Popular Science Monthly/Volume 17/May 1880/The Carbon Button

By E. A. ENGLER, A. M.

ALTHOUGH the telephone seems to have sprung up among us very suddenly, there have been steps in its development which show that the difficulties encountered in devising a means for the transmission of articulate speech have not been overcome altogether by a single stroke of individual genius, but singly by the patient and, for the most part, unrewarded labor of many. Each stage of its development was the outgrowth of suggestions obtained from previous experiments. Of the instruments which served their purpose in the discovery of the properties of the carbon button, a brief description will be given in this paper.

Sound is known to be produced by vibrations, generally of air; differences between sounds are due to differences in vibration. There are but three essential characteristics to be noted, all dependent upon the vibrations of the air: 1. The pitch, by virtue of which a sound is called high or low, and which depends upon the number or rapidity of the vibrations; 2. The intensity or loudness, which is determined by the amplitude of the vibration; 3. The quality by which we distinguish the corresponding tones of different instruments, and which depends on the form of the vibration. In order to obtain an exact reproduction of any sound, its pitch, intensity, and quality must be exactly reproduced; and, to render this possible, the rapidity, amplitude, and form of the vibration must be exactly reproduced.

For producing sound at a distant place two methods suggest themselves: 1. Actually to transmit the sound vibrations through the air; this is the method employed in the speaking tube. 2. To reproduce the sound vibrations at the distant station; this is the method employed in the telephone. The previous development of the telegraph naturally suggested electricity as the agent to carry the vibrations from one place to another. It thus became necessary to convert sound waves into electric waves and vice versa, and experiments looking to the accomplishment of that end were begun nearly twenty-years ago.

The first successful experiments were made by Philip Reis, of Fredericksdorf, Germany, in 1861. He argued that if it could be found practicable to convert sound pulsations into electric pulsations, and then convert these pulsations back again into sound pulsations, the same effect would be produced as if the vibrations had been actually transmitted through the air. In his instruments a membrane rigidly secured on the sides was caused to vibrate in the center by the motion of the air produced by any sound. In the center of this membrane was a delicate circuit-breaker so arranged as to break the circuit of an electric telegraph line at every vibration, thus successively magnetizing and demagnetizing an electro-magnet at the receiving station, and causing its armature to vibrate in accordance with the vibrations of the membrane at the transmitting station. The vibrations of this armature, properly mounted on a sounding-board, set into vibration the surrounding air, which carried the sound to the ear. His first instrument is represented in Fig. 1. A is the transmitting and B the receiving instrument, supposed to be placed at different stations and connected with each other by a metallic conductor. A conical tube, a b, six inches long, four inches in diameter at the larger, and one and a half inch in diameter at the smaller end, is closed at b by a collodion membrane o, against the center of which rests one end, c, of the lever c d. This lever has electric connection with the wire of the line joining the two stations at its point of support, e. The end d of the lever rests against the flat springy, which can be properly adjusted by means of the screw h, and which, through the metal standard f, is connected with the battery C. At station B the conducting wire passed around the electro-magnet m, which is mounted on a sounding-box W; thence to the ground. Attached to the armature at the pole of the magnet is a thin plate i, which is hung on an horizontal axis projecting from the upright k; the motion of the plate can be regulated by the screw l and the spring s. The best dimensions and most suitable adjustments of the instrument were determined by experiment. Its operation is as follows: When at rest the small spring n keeps the lever c d in contact at g, the circuit is closed, and the magnet in attracts the armature i. But, when by speaking into the tube a b, the air in the tube and therefore the membrane o is set into vibration, the contact at g is alternately broken and closed, and consequently the magnet at B is demagnetized and magnetized, alternately releasing and attracting the armature i. It is evident that the vibrations of i correspond in number and interval to the vibrations of the membrane o; so that the sound which enters the tube a b is reproduced at B so far as its pitch is concerned. But as the strength of the current is constant, neither the intensity nor the quality of the sound is reproduced.

