102 Frof. W. Siemens on Telephony. 



For giving back the sounds of speech the tympanic form 

 of membrane is not so well suited. It also appears generally 

 more to the purpose to employ larger and more powerful in- 

 struments for giving, and smaller instruments of more delicate 

 and lighter construction for receiving, at the same time bring- 

 ing the instrument into the most suitable position for the ear. 



Too powerful receiving-apparatus have the drawback, that 

 the countercurrents produced by the vibrations of their mem- 

 brane weaken the moving currents and displace the trains of 

 sinusoid waves of the induced currents, by which the speech 

 is made indistinct and assumes strange shades of sound. 



It is scarcely to be assumed generally that telephones on 

 Bell's principle (in which the sound-waves themselves have to 

 perforin the work of exciting the currents required for their 

 conveyance) will be successfully produced so as to utter speech 

 distinctly intelligible at a greater distance from the telephone ; 

 and, as we have already insisted, it is quite impossible of at- 

 tainment that they should reproduce not weakened the mass 

 of sound by which their membrane is struck, or even re- 

 inforced. This possibility, however, is not excluded when a 

 galvanic battery is used for putting in motion the membrane 

 of the receiving- apparatus, which then accomplishes the work 

 to be expended. Reis endeavoured to effect this by means of 

 contacts, Edison with the aid of powdered graphite inserted 

 in the conduction-circuit of the battery. 



Contacts will hardly operate with sufficient constancy and 

 certainty for the sounds of speech to be given back with purity. 

 But it is possible that the solution of the problem lies in the 

 course taken by Edison ; it therein only depends on the dis- 

 covery of a material or an arrangement by means of which 

 changes in the resistance of the circuit may be produced con- 

 siderable in amount and proportional to the amplitude of the 

 vibrations of the membrane. The form and quality of gra- 

 phite powder are too variable to accomplish this with certainty. 

 Experiments which I have commenced with other arrange- 

 ments have not at present given any satisfactory result. 

 Nevertheless Edison's procedure remains well worthy of con- 

 sideration, as it possibly forms the key to a future important 

 development of telephony. 



If, however, telephonic instruments are susceptible of fur- 

 ther extensive improvement, the conducting-lines will always 

 confine the circle of their application within rather narrow 

 limits. Even if, as we have already shown to be indispen- 

 sable, special posts be appropriated to telephone-lines, carrying 

 no telegraph-wires, and double lines be everywhere employed 

 for the telephones, yet even the telephonic messages on several 



