Action of the Microphone. 45 



would require too much time to describe even a few of them ; 

 and as I am anxious in this paper to confine myself to general 

 considerations, I will take it for granted that some of the forms 

 of instrument and the results produced are already known. 



The problem which the microphone solves is this — To in- 

 troduce into an electrical circuit an electrical resistance, which 

 resistance shall vary in exact accord with sonorous vibrations 

 so as to produce an undulatory current of electricity from a 

 constant source, whose wave-length, height, and form shall 

 be an exact representation of the sonorous waves. In the mi- 

 crophone we have an electric conducting material susceptible 

 of being influenced by sonorous vibrations ; and thus we have 

 the first step of the problem. 



The second step is one of the highest importance : it is es- 

 sential that the electrical current flowing be thrown into waves 

 of determinate form by the sole action of the sonorous vibra- 

 tions. I resolved this by the discovery that when an electric 

 conducting matter in a divided state, either in the form of 

 powder, filings, or surfaces, is put under a certain slight pres- 

 sure, far less than that which would produce cohesion and 

 more than would allow it to be separated by sonorous vibra- 

 tions, the following state of things occurs. The molecules 

 at these surfaces being in a comparatively free state, although 

 electrically joined, do of themselves so arrange their form, 

 their number in contact, or their pressure (by increased size 

 or orbit of revolution) that the increase and decrease of elec- 

 trical resistance of the circuit is altered in a very remarkable 

 manner, so much so as to be almost fabulous. 



The problem being solved, it is only necessary to observe 

 certain general considerations to produce an endless variety of 

 microphones, each having a special range of resistance. 



The tramp of a fly or the cry of an insect requires little range 

 but great sensitiveness ; and two surfaces, therefore, of chosen 

 materials under a very slight pressure, such as the mere weight 

 of a small superposed conductor, suffice ; but it would be un- 

 suitable for a man's voice, as the vibrations would be too 

 powerful, and would, in fact, go so far beyond the legitimate 

 range that interruptions of contact amounting to the well- 

 known " make and break " would be produced. 



A man's voice requires four surfaces of pine charcoal, as is 

 described in my paper to the Royal Society, six of willow 

 charcoal, eight of boxwood, and ten of gas-carbon. The 

 effects, however, are far superior with the four of pine than 

 with either the ten of gas-carbon or any other material as yet 

 used. It should be noted that pine wood is the best resonant 

 material we possess ; and it preserves its structure and quality 



