276 PLEASANT WAYS IN SCIENCE. 



In the telephone a stretched membrane, or a diaphragm 

 of very flexible iron, vibrates when words are uttered in its 

 neighbourhood. When a stretched membrane is used, with 

 a small piece of iron at the centre, this small piece of iron, 

 as swayed by the vibrations of the membrane, causes elec- 

 trical undulations to be induced in the coils round the poles 

 of a magnet placed in front of the membrane. These undu- 

 lations travel along the wire and pass through the coils of 

 another instrument of similar construction at the other end 

 of the wire, where, accordingly, a stretched membrane 

 vibrates precisely as the first had done. The vibrations of 

 this membrane excite atmospheric vibrations identical in 

 character with those which fell upon the first membrane 

 when the words were uttered in its neighbourhood ; and 

 therefore the same words appear to be uttered in the neigh- 

 bourhood of the second membrane, however far it may be 

 from the transmitting membrane, so only that the electrical 

 undulations are effectually transmitted from the sending to 

 the receiving instrument. 



I have here described what happened in the case of that 

 earlier form of the telephone in which a stretched membrane 

 of some sucn substance as goldbeaters sicin was employed, 

 at the centre of which only was placed a small piece of iron. 

 For in its bearing on the subject of the phonograph, this 

 particular form of telephonic diaphragm is more suggestive 

 than the later form in which very flexible iron was employed. 

 We see that the vibrations of a small piece of iron at the 

 centre of a membrane are competent to reproduce all the 

 peculiarities of the atmospheric waves which fall upon the 

 membrane when words are uttered in its neighbourhood. 

 This must be regarded, I conceive, as a remarkable acous- 

 tical discovery. Most students of acoustics would have 

 surmised that to reproduce the motions merely of the central 

 parts of a stretched diaphragm would be altogether insuffi- 

 cient for the reproduction of the complicated series of 

 sound-waves corresponding to the utterance of words. I 

 apprehend that if the problem had originally been suggested 



