in the other 

 telephone, even when the watch actually touched the telephone- 

 case. On the other hand, the above-mentioned thread tele- 

 phone transmitted the ticking through a thread about 20 



Phil. Mag. S. 5. Vol. 6. No. 35. Aug. 1878. H 



