1878.] in varying the Force of an Electric Current. 3 Go 



obtained from willow charcoal containing iron in a fine state of 

 division. 



Pine charcoal treated in this manner (although a non-conductor as 

 a simple charcoal) has high conductive powers, due to the iron; and 

 from the minute division of the iron in the pores, is a most excellent 

 material for the purpose. 



Any one of these preparations confined in a glass tube or a box, and 

 provided with wires for insertion in a circuit, I call a " transmitter." 



Reis, in 1860, showed how, by the movement of a diaphragm, inter- 

 mittent voltaic currents could be transmitted, agreeing in exact number 

 with the sonorous waves impinging on the diaphragm, and thus repro- 

 ducing music at a distance by causing an electro-magnet to vibrate in 

 unison with the diaphragm; and, with an iron diaphragm, Graham 

 Bell showed how the vibrations of that diaphragm in front of a 

 polarised electro-magnet could similarly induce magneto- currents, cor- 

 responding in number, amplitude, and form, with the sonorous vibra- 

 tion, and thus reproduce all the delicacies of the human voice. Edison 

 and others have produced variations in the strengths of a constant 

 current by causing the diaphragm to press directly upon some elastic 

 conductor, such as carbon, spongy platinum, &c, the varying pressure 

 upon these materials varying the resistance of the circuit, and conse- 

 quently the strength of current flowing. Graham Bell and others 

 have produced the same effect, by causing the vibrations of the dia- 

 phragm to vary the electromotive force in the circuit. It will be 

 seen, however, that in the experiments made by myself, the diaphragm 

 has been altogether discarded, resting as it does upon the changes' 

 produced by molecular action, and that the variations in the strengths 

 of the currents flowing are produced simply and solely by the direct 

 effect of the sonorous vibrations. •' ; - 



I have found that any sound, however feeble, produces vibrations 

 which can be taken up by the matter interposed in the electrical 

 circuit. Sounds absolutely inaudible to the human ear affect the re- 

 sistance of the conductors described above. In practice, the effect' is 

 so sensitive, that a slight touch on the board, by the finger nailybii 

 which the transmitter is placed, or a mere touch with the soft part of 

 a feather, would be distinctly heard at the receiving station. The 

 movement of the softest camel hair brush on any part of the board is. 

 distinctly audible. If held in the hand, several feet from a piano, the 

 whole chords — the highest as well as the lowest — can be distinctly- 

 heard at a distance. If one person sings a song, the distant station; 

 provided with a similar transmitter, can sing and speak at the same 

 time, and the sounds will be received loud enough for the person sing- 

 ing to follow the second speech or song sent from the distant end. : ' 



Acting on these facts, I have also devised an instrument suitable for 

 magnifying weak sounds, which I call a microphone. The microphone^ 



2 b 2 