In 1874 Elisha Gray, of Chicago, accomplished the reproduction of intensity and quality as well as pitch of sound by means of an instrument in which the strength of the current could be varied in exact accordance with the tone to be transmitted, and was thus enabled to

PSM V17 D022 Speaker design by philip reis 1861.jpg
Fig. 1.

reproduce any number of tones simultaneously without losing their specific character—a thing plainly impossible with the Reis instrument. The device used is shown in Fig. 2. T1 is a mouthpiece into which the person transmitting sounds speaks. D1 is a tense thin diaphragm connected with the line joining the two stations. To the center of the diaphragm is fastened one end of a metal rod N, whose other end dips into a jar J containing acidulated water. A metal plug p at the bottom of this jar is connected with one wire of the battery E, the other going to the ground. At the receiving station the wire simply passes over an electro-magnet H, thence to the ground. Close to H is placed the diaphragm D, properly provided at its center with a metal plate which serves as armature for the electro-magnet, and fastened at its circumference in the holder T. The action of the instrument

PSM V17 D023 Microphone and speaker by elisha gray 1874.jpg
Fig. 2.

is as follows: The person sending the message speaks into the mouthpiece T1 thus causing the diaphragm D1 with the plunger N, to vibrate. The greater the amplitude of vibration the deeper the rod N descends into the liquid, and therefore the thinner the stratum of liquid through which the current will have to pass; thus the resistance to the passage of the current is varied inversely as the intensity of the sound. At the receiving station the current magnetizes the electro-magnet H, and thus reproduces in the diaphragm D the vibrations of the diaphragm D1.

A number of telephones have since been invented, differing from each other in method of application and details of construction, but all embodying the scientific principle used by Gray.

Using the instrument invented by Reis, and the suggestions which Gray's experiments afforded, Thomas A. Edison began his attempts to construct a new form of telephone. Inasmuch as his experiments in this direction "cover many thousand pages of manuscript," only a few of the more characteristic ones will be given.

In the Reis transmitter a platinum screw was made to face the diaphragm, and a drop of water was put between them. The only result, however, was the decomposition of the water and the deposit of a sediment on the platinum. Two disks of platinum, one on the diaphragm and the other on the screw, so placed as to hold several drops of water by capillary attraction, were then tried. Acidulated solutions were substituted for water; paper and other materials, saturated with various solutions, were tried; sharp edges were substituted for disks. The result of all these experiments was complete failure, on account of the decomposition of the fluids. These were therefore abandoned and the attempt was made to vary the strength of the current by the use of platinum points, springs, and other devices. The number of these points which was to be brought into the electric circuit was to be dependent upon the amplitude of the vibration, and thus the resistance of the circuit was to be varied inversely as the intensity of the sound producing the vibration. All of these conrivances were of no avail. Subsequently plumbago and white Arkansas oil-stone were tried on account of their great resistance, and with these fair success was attained. Various expedients were used to make the portion of the material employed in the circuit proportional to the amplitude of vibration, but the confusion introduced by the devices themselves rendered the apparatus practically useless. All these experiments were conducted before the close of the year 1876.

