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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 magnetotelephone. 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 deter

FIG. 3.

mination 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 of an ohm, while the normal resistance of the carbon itself was three ohms.

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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 resist

ance 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

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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 sub

VOL. XVII.-2

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 suc

ceeded 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.* For the follow

* For a description of the motograph the reader is referred to Edwin M. Fox's article in "Scribner's Monthly," June, 1879.

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ing 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 dia

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LOCAL
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