While the princ.i.p.al object of this invention was to accomplish the repet.i.tion of signals without the aid of an electromagnetic relay, the instrument devised by Edison was capable of use as a recorder also, by employing a small wheel inked by a fountain wheel and attached to the vibrating arm through suitable mechanism. By means of this adjunct the dashes and dots of the transmitted impulses could be recorded upon a paper ribbon pa.s.sing continuously over the drum.

The electromotograph is shown diagrammatically in Figs. 1 and 2, in plan and vertical section respectively. The reference letters in each case indicate identical parts: A being the chalk drum, B the paper tape, C the auxiliary cylinder, D the vibrating arm, E the frictional pad, F the spring, G and H the two contacts, I and J the two wires leading to local circuit, K a battery, and L an ordinary telegraph key. The two last named, K and L, are shown to make the sketch complete but in practice would be at the transmitting end, which might be hundreds of miles away. It will be understood, of course, that the electromotograph is a receiving and relaying instrument.

Another notable use of the electromotograph principle was in its adaptation to the receiver in Edison's loud-speaking telephone, on which United States Patent No. 221,957 was issued November 25, 1879. A chalk cylinder moistened with a chemical solution was revolved by hand or a small motor. Resting on the cylinder was a palladium-faced pen or spring, which was attached to a mica diaphragm in a resonator. The current pa.s.sed from the main line through the pen to the chalk and to the battery. The sound-waves impinging upon the distant transmitter varied the resistance of the carbon b.u.t.ton therein, thus causing corresponding variations in the strength of the battery current. These variations, pa.s.sing through the chalk cylinder produced more or less electrochemical decomposition, which in turn caused differences of adhesion between the pen and cylinder and hence gave rise to mechanical vibrations of the diaphragm by reason of which the speaker's words were reproduced. Telephones so operated repeated speaking and singing in very loud tones. In one instance, spoken words and the singing of songs originating at a distance were heard perfectly by an audience of over five thousand people.

The loud-speaking telephone is shown in section, diagrammatically, in the sketch (Fig. 3), in which A is the chalk cylinder mounted on a shaft, B. The palladium-faced pen or spring, C, is connected to diaphragm D. The instrument in its commercial form is shown in Fig. 4.

VI. THE TELEPHONE

ON April 27, 1877, Edison filed in the United States Patent Office an application for a patent on a telephone, and on May 3, 1892, more than fifteen years afterward, Patent No. 474,230 was granted thereon.

Numerous other patents have been issued to him for improvements in telephones, but the one above specified may be considered as the most important of them, since it is the one that first discloses the principle of the carbon transmitter.

This patent embodies but two claims, which are as follows:

"1. In a speaking-telegraph transmitter, the combination of a metallic diaphragm and disk of plumbago or equivalent material, the contiguous faces of said disk and diaphragm being in contact, substantially as described.

"2. As a means for effecting a varying surface contact in the circuit of a speaking-telegraph transmitter, the combination of two electrodes, one of plumbago or similar material, and both having broad surfaces in vibratory contact with each other, substantially as described."

The advance that was brought about by Edison's carbon transmitter will be more apparent if we glance first at the state of the art of telephony prior to his invention.

Bell was undoubtedly the first inventor of the art of transmitting speech over an electric circuit, but, with his particular form of telephone, the field was circ.u.mscribed. Bell's telephone is shown in the diagrammatic sectional sketch (Fig. 1).

In the drawing M is a bar magnet contained in the rubber case, L. A bobbin, or coil of wire, B, surrounds one end of the magnet. A diaphragm of soft iron is shown at D, and E is the mouthpiece. The wire terminals of the coil, B, connect with the binding screws, C C.

The next ill.u.s.tration shows a pair of such telephones connected for use, the working parts only being designated by the above reference letters.

It will be noted that the wire terminals are here put to their proper uses, two being joined together to form a line of communication, and the other two being respectively connected to "ground."