In January of the next year the idea occurred to Mr. Edison to make use of the fact that semi-conductors vary their resistance with the pressure to which they are subjected—a thing which he had accidentally discovered while constructing some apparatus for artificial cables about four years before. He immediately set to work to construct an instrument. A diaphragm carrying at its center a spring faced with platinum was placed opposite to a small cup containing the semi-conductor to be tried. The adjustment was secured by means of a screw fastened to the cup. The vibrations of the diaphragm produced by the tones of the voice determined the pressure of the spring upon the semi-conductor. The materials first experimented upon were crude plumbago mixed with dry powders of different kinds. The results obtained were encouraging, the volume of sound being great, but the articulation so poor that some practice was necessary before the peculiar sound of the instrument could be caught with ease. An improvement was effected when, after much experimenting, solid materials were abandoned and tufts of gloss silk coated with semi-conductors were substituted. But, with all the improvement that could be devised, the instrument was still very inferior to the magneto-telephone of Professor Bell, and required such frequent adjustment as to make it very objectionable. Experiment developed the fact that the change in resistance in the semi-conductor, due to the impact of sound-vibrations, was very small, and, in order to make this change of resistance as important a factor as possible, Mr. Edison determined to make the resistance of his circuit very small: to that end he tried the primary circuit of an induction-coil, but the experiment failed. The cause of failure was at first only a matter of conjecture; but, by trying one thing after another as they suggested themselves, without any very definite purpose, conjecture finally condensed into the belief that the resistance of the semi-conductor was too great to be used with the primary circuit of an induction-coil. The effort then was to reduce the resistance of the semi-conductor to a few ohms and still be able to vary its resistance by the pressure caused by the vibrations of the diaphragm. To effect this a small circular piece, technically termed "button," of the semi-conductor was placed between two platinum disks in a small cup. Electric connection between the disks and the button was secured by inserting a small piece of rubber tubing. The first button was made of solid plumbago, and the results were quite excellent; but still the instrument was inferior to the Bell telephone. Experiments were then made upon many materials in order to obtain a button whose resistance, though small, could be greatly varied; and, when the list of substances, natural and artificial, had been wellnigh exhausted, without very satisfactory result, a fortunate accident led to the solution of the difficulty. A small quantity of lampblack had been taken from the chimney of a smoking petroleum-lamp and preserved as a curiosity on account of its intensely black color. This substance was now tried as, it would seem, a dernier ressort. The results were excellent beyond all hope, the articulation very distinct, and the volume several times as great as could be obtained with a magneto-telephone. It was found that the resistance could be varied by pressure alone from three hundred ohms to the fractional part of a single ohm. Fig. 3 shows an instrument used for the experimental determination

PSM V17 D025 Instrument to determine the change of resistance due to pressure.jpg
Fig. 3.

of the change of resistance due to pressure only. C is a piece of carbon placed between two metallic plates which are connected with the battery, B, in whose circuit is also the galvanometer G. As the current passes it must go through the carbon, the pressure on which can be varied by changing the weights placed upon it. The deflections of the galvanometer-needle indicated that the resistance of the carbon varied inversely as the pressure to which it is subjected. The best arrangement proved to be to make the resistance of the circuit 610 of an ohm, while the normal resistance of the carbon itself was three ohms.

Good results were obtained with other materials besides carbon; the following is a list of the six most useful substances for this purpose in the order of their value: 1. Lampblack; 2. Hyperoxide of lead; 3. Iodide of copper; 4. Graphite; 5. Gas-carbon; 6. Platinum-black.

In the manufacture of the carbon button great care has to be taken that the deposit of lampblack be obtained at the lowest possible temperature, and untouched by the flame; otherwise it is utterly useless for the purpose. Thus commercial lampblack offers very great resistance to the passage of the electric current, and for that reason can not be used at all. The lampblack taken from the chimney is laid upon a white slab, where the brown portions are readily detected and removed. The pure black portion is then ground and subjected to a pressure of several thousand pounds in a mold. It is then repowdered and repressed several times, and finally molded into buttons weighing three hundred milligrammes each.

The special advantages of the carbon button over buttons of other materials are notably its sensitiveness to very slight changes of pressure, its remarkable elasticity and its delicacy over a long range of absolute pressures. These properties it possesses in a higher degree than any other substance, and the explanation of this peculiarity has been found in certain of its physical characteristics. Microscopic examination has shown that, of all finely divided substances, whether obtained by chemical or mechanical means, lampblack is the most finely divided. Now, it is known that the change in resistance of any piece of finely divided material, caused by change of pressure, is due to the increase or diminution of the number of particles brought into contact with each other. On this account a given change of pressure will show a greater change of resistance in carbon than in any other substance. Moreover, with other materials, a point is soon reached when additional pressure ceases to produce any appreciable change in resistance, doubtless because all the particles are already in contact. But the fact that lampblack is so finely divided enables it to respond to changes of pressure long after other materials have lost their sensitiveness. For this reason a comparatively large initial pressure can be used with the carbon, and the instrument is not so easily thrown out of adjustment. That the greater delicacy of the lampblack is due to the fact that it is so finely divided has been confirmed by experiments made with gas-retort carbon, the particles of which are comparatively coarse, graphite, which is more finely divided, and lampblack, whose particles are the finest of all. The changes of resistance for a given change of pressure were found to be proportional to the number of particles in a given volume, or inversely proportional to the size of the particles. By microscopic comparison between a Rutherford diffraction grating having 17,291 lines ruled to the inch on a piece of speculum metal, Mr. Edison estimated that there could not be less than 10,000,000 points in contact in the carbon-button when used in the telephone. This must, however, be regarded only as an approximation.

