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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.* 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 :

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

“ 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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gram, 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

i 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.

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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

l demonstrated, measurement of it was impossible. The instrument has also been used in measuring the beat of some of the stars.

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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.

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