Now, if we imagine a person at each one of the instruments (Fig. 2) we shall find that when one of them speaks the sound vibrations impinge upon the diaphragm and cause it to act as a vibrating armature. By reason of its vibrations, this diaphragm induces very weak electric impulses in the magnetic coil. These impulses, according to Bell's theory, correspond in form to the sound-waves, and, pa.s.sing over the line, energize the magnet coil at the receiving end, thus giving rise to corresponding variations in magnetism by reason of which the receiving diaphragm is similarly vibrated so as to reproduce the sounds. A single apparatus at each end is therefore sufficient, performing the double function of transmitter and receiver. It will be noticed that in this arrangement no battery is used The strength of the impulses transmitted is therefore limited to that of the necessarily weak induction currents generated by the original sounds minus any loss arising by reason of resistance in the line.

Edison's carbon transmitter overcame this vital or limiting weakness by providing for independent power on the transmission circuit, and by introducing the principle of varying the resistance of that circuit with changes in the pressure. With Edison's telephone there is used a closed circuit on which a battery current constantly flows, and in that circuit is a pair of electrodes, one or both of which is carbon. These electrodes are always in contact with a certain initial pressure, so that current will be always flowing over the circuit. One of the electrodes is connected with the diaphragm on which the sound-waves impinge, and the vibrations of this diaphragm cause corresponding variations in pressure between the electrodes, and thereby effect similar variations in the current which is pa.s.sing over the line to the receiving end. This current, flowing around the receiving magnet, causes corresponding impulses therein, which, acting upon its diaphragm, effect a reproduction of the original vibrations and hence of the original sounds.

In other words, the essential difference is that with Bell's telephone the sound-waves themselves generate the electric impulses, which are therefore extremely faint. With Edison's telephone the sound-waves simply actuate an electric valve, so to speak, and permit variations in a current of any desired strength.

A second distinction between the two telephones is this: With the Bell apparatus the very weak electric impulses generated by the vibration of the transmitting diaphragm pa.s.s over the entire line to the receiving end, and, in consequence, the possible length of line is limited to a few miles, even under ideal conditions. With Edison's telephone the battery current does not flow on the main line, but pa.s.ses through the primary circuit of an induction-coil, from the secondary of which corresponding impulses of enormously higher potential are sent out on the main line to the receiving end. In consequence, the line may be hundreds of miles in length. No modern telephone system is in use to-day that does not use these characteristic features: the varying resistance and the induction-coil. The system inaugurated by Edison is shown by the diagram (Fig. 3), in which the carbon transmitter, the induction-coil, the line, and the distant receiver are respectively indicated.

In Fig. 4 an early form of the Edison carbon transmitter is represented in sectional view.

The carbon disk is represented by the black portion, E, near the diaphragm, A, placed between two platinum plates D and G, which are connected in the battery circuit, as shown by the lines. A small piece of rubber tubing, B, is attached to the centre of the metallic diaphragm, and presses lightly against an ivory piece, F, which is placed directly over one of the platinum plates. Whenever, therefore, any motion is given to the diaphragm, it is immediately followed by a corresponding pressure upon the carbon, and by a change of resistance in the latter, as described above.

It is interesting to note the position which Edison occupies in the telephone art from a legal standpoint. To this end the reader's attention is called to a few extracts from a decision of Judge Brown in two suits brought in the United States Circuit Court, District of Ma.s.sachusetts, by the American Bell Telephone Company against the National Telephone Manufacturing Company, et al., and Century Telephone Company, et al., reported in Federal Reporter, 109, page 976, et seq.

These suits were brought on the Berliner patent, which, it was claimed, covered broadly the electrical transmission of speech by variations of pressure between opposing electrodes in constant contact. The Berliner patent was declared invalid, and in the course of a long and exhaustive opinion, in which the state of art and the work of Bell, Edison, Berliner, and others was fully discussed, the learned Judge made the following remarks: "The carbon electrode was the invention of Edison....

