Action of the Microphone. 49 



ment whenever I spoke. As yet I have found no such insu- 

 lator for sound as gutta-percha is for electricity. Caoutchouc 

 seems to be the best; but I have never been able by the use of 

 any amount at my disposal to prevent the microphone report- 

 ing all it heard. 



The question of insulation has now become one of necessity, 

 as the microphone has opened to us a world of sounds, of the 

 existence of which we were unaware. If we can insulate the 

 instrument so as to direct its powers on any single object, as 

 at present I am able to do on a moving fly, it will be possible 

 to investigate that object undisturbed by the pandemonium of 

 sounds which at present the microphone reveals where we 

 thought complete silence prevailed. 



I have recently made the following curious observation : — 

 A microphone on a resonant board is placed in a battery-cir- 

 cuit together with two telephones. When one 'of these is 

 placed on the resonant board, a continuous sound will emanate 

 from the other. The sound is started by the vibration which 

 is imparted to the board when the telephone is placed on it ; 

 this impulse, passing through the microphone, sets both tele- 

 phone-disks in motion ; and the instrument on the board, 

 reacting through the microphone, causes a continuous sound 

 to be produced, which is permanent so long as the indepen- 

 dent current of electricity is maintained through the micro- 

 phone. It follows that the question of providing a relay for 

 the human voice in telephony is thus solved. 



The transmission of sound through the microphone is per- 

 fectly duplex ; for if two correspondents use microphones as 

 transmitters and telephones as receivers, each can hear the 

 other, but his own speech is inaudible ; and if each sing a dif- 

 ferent note, no chord is heard. The experiments on the deaf 

 have proved that they can be made to hear the tick of a watch, 

 but not, as yet, human speech distinctly ; and my results in 

 this direction point to the conclusion that we only hear our- 

 selves speak through the bones and not through the ears. 



However simple the microphone may appear at first glance, 

 it has taken me many months of unremitting labour and study 

 to bring to its present state through the numerous forms each 

 suitable for a special object. The field of usefulness for it 

 widens every day. Sir Henry Thompson has succeeded in 

 applying it to surgical operations of great delicacy ; and by its 

 means splinters, bullets, in fact all foreign matter, can be at 

 once detected. Dr. llichardson and myself have been experi- 

 menting in lung- and heart-diseases ; and although the applica- 

 tion by Sir H. Thompson is more successful, I do not doubt 

 that we shall ultimately succeed. There is also hope that 



Phil Mag. S. 5. Vol. 6. No. 34. July 1878, E 