The only defect in the carbon button is its friability. But, when properly armatured, it need receive no violent shock, and will last as long as necessary. Even if it should happen to become cracked, the volume of sound would not be materially lessened. Experiments have been made to harden the button by mixing various substances with the carbon, and then subjecting the mixtures to high temperatures. Though all these processes tend to impair the delicacy of the button, it is still far superior to a button made of any other substance.

The first application made of the carbon button was in the telephone. The arrangement of the apparatus is shown in Fig. 4. The carbon button, E, is placed between two platinum plates, D and G, which are in the circuit of a battery, as shown by the figure. Upon the upper platinum, D, is placed an ivory plate, C. A piece of rubber

PSM V17 D027 Carbon microphone of Elisha Gray.jpg
Fig. 4.

tubing, B, connects the ivory with the vibrating diaphragm, A A. All this is inclosed in a hard-rubber case with suitable mouthpiece and adjusting apparatus. The vibrations of the diaphragm communicated through the rubber cause variations in the pressure upon the carbon, and corresponding variations in the strength of the current which traverses the wire. At the receiving station an instrument similar to the one already described, invented by Gray, may be used.

At first the diaphragm was made so delicate that it continued to vibrate an appreciable time after the cause which set it in vibration ceased to act, at least long enough to cause an interference in articulation due to the mingling of successive vibrations. The object of the piece of rubber was to dampen the vibrations of the diaphragm, or to bring the diaphragm quickly to rest after it has been set in motion by a sound. The rubber was found to be somewhat tardy in its action; at best the sound emitted was muffled. The rubber had the additional disadvantage of becoming somewhat flattened with use, thus necessitating readjustment. Experiments were then made to find something which would bring the diaphragm to rest more quickly than the rubber could, and for that purpose a thin spiral metallic spring was stituted. But the spring itself gave out a tone when the diaphragm was in vibration, and was therefore objectionable. To overcome this difficulty thicker wire was used for the spring, and with better results. Trials were made with wires of different thicknesses, and it was found that the results improved as the thickness of the wire was increased, until finally the best results were obtained by using a piece of solid material rigidly secured to the diaphragm and ivory plate. It then occurred to Mr. Edison that, inasmuch as the working of his instrument depended upon changes of pressure only, there would be no need of having a vibrating diaphragm at all. A heavy diaphragm was therefore constructed and rigidly fastened to the carbon disk, so that the loudest tones would produce no vibration in it. With this arrangement the articulation was perfect, and, because the comparatively large area of the inflexible plate produced a greater pressure upon the carbon for a given tone than could be obtained when only the one point of the plate or diaphragm was used, the volume of sound was so magnified that a whisper three feet from the instrument was distinctly intelligible at the other end of the line.

Besides greater simplicity of construction, the carbon telephone possesses advantages over all others. With the telephone, as with an ordinary telegraphic instrument, there is a limit beyond which it fails to be of service, but with the telephone this limit is sooner reached than with the ordinary instruments. For this two causes are assigned: 1. The greater rapidity with which the electric impulses are sent over the line in the use of the telephone allows the line less time for charge and discharge than in Morse circuits where the transmission is done by hand; 2. The inductive action of currents passing through neighboring wires often renders the signals indistinguishable. These disturbances occur with all telephones, but they are least noticeable with the carbon telephone, because with it a stronger current is used, and therefore less sensitive receivers are required. Mr. Henry Bentley, President of the Local Telegraph Company at Philadelphia, made a set of experiments with this apparatus upon the lines of the Western Union Telegraph Company, which were on poles along with other wires through which currents were passing sufficiently strong to render the magneto-telephone useless, and found it entirely successful for a distance of from one hundred to two hundred miles. He has succeeded in using it upon a line seven hundred and twenty miles long. His experiments also show that the instrument can be used in a Morse circuit with a battery and eight or ten way-stations, using the ordinary telegraphic apparatus. It can also be used upon a wire which is at the same time being worked quadruplex.

The carbon telephone is rendered even more efficient when used in connection with the electro-motograph receiver.[2] For the following drawing and description, given with the sanction and approval of Mr. Edison, the writer is indebted to the courtesy of Mr. S. D. Mott, of Edison's laboratory:

"The course and action of the currents in Edison's loud-speaking telephone are as follows: Reference is made to the accompanying

PSM V17 D029 Circuits between two phones using the motograph.jpg
Fig. 5.
diagram, Fig. 5, which represents the termini of a telephone-line; C, the induction coil, consisting of a primary, secondary, and tertiary circuit; T, the carbon transmitter; R, the electro-motograph receiver; B, battery; r, relay; b, bell; p, push-button; and p’, bell-button. The local circuit is represented in dotted lines (- - - -); the primary thus —— - —— - ——; the secondary thus —— - - - —— - - - ——; and the tertiary thus —— —— —— ——. Suppose A, station 1, wishes to communicate with B, station 2. He depresses the bell-button p’, when, it will be seen, a circuit is completed over the line through B's relay, closing his local circuit and ringing his bell; B then answers by depressing his bell-button and ringing A's bell. When A speaks he depresses his push-button p connecting his primary and tertiary, which completes his local primary circuit passing through the transmitter, where the electric impulse is transformed, as it were, into electric waves of varying number and amplitude by the peculiar property of the carbon button as varying pressure is put upon it by the vibrating diaphragm actuated by the voice. This electric wave-impulse, in passing through A's primary coil, induces a corresponding current in his secondary, which is transmitted, as may be traced over the line, into B's coil, when induction again takes place in B's tertiary, and B will then hear from his receiver what A has to say, and transmits his answer by the same modus operandi. The second connection that A makes when he depresses his push-button p is for the purpose of keeping his tertiary closed in order that B might interrupt him at any time during the communication. The reason for the alternate contact of the primary and tertiary at p is that each contact gives a slight but harmless knock upon the chalk cylinder of the motograph receiver, which, if occurring simultaneously, tends to disrupt its surface. For talking, one of the two Callaud cells is used; for the bell the two are required. Mr. Edison has lately adopted a small electric engine instead of a crank for the motograph purposes, which occasions the use of an extra cell."

While Mr. Edison was experimenting with his telephone in order to ascertain the proper arrangement of the diaphragm, he found that the expansion or contraction of the rubber handle caused such variations of pressure on the carbon button as to render the instrument inarticulate and sometimes even inoperative. He then tried iron handles. The same trouble was experienced, and, in addition, the receiving instrument was found to emit a kind of sound, which was attributed to the molecular action of the iron during the process of expansion. The immediate result of this discovery was that the handle of the instrument was dispensed with; but it also furnished a suggestion which, calling prominent attention to the extreme delicacy of the carbon button, led to the invention of the micro-tasimeter. If the carbon button would respond to changes of pressure as small as those caused by molecular action in the handle of the telephone, it would also serve as a means of measuring such small differences of pressure, and thus furnish a comparison between the causes which produced them. The essential principle of the tasimeter is shown in Fig. 6.

PSM V17 D031 Microtasimeter measuring carbon sensitivity.jpg
Fig. 6.

