Prof. W. Siemens on Telephony. 101 



ders the speech-sounds indistinct and accompanies them with 

 a strange, unpleasant clang. 



Hence, for the construction of larger telephones delivering 

 much more powerful currents, I do not use any vibrating plate 

 of iron, but I fix to the membrane that receives the sound- 

 waves (which is made of non-magnetic material) a light coil 

 of wire which waves freely in a ring-shaped strongly magnetic 

 field. By the vibrations of the coil, intense currents alterna- 

 ting in direction are induced in it, which at the other extre- 

 mity of the conduction set in similar vibrations either the coil 

 of a similar instrument or the iron membrane of a Bell tele- 

 phone. 



As the breadth of a flat membrane cannot exceed rather 

 narrow limits without confusing the speech-sounds trans- 

 mitted, by the advice of Prof. Helmholtz I have given to the 

 membrane the form of the tympanum of the ear. This form 

 is obtained, according to Helmholtz, when a moist skin of 

 parchment or a bladder is stretched over the rim of a ring, and 

 then its centre gradually depressed to the desired depth by a 

 screw or otherwise. The membrane will then retain this form 

 after drying. If now a model be made after this form in 

 metal, with its aid a membrane of sheet-brass, or, better, alu- 

 minium can be pressed so as to have the same form as the 

 former. Membranes of this shape are especially suitable for 

 the reception of sound-waves and for the transference of their 

 vis viva to masses that are to be set vibrating (a purpose which 

 they have to fulfil in the ear also), since their flexion results 

 chiefly near the margin of the membrane — while in flat mem- 

 branes it takes place more in the vicinity of the centre, and 

 hence with these only those sound-waves which strike the 

 middle of the plate come into full action. Such a telephone 

 with a parchment membrane 20 centims. in diameter, a wire 

 coil of 25 millims. diameter, 10 millims. height, and 5 millims. 

 thickness, in a magnetic field of great intensity generated by 

 a powerful electromagnet, transmits with perfect distinctness 

 to a great number of smaller telephones every sound produced 

 in any part of a room of moderate size ; and the purity and 

 clearness with which it transmits the sounds of speech and 

 musical notes are remarkable — which may arise partly from 

 the appropriate form of the membrane, and partly from the 

 coil, on moving in the cylindrical magnetic field, generating 

 more regular sinusoid currents than a vibrating iron plate. 

 An apparatus in which such a wire coil is moved rapidly up 

 and down by means of a winch with a long connecting-rod 

 could be used with advantage for the generation of sine-cur- 

 rents of great intensity. 



