In 1831 Wheatstone, by his "magic lyre" experiment, showed that, when the sounding-boards of two musical instruments are connected together by a rod of pine wood, a tune played on one will be faithfully reproduced by the other. This only answers, however, for telephoning musical sounds to short distances. Another and somewhat similar example is furnished by what has been variously designated as the "string," "toy," "lovers'," and "mechanical" telephone. Two disks of thin metal, or two stretched membranes, each furnished with a mouthpiece, are connected together by a thin string or wire attached at each end to the centers of the membranes. A good example may be made with two cylindrical tin cups; the bottoms form the membranes and the cups the mouthpieces. . . .
In July, 1837, Dr. C. G. Page, of Salem, Massachusetts drew attention to the sound given out by an electromagnet at the instant when the electric circuit is closed or broken, and in October of the same year he discust, in a short article entitled "Galvanic Music," the musical note produced by rapidly revolving the armature of an electromagnet in front of the poles. Experiments bearing on this subject were subsequently made by a great number of investigators. Page's discovery is of considerable importance in connection with the theory of action of various forms of telephone, and was a very important feature in the early attempts by Reis to transmit music and speech.
On August 26, 1854, there appeared in L'Illustration (Paris) an interesting article by Charles Bourseul on the electric transmission of speech. The writer recommended the use of a flexible plate at the source of sound, which would vibrate in response to the varying pressure of the air, and thus open and close an electric circuit, and of a similar plate at the receiving station, which would be acted on electromagnetically and thus give out as many pulsations as there are breaks in the current. These suggestions were to some extent an anticipation of the work of Reis; but the conditions to be fulfilled before the sounds given out at the receiving station can be similar in pitch, quality, and relative intensity to those produced at the transmitting station are not stated, and do not seem to have been appreciated.
In Reis's lecture an apparatus was described which has given rise to much discussion as to priority in the invention of the telephone. The instrument was described in over fifty publications in various countries, and was well known to physicists previous to Bell's introduction of the electric telephone as a competitor with the electric telegraph. Reis caused a membrane to open and close an electric circuit at each vibration thus transmitting as many electric pulses through the circuit as there were vibrations in the sound. These electric pulses were made to ad on an electromagnet at the receiving station, which, in accordance with Page's discovery, gave out a sound of a pitch corresponding to the number of times it was magnetized or demagnetized per second. . . .
The next worker at the telephone, and the one to whom the present great commercial importance of the instrument is due, was Bell.2 His aim was the production, by means of the undulations of pressure on a membrane caused by sound, of an electric current the strength of which should at every instant vary directly as the pressure varied. His first idea seems to have been to employ the vibrations of the current in an electric circuit, produced by moving the armature of an electromagnet included in the circuit nearer to or farther from the poles of the magnet. He proposed to make the armature partake of the vibrations of the atmosphere either by converting it into a suitable vibrator or by controlling its vibrations by a stretched membrane of parchment. In the early trials the armature had the form of a hinged lever of iron carrying a stud at one end, which prest against the center of a stretched membrane.
The experiments with this form were not successful, and, with the view of making the moving parts as light as possible, he substituted for the comparatively heavy lever armature a small piece of clock-spring, about the size of a sixpence, glued to the center of the diaphragm. The magnet was mounted with its end carrying the coil opposite, and very close to, the center of the piece of clock-spring. This answered sufficiently well to prove the feasibility of the plan, and subsequent experiments were directed to the discovery of the best form and arrangement of the parts. An increase in the size of the iron disk attached to the membrane augmented both the loudness and the distinctness of the sounds, and this finally led to the adoption of the thin iron disk now in use, which is supported around its edge, and acts as both membrane and armature. Again, the form of the opening or mouthpiece in front of the membrane exercised considerable influence on the efficiency of the instrument, and it was ultimately ascertained that a small central opening, with a thin air space extending across the face of the membrane, was best.
It was also found that comparatively small magnets were sufficient, and that there was no particular virtue in the closed circuit and electromagnet, but that a small permanent magnet having one pole in contact with the end of the core of a short electromagnet, the coil of which was in circuit with the line, but which had no permanent current flowing through it, answered the purpose quite as well. In fact, the effect of keeping a permanent current flowing through the line, and the coils of the electromagnet was to keep the core of the electromagnet magnetized. This seems to have been almost simultaneously pointed out by Bell and others who were working in conjunction with him, and by Professor Dolbear.3 Many experiments were made for ascertaining the best length of wire to use in the coil of the transmitting and the receiving instrument; but this is clearly a question dependent to a large extent on the nature of the line and the system of working adopted.
After Bell's success a large number of experimenters entered the field, and an almost endless variety of modifications have been described. But few possess any real merit, and almost none have any essentially new principles.
A telephone transmitter and a receiver on a novel plan were patented in July, 1877, by Edison, shortly after the introduction of Bell's instruments. The receiver was based on the change of friction produced by the passage of an electric current through the point of contact of certain substances in relative motion. In one form a drum, mounted on an axis and covered by a band of paper soaked in a solution of caustic potash, is turned under a spring, the end of which is in contact through a platinum point with the paper. The spring is attached to the center of a diaphragm in such a way that, when the drum is turned the friction between the point of the spring and the paper deflects the diaphragm. The current from the line is made to pass through the spring and paper to the cylinder.
Now it had been previously shown by Edison that, when a current is made to pass through an arrangement like that just described, the friction between the paper and the spring in greatly diminished. Hence, when the undulating telephonic currents are made to pass through the apparatus, the constant variation of the friction of the spring causes the deflections of the diaphragm to vary in unison with the variation of the electric currents, and sounds are given out corresponding in pitch, and also to some extent in quality, with the sounds produced at the transmitting station. A cylinder of chalk was used in some of Edison's later experiments.
Experiments very similar to those of Edison were made by Elisha Gray, of Chicago, Illinois, and described by him in papers communicated to the American Electrical Society in 1875 and 1878. In these experiments the electric current passed through the fingers of the operator's hand, which thus took the place of the spring in Edison's apparatus. The diaphragm was itself used as the rubbing surface, and it was either mounted and rotated, or the fingers were moved over it. When the current passed, the friction was felt to increase, and the effect of sending a rapidly undulating current through the arrangement was to produce a sound. The application of this apparatus to the transmission of music is described by Gray . . . . .
On April 4, 1877, Mr. Emile Berliner4 filed a caveat in the United States patent office, in which he stated that, on the principle of the variation with pressure of the resistance at the contact of two conductors, he had made an instrument which could be used as a telephone transmitter, and, that, in consequence of the mutual forces between the two parts of the current on the two sides of the point of contact, the instrument was capable of acting as a receiver. The caveat was illustrated by a sketch showing a diaphragm with a metal patch in the center, against which a metal knob was slightly prest by an adjusting screw. This seems to have been the first transmitter in which it was proposed to use the resistance the contact of two conductors.
Almost simultaneously with Berliner, Edison conceived the idea of using a variable resistance transmitter. He proposed to introduce into the circuit a cell containing carbon powder, the pressure on which could be varied by the vibrations of a diaphragm. He sometimes held the carbon powder against the diaphragm in a small shallow cell (from a quarter to half an inch in diameter and about an eighth of an inch deep), and sometimes he used what he describes as a fluff, that is, a little brush of silk fiber with plumbago rubbed into it. In another form the plumbago powder was worked into a button cemented together with syrup and other substances.
1 From an article in the ninth edition of the "Britannica."
2 Alexander Graham Bell was born in Edinburgh in 1847, and came to America in 1870. He settled in Boston the next year as a professor of vocal physiology. Six years later his invention of the telephone was announced. His home is now in Washington