36 



REPORT — 1877. 



receiver which emits the sounds received from a distance by means of Keiss's 

 diaphragm. 



It remained for Professor Graham Bell, of Boston, who has been worMng at this 

 question with the true spirit of a philosopher since 1872, to make the discovery by 

 which tone, intensity, and quality of sounds can all be sent. He has rendered it 

 possible to reproduce the human voice with all its modulations at distant points. I 

 have spoken with a person at various distances up to 32 mile3 ; and through about 

 a quarter of a mile I have heard Professor Bell breathe, laugh, sneeze, cough, and, 

 in fact, make any sound the human voice can produce. Without explaining the 

 various stages through which his apparatus has passed, it will be sufficient to explain 

 it in its present form. Like Reiss he throws a diaphragm into vibration, but 

 Professor Bell's diaphragm is a disk of thin iron (a), which vibrates in front of a 

 soft iron core (b), attached to the pole of a permanent bar-magnet N S (see Fig. 2). 



T.inc: 



M g .z. 



K 



This core becomes magnetized by the influence of the bar-magnet N S, inducing all 

 around it a magnetic field and attracting the iron diaphragm towards it. Around 

 this core is wound a small coil (c) of No. 38 silk-covered copper wire. One end of 

 this wire is attached to the line wire, the other is connected to the earth. The 

 apparatus at each end is identically similar, so that it becomes alternately trans- 

 mitter and receiver, first being put to the mouth to receive sounds and then to the 

 ear to impart them. Now the operation of this apparatus depends upon the simple 

 fact that any motion of the diaphragm a alters the condition of the magnet field 

 surrounding the core b, and any alteration of the magnet field (that is, either strength- 

 ening or weakening) means the induction of a current of electricity in the coil c. 

 Moreover, the strength of this induced current depends upon the amplitude of the 

 vibration, and its form on the rate of vibration. The number of currents sent of 

 course depends upon the number of vibrations of the diaphragm. Now each current 

 induced in the coil c passes through the line wire to the coil c, and then it alters 

 the magnetization of the core &', increasing or diminishing its attraction for the iron 

 diaphragm a\ Hence the diaphragm a! is vibrated also, and every vibration of 

 the diaphragm a must be repeated on the diaphragm a' with a strength and 

 form that must vary exactly together. Hence, whatever sound produces the vibra- 

 tion of a is repeated by «', because its vibrations are an exact repetition of those 

 of a. 



It is quite evident, however, that Bell's telephone is limited in its range. The 

 currents operating it are very weak, and it is so sensitive to currents that, when at- 

 tached to a wire which passes in the neighbourhood of other wires, it is subject to 

 be acted upon by every current that passes through any one of those wires. Hence 

 on a busy line it emits sounds that are very like the pattering of hail against a 

 window, and which are so loud as to overpower the effects of the human voice. 



Now Mr. T. A. Edison, of New York, has endeavoured to remedy those defects in 

 Bell's by introducing a transmitter which is operated on by a battery-current, whoso 

 strength is made to vary directly with the quality and intensity of the human 

 voice. In carrying out his investigations in this field, he has discovered the curious 

 fact that the resistance of plumbago varies in some ratio inversely with the pressure 



