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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 in
strument is as follows : The person sending the message speaks into the mouthpiece Tı, thus causing the diaphragm D,, 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 electromagnet H, and thus reproduces in the diaphragm D the vibrations of the diaphragm D.
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 contrivances 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. sults 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
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.
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
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