96 Prof. W. Siemens on Telephony. 



feci quiet is necessary around it, in order that the ear may 

 not be dulled and disturbed by extraneous noises. A still 

 graver obstacle to its practical employment consists in this — 

 that it needs complete electrical calm. As the currents are 

 extraordinarily feeble which are generated by the vibrating 

 iron membrane and put the membrane of the other instru- 

 ment into similar vibrations, therefore also very weak extra- 

 neous currents are sufficient to disturb the latter and bring to 

 the ear confusing noises of other origin. 



In order to procure fixed points for judging the intensity 

 of the currents which are effective in the telephone, I placed 

 one of Bell's telephones, the magnet-pole of which was wound 

 round with 800 turns of copper wire 0*1 millim. thick and 

 possessing a resistance of 110 mercury units, in a circuit con- 

 taining a Daniell's cell, with a commutator, by which the di- 

 rection of the current w T as reversed 200 times in a second. 



Without an inserted resistance, these current-waves pro- 

 duced in the telephone an extremely inharmonious noise, 

 audible at a long distance, and almost intolerable close to the 

 ear. By the insertion of a resistance, this noise was dimi- 

 nished, but was still very loud after the insertion of 200,000 

 units. If 6 Daniells were inserted, the noise was still di- 

 sinctly audible through ten million units of resistance. If 12 

 Daniells and twenty million units of resistance were inserted, 

 the sound was decidedly more distinct than in the last prece- 

 ding case. In like manner an increase in its intensity took 

 place when thirty and fifty millions of units were inserted 

 with 18 and 30 Daniells respectively. This corroborates 

 Beetz's observation that electromagnetism, with equal cur- 

 rent-intensity, is more quickly called forth in circuits of great 

 resistance by correspondingly more intense electromotive 

 forces, than in circuits with little resistance and proportion- 

 ately less electromotive forces, because the countercurrents 

 which arise in the windings of the electromagnet count for 

 more in the latter case than in the former. 



If in the circuit of the commutator the primary spiral of a 

 small voltaic induction-coil was inserted, such as is ordinarily 

 employed by physicians, while the telephone and resistance- 

 scale were in the circuit of the secondary wire, with one 

 Daniell element a loud-sounding noise was still obtained when 

 fifty million mercury units were inserted ; and this remained 

 distinctly audible even when the secondary spiral was pushed 

 back right to the end of the primary. 



This sensitiveness of BelPs telephone to feeble currents ren- 

 ders it very useful as a galvanoscope, especially for the de- 

 tection of feeble and rapidly changing currents, for which there 