A firm standard, A, holds at its upper end a screw which works against a follower, H, to which is attached the metal cup, I. At the base, between two platinum plates, a, a, is the carbon, C; the platinum plates are in a battery circuit provided with a galvanometer. Upon the upper platinum rests a metallic cup, D. Between the two cups, I and D, is placed a piece, E, of any material upon which experiment is to be made. The expansions and contractions of E cause changes of pressure upon the carbon, and thus changes of resistance in the electric circuit which are indicated by the galvanometer. The screw-head is turned until the initial pressure is sufficient to deflect the needle a few degrees. After the needle comes to rest, the slightest change of pressure will be indicated. The delicacy of the instrument depends largely upon the coefficient of expansion of the material used at E. With a piece of hard rubber, upon which the heat from the hand placed a few inches away is allowed to act, there is a deflection in the needle of a galvanometer which is insensible to the action of a thermopile facing a red-hot iron near at hand. When extreme delicacy is required, a Thomson's reflecting galvanometer is employed in a Wheatstone bridge in the way indicated in Fig. 7. The tasimeter is placed at i, and adjusted to a given resistance. The resistance at a, b, c, is made the same. The galvanometer is placed at G, and the minutest change of resistance at i is indicated at the galvanometer scale.

The instrument is of service for a variety of uses. It is an excellent device for detecting and measuring small and almost inappreciable quantities of heat. In the total eclipse of the sun in 1878, by the aid of the tasimeter, what was previously only a matter of conjecture was proved to be a certainty—that the corona of the sun emits heat. The apparatus above described was arranged with as much care as possible, so that the smallest amount of heat might be detected. So great was the delicacy of the instrument that, at the time of total eclipse, when the beam from the corona was allowed to fall upon the tasimeter, the spot of light reflected from the galvanometer mirror not only changed its position, but moved completely off the scale which had been provided; so that, while the presence of heat in the corona was demonstrated, measurement of it was impossible. The instrument has also been used in measuring the heat of some of the stars.

PSM V17 D032 Wheatstone bridge using Thomson galvanometer.jpg
Fig. 7.

Besides being used as a delicate thermometer, the tasimeter also serves as a means of determining the coefficient of expansion of bodies; for, by having a micrometer screw attachment, the amount of expansion can be readily determined. By turning the screw, when the needle has been deflected, until it is brought back to zero, the increase in length can be read by the number of turns or parts of a turn the screw has been moved. Fig. 8 gives a section of the tasimeter, showing the micrometer screw. The piece of material to be tested is seen at A, being clamped rigidly at B, and resting in a metal socket M, which rests upon the carbon placed in the battery circuit as indicated. The object of the funnel-shaped opening, with a small slit facing A, is to cut off all heat which is not wanted.

The tasimeter can also be used as a delicate hygrometer; by inserting

PSM V17 D033 Mechanism of the micro tasimeter.jpg
Fig. 8.

a piece of any substance which is capable of changing in volume as the effect of a change in moisture in the air, the pressure on the carbon will be varied accordingly and indicated by the galvanometer.

PSM V17 D033 Perspective view of the micro tasimeter.jpg
Fig. 9.—Perspective view of Micro-tasimeter.

The great sensitiveness of this instrument makes it seem probable that it will be used for many purposes not at present thought of. Special modifications for particular purposes are always possible. Fig. 10 shows a special application of the principle of the tasimeter devised at my suggestion by Professor C. A. Smith, of Washington University, St. Louis. A is a silver tube securely fastened at the top into a brass collar E. At the lower end, o, a steel rod, B, is firmly joined to the silver tube and runs up within it through the collar, E, and the carbon,

PSM V17 D034 Micro tasimeter used to measure expansion of tubes.jpg
Fig. 10.

C, to a nut, D, by which the whole is clamped together. At E a screw-thread is cut, so that a brass tube, somewhat larger and longer than the silver tube, may be joined with the instrument for purposes of protection. The expansion or contraction of the silver tube, or, if the change in temperature is not sudden, the difference in expansion or contraction between the silver and the steel determines the variation of pressure on the carbon. It is proposed to use this instrument in determining the changes of temperature in steam cylinders, the laws of motion of a fluid whose temperature is not uniform, the rapidity of mixture when fluids of different temperatures are brought together, and the number of thermal units in any given volume of fluid.