Edison preceded Berliner in the transmission of speech.... The carbon transmitter was an experimental invention of a very high order of merit.... Edison, by countless experiments, succeeded in advancing the art. . . . That Edison did produce speech with solid electrodes before Berliner is clearly proven.... The use of carbon in a transmitter is, beyond controversy, the invention of Edison. Edison was the first to make apparatus in which carbon was used as one of the electrodes....

The carbon transmitter displaced Bell's magnetic transmitter, and, under several forms of construction, remains the only commercial instrument.... The advance in the art was due to the carbon electrode of Edison.... It is conceded that the Edison transmitter as apparatus is a very important invention.... An immense amount of painstaking and highly ingenious experiment preceded Edison's successful result. The discovery of the availability of carbon was unquestionably invention, and it resulted in the 'first practical success in the art.'"

VII. EDISON'S TASIMETER

THIS interesting and remarkable device is one of Edison's many inventions not generally known to the public at large, chiefly because the range of its application has been limited to the higher branches of science. He never applied for a patent on the instrument, but dedicated it to the public.

The device was primarily intended for use in detecting and measuring infinitesimal degrees of temperature, however remote, and its conception followed Edison's researches on the carbon telephone transmitter. Its principle depends upon the variable resistance of carbon in accordance with the degree of pressure to which it is subjected. By means of this instrument, pressures that are otherwise inappreciable and undiscoverable may be observed and indicated.

The detection of small variations of temperatures is brought about through the changes which heat or cold will produce in a sensitive material placed in contact with a carbon b.u.t.ton, which is put in circuit with a battery and delicate galvanometer. In the sketch (Fig. 1) there is ill.u.s.trated, partly in section, the form of tasimeter which Edison took with him to Rawlins, Wyoming, in July, 1878, on the expedition to observe the total eclipse of the sun.

The substance on whose expansion the working of the instrument depends is a strip of some material extremely sensitive to heat, such as vulcanite. shown at A, and firmly clamped at B. Its lower end fits into a slot in a metal plate, C, which in turn rests upon a carbon b.u.t.ton.

This latter and the metal plate are connected in an electric circuit which includes a battery and a sensitive galvanometer. A vulcanite or other strip is easily affected by differences of temperature, expanding and contracting by reason of the minutest changes. Thus, an infinitesimal variation in its length through expansion or contraction changes the pressure on the carbon and affects the resistance of the circuit to a corresponding degree, thereby causing a deflection of the galvanometer; a movement of the needle in one direction denoting expansion, and in the other contraction. The strip, A, is first put under a slight pressure, deflecting the needle a few degrees from zero.

Any subsequent expansion or contraction of the strip may readily be noted by further movements of the needle. In practice, and for measurements of a very delicate nature, the tasimeter is inserted in one arm of a Wheatstone bridge, as shown at A in the diagram (Fig. 2). The galvanometer is shown at B in the bridge wire, and at C, D, and E there are shown the resistances in the other arms of the bridge, which are adjusted to equal the resistance of the tasimeter circuit. The battery is shown at F. This arrangement tends to obviate any misleading deflections that might arise through changes in the battery.

The dial on the front of the instrument is intended to indicate the exact amount of physical expansion or contraction of the strip. This is ascertained by means of a micrometer screw, S, which moves a needle, T, in front of the dial. This screw engages with a second and similar screw which is so arranged as to move the strip of vulcanite up or down. After a galvanometer deflection has been obtained through the expansion or contraction of the strip by reason of a change of temperature, a similar deflection is obtained mechanically by turning the screw, S, one way or the other. This causes the vulcanite strip to press more or less upon the carbon b.u.t.ton, and thus produces the desired change in the resistance of the circuit. When the galvanometer shows the desired deflection, the needle, T, will indicate upon the dial, in decimal fractions of an inch, the exact distance through which the strip has been moved.

With such an instrument as the above, Edison demonstrated the existence of heat in the corona at the above-mentioned total eclipse of the sun, but exact determinations could not be made at that time, because the tasimeter adjustment was too delicate, and at the best the galvanometer deflections were so marked that they could not be kept within the limits of the scale. The sensitiveness of the instrument may be easily comprehended when it is stated that the heat of the hand thirty feet away from the cone-like funnel of the tasimeter will so affect the galvanometer as to cause the spot of light to leave the scale.

This instrument can also be used to indicate minute changes of moisture in the air by subst.i.tuting a strip of gelatine in place of the vulcanite. When so arranged a moistened piece of paper held several feet away will cause a minute expansion of the gelatine strip, which effects a pressure on the carbon, and causes a variation in the circuit sufficient to throw the spot of light from the galvanometer mirror off the scale.

The tasimeter has been used to demonstrate heat from remote stars (suns), such as Arcturus.

VIII. THE EDISON PHONOGRAPH

THE first patent that was ever granted on a device for permanently recording the human voice and other sounds, and for reproducing the same audibly at any future time, was United States Patent No. 200,251, issued to Thomas A. Edison on February 19, 1878, the application having been filed December 24, 1877. It is worthy of note that no references whatever were cited against the application while under examination in the Patent Office. This invention therefore, marked the very beginning of an entirely new art, which, with the new industries attendant upon its development, has since grown to occupy a position of worldwide reputation.

That the invention was of a truly fundamental character is also evident from the fact that although all "talking-machines" of to-day differ very widely in refinement from the first crude but successful phonograph of Edison, their performance is absolutely dependent upon the employment of the principles stated by him in his Patent No. 200,251. Quoting from the specification attached to this patent, we find that Edison said:

"The invention consists in arranging a plate, diaphragm or other flexible body capable of being vibrated by the human voice or other sounds, in conjunction with a material capable of registering the movements of such vibrating body by embossing or indenting or altering such material, in such a manner that such register marks will be sufficient to cause a second vibrating plate or body to be set in motion by them, and thus reproduce the motions of the first vibrating body."

It will be at once obvious that these words describe perfectly the basic principle of every modern phonograph or other talking-machine, irrespective of its manufacture or trade name.

Edison's first model of the phonograph is shown in the following ill.u.s.tration.

It consisted of a metallic cylinder having a helical indenting groove cut upon it from end to end. This cylinder was mounted on a shaft supported on two standards. This shaft at one end was fitted with a handle, by means of which the cylinder was rotated. There were two diaphragms, one on each side of the cylinder, one being for recording and the other for reproducing speech or other sounds. Each diaphragm had attached to it a needle. By means of the needle attached to the recording diaphragm, indentations were made in a sheet of tin-foil stretched over the peripheral surface of the cylinder when the diaphragm was vibrated by reason of speech or other sounds. The needle on the other diaphragm subsequently followed these indentations, thus reproducing the original sounds.

Crude as this first model appears in comparison with machines of later development and refinement, it embodied their fundamental essentials, and was in fact a complete, practical phonograph from the first moment of its operation.

The next step toward the evolution of the improved phonograph of to-day was another form of tin-foil machine, as seen in the ill.u.s.tration.

It will be noted that this was merely an elaborated form of the first model, and embodied several mechanical modifications, among which was the employment of only one diaphragm for recording and reproducing.

Such was the general type of phonograph used for exhibition purposes in America and other countries in the three or four years immediately succeeding the date of this invention.

In operating the machine the recording diaphragm was advanced nearly to the cylinder, so that as the diaphragm was vibrated by the voice the needle would p.r.i.c.k or indent a wave-like record in the tin-foil that was on the cylinder. The cylinder was constantly turned during the recording, and in turning, was simultaneously moved forward. Thus the record would be formed on the tin-foil in a continuous spiral line.