: 1 1 Prof. W. Siemens on Telephony. 



be supported at any number of points by elastic thread's of a 

 low inches length, and also, with similar elastic fastenings at 

 the angles, from any number of angles, without the apparatus 

 losing the capability of conveying with perfect distinctness 

 and correctness completely toneless whispered speech — a per- 

 formance which previously no electric telephone could accom- 

 plish. Although this " spea*king telegraph," or, more cor- 

 rectly, "thread telephone," possesses no practical value (since 

 its working is still limited to short distances and is interrupted 

 by wind and rain), yet it is most deserving of notice, because 

 it proves that stretched membranes are fitted to take up, 

 almost completely, all the air-vibrations by which they are 

 struck, and to reproduce in another place all speech-sounds 

 and noises when mechanically put into similar vibrations. 



Reis, as is well known, was the first to endeavour to operate 

 the conveyance of tones by electric currents instead of a 

 stretched thread. He made use of the vibrations of a mem- 

 brane exposed to sound-waves to produce closing contacts of 

 a galvanic series. The current-waves hereby generated tra- 

 versed, at the other end of the conduction, the coil of an 

 electromagnet, which, provided with a suitable resonance- 

 arrangement, again produced approximately the same tones 

 by which the membrane, struck by the sound-waves, had been 

 set vibrating. This could only be done very imperfectly, 

 since the contact-arrangements only became effective with the 

 greater vibrations of the membrane, and could only imperfectly 

 render even these. 



Bell appears first to have had the happy thought to let the 

 vibrating membrane itself call forth the currents serving for 

 the transmission of its vibrations — making it of soft iron, and 

 placing its centre opposite and very near to the end of a steel 

 magnet wound round with insulated wire. By the vibrations 

 of the membrane the attraction between the plate and the 

 magnet, and therewith the magnetic potential of the wire- 

 enveloped end of the bar-magnet, were alternately augmented 

 and diminished ; by this, in the wire of the coil and in the 

 conduction, currents were produced which, with the minute- 

 ness of the vibrations of the plate, generated electrical sine 

 vibrations corresponding to the vibrations of the mass of air, 

 which were thus in a condition to call forth again membrane- 

 and air-vibrations in a similar apparatus at the other extremity 

 of the conduction. The result was unaffected by the circum- 

 stance that, as Du Bois-Reymond * has pointed out, in the 

 receiving membrane the phases and ratios of amplitude of the 

 partial tones are different from those in the emitting membrane. 

 * Archivfur Physiologic, 1877, pp. 573, 582, 