After having discovered the peculiar properties of the carbon button, Mr. Edison made the current pass through several carbon disks instead of one. Increase in the intensity of the sound was noticed, but the articulation was impaired. The experiment was tried in a number of different devices, one of which is shown in Fig. 11. Instruments

PSM V17 D034 Edison microphone employing several carbon discs.jpg
Fig. 11.

of this class, whose object is to magnify the sound, have come to be known as microphones, though it is doubtful if any of them succeeded in transmitting very faint sounds so that they could be intelligible at a distance.

Intimately associated with Mr. Edison's discovery and use of the properties of the carbon button are the experiments of Professor Hughes, of London. In May, 1878, Professor Hughes made the following discovery: He took a short glass tube and filled it with white silver powder, a mixture of tin and zinc. The ends of the tube were closed with plugs of gas-carbon, and the plugs secured by covering them with sealing-wax. The carbon plugs were connected with the wires of a battery in whose circuit was a galvanometer, Fig. 12. When the tube is held in the hand and subjected to a longitudinal tensile

PSM V17 D035 Galvanometer measuring current variations of tensility tests.jpg
Fig. 12.

strain, the needle of the galvanometer is deflected in one direction; when the tensile strain is changed to a compression, the needle is deflected in the opposite direction. The explanation offered is that, when the tube is stretched, the number of particles of powder in contact with each other, and therefore the intensity of the current, is diminished; when the tube is compressed, the number of particles of powder in contact with each other, and therefore the intensity of the current, is increased. Moreover, this instrument is so sensitive that it is capable of taking up the vibrations of any sound, and of varying the electric current in accordance with them, so that the sound is reproduced at a distance in an ordinary telephone which may be placed in the circuit. If the tube is placed upon a resonating-box, the delicacy is increased. The tube in such an arrangement serves as the transmitting and the telephone as the receiving instrument. Other substances may be substituted for the white silver powder with good results. It is essential, however, that the substance used be not homogeneous. A piece of vegetable carbon plunged when incandescent into a mercury-bath, so that it becomes impregnated with particles of mercury, when placed in the tube, works quite well; pure vegetable carbon, on the contrary, is useless on account of its high resistance. Another form of transmitter is shown in Fig. 13. A piece of carbon, A, is hung on two arms, C, by a metal pivot, and rests at one end on a piece of metallized carbon, D, placed upon a piece of sealing-wax. The arrangement of the wires can be understood by the figure. Variations of pressure upon D produce variations of intensity in the current. This crude instrument is so delicate that even the tread of a fly produces a sufficient change of pressure, and consequent change of intensity in

PSM V17 D036 Resistance tests with metallized carbon.jpg
Fig. 13.

the current, to be distinctly heard in the receiving telephone. Fig. 14 shows another form of transmitter. Two pieces of gas-carbon, C C, are stuck to a pine board with sealing-wax, and connected with the wires of a battery. A third piece of carbon, A, pointed at the ends, rests loosely between them. This instrument will transmit low sounds

PSM V17 D036 Measuring capacitance based on contact area.jpg
Fig. 14.

uttered at a distance from it of several yards. The capacity of the two instruments last described for transmitting sounds seems to depend upon the fact that the current is made to pass through an imperfect contact, which, when acted upon by the vibrations of sound, gives to the current an undulatory character. Successful experiments have been made with loose-jointed machinery used as a transmitter. Even a common nail laid loosely across two other nails, insulated from each other but connected with a battery, will make a good transmitter.

That the full outcome of these discoveries has not yet been reached there can be no doubt. Speculation in such matters is easy; but facts developed are sufficiently wonderful to command deepest admiration, without conjecture as to future possibilities.

  1. This paper, at first intended for a special occasion, has been published at the suggestion of several friends. In its preparation, use has been made of information to be found in George B. Prescott's work on the telephone, and in the journals of science. Most of the illustrations are from Prescott's work.
  2. For a description of the motograph the reader is referred to Edwin M. Fox's article in "Scribner's Monthly," June, 1879.