Jti7ie 2'], 1878] 



NATURE 



239 



ON THE PHYSICAL ACTION OF THE 

 MICROPHONE ' 



IN the paper read on May 9 before the Royal Society I gave a 

 general outline of the discoveries I had made, the materials 

 used, and the forms of microphone employed in demonstrating 

 important points. I have made a great number of microphones 

 each for some special purpose, varying in form, mechanical 

 arrangement, and materials. It 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 resolves is this — to intro- 

 duce into an electrical circuit an electrical resistance, which resist- 

 ance 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 re- 

 presentation of the sonorous waves. In the microphone 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 

 essential 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 is in a divided state, either in the form of 

 powder, filings, or siu-faces, and 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 vibrations, the 

 following state of things occurred : — The molecules at these sur- 

 faces being in a comparatively free state, although electrically 

 joined, do of themselves so arrange their form, their number in 

 contact, ©r their pressure (by increased size or orbit of revolu- 

 tion), that the increase and decrease^of electricalVesistance of the 

 circuit is altered in a very remarkable manner, so much so as to 

 be almost fabulous. 



The problem being resolved it is only necessary to observe cer- 

 tain 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 sxirfaces therefore of chosen 

 materials under a very slight pressiure, 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 \\ould be too power- 

 ful, 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 

 efi"ects are, however, 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 

 when converted into the peculiar charcoal I have discovered and 

 described. 



It is not only necessary to vary the number of surfaces and 

 materials in accordance with the range and power of the vibra- 

 tions, but these siurfaces and materials must be put under more 

 or less pressure in accordance with the force of the sonorous 

 vibrations. Thus, for a man's voice the surfaces must be under 

 a far greater pressure than for the movements of insects ; still 

 the range of useful effect is very great, as the boxes which I have 

 specially arranged for a man's voice are still sensitive to the tick 

 of a watch. 



In all cases it should be so arranged that a perfect undulatory 

 current is obtained from the sonorous vibrations of a certain 

 range. Thus, when speaking to a microphone transmitter of 

 human speech, a galvanometer should be placed in the circuit, 

 and, while speaking, the needle should not be deflected, as the 

 waves of -f and - electricity are equal, and are too rapid to 

 disturb the needle, which can only indicate a general weakening 

 or strengthening of the current. If the pressure on the mate- 

 rials is not sufficient, we shall have a constant succession of 

 interruptions of contact, and the galvonometer-needle will indi- 

 cate the fact. If the presssure on the materials is gradually 

 increased, the tones will be loud but wanting in distinctness, the 

 galvanometer indicating interruptions ; as the pressure is still 



■• By Prof. Hughes. Cca.municated to the Physical Society, June 8, 1878. 



increased, the tone becomes clearer, and the galvanometer will 

 be stationary when a maximum of loudness and clearness is 

 attained. If the pressure be further increased, the sounds 

 become weaker, though very clear, and, as the pressure is still 

 further augmented, the sounds die out (as if the speaker was 

 talking and walking away at the same time) until a point is 

 arrived at where there is complete silence. 



When the microphone is fixed to a resonant board the lower 

 contact should be fixed to this board, so that the sonorous 

 vibrations act directly on it. The upper contact, where the 

 pressure is applied, should be as free as possible from the 

 influence of the vibrations, except those .directly transmitted to 

 it by the surfaces underneath; it (the upper surface) should 

 have its inertia supplemented by that of a balanced weight. 

 This inertia I find necessary to keep the contact unbroken by 

 powerful vibrations. No spring can supply the required inertia, 

 but an adjustable spring may be used to ensvure that the com- 

 paratively heavy lever shall duly press on the contacts. 



The superposed surfaces in contact may be screwed down 

 by an insulated screw passing through them all, thus doing 

 away with the lever and spring; but this arrangement is far 

 more difficult to adjust, and the expansion by heat of the screw 

 causes a varying pressure. It is exceedingly simple, however, 

 easily made, and illustrates the theoretical conditions better 

 than the balanced lever I have adopted in practice. In order 

 to study the theoretical considerations, and that with the most 

 simple form of microphone, freed from aU surrounding mecha- 

 nisms, let us take a flat piece of charcoal two millims, thick and 

 one centim. square, and, after making electrical contact by 

 means of a copper wire on the lower surface, glue that to a 

 small resonant board, or, better for the purpose of observation, 

 to a block or cube of wood ten centims. square. Upon this 

 superpose one or more similar blocks, the upper surface in 

 communication with a wire, the lower resting flat, or as nearly 

 so as possible, on the lower block. 



The required pressure is put on the upper block, and while 

 in this state the two may be fastened together with glue at 

 the sides, or, better, by an insulated screw. The pressure can 

 then be removed, as the screw or glue equally preserves the 

 force. Let the lower piece be called A and the upper B : 

 when we put this block or board under sonorous vibrations, 

 we cannot suppose an undulatory movement of the actual 

 wave-length in such a mass, that is a length comparable with 

 the real wave-length of the sonorous wave which may be several 

 feet. Now we cannot suppose a wave of any length without 

 admitting that the force must be transmitted from molecule to 

 molecule throughout the entire length : thus any portion of a 

 wave, of which this block represents a fraction, must be in 

 molecular activity. The lower portion of the charcoal A, being 

 part of the block itself, has this molecular action throughout, 

 transmitting it also to the upper block. How is it that the 

 molecular action at the surfaces A and B should so vary the 

 conductivity or electrical resistance as to throw it into waves in 

 the exact form of the sonorous vibrations ? It cannot be because 

 it throws up the upper portion, making an intermittent current, 

 because the upper portion is fastened to the lower, and the 

 galvanometer does not indicate any interruption of current what- 

 ever. It cannot be because the molecules arrange themselves in 

 stratified lines, becoming more or less conductive, as then sur- 

 faces would not be required— that is, we should not require 

 discontinuity between the blocks A and B ; nor would the upper 

 surface be thrown up if the pressure be removed, as sand is on a 

 vibrating glass. The throwing up of this upper piece B when 

 pressure is removed proves that a blow, pressure, or upheaval of 

 the lower portion takes place— that this takes place there cannot 

 be any doubt, as the surface, considered alone (having no depth), 

 could not bodily quit its mass. In fact, there must have been 

 a movement to a certain depth ; and I am inclmed to believe, 

 from numerous experiments, that the whole block mcreases and 

 diminishes in size at all points, in the centre as well as the 

 surface, exactly in accordance with the form of the sonorous 

 wave Confining our attention, however, to the pomts A and 

 B, how can this increased molecular size or form produce a 

 change in the electrical waves ? This may happen in two ways : 

 first,%y increased pressure on the upper surface, due to its en- 

 lar<Tement; or, second, the molecules themselves, finding a 

 certain resistance opposed to their upward movement, spread 

 themselves, making innumerable fresh points of contact. Thus, 

 an undulatory current would appear to be produced by infinite 

 chan<Te in the number of fresh contacts. I am inclined to believe 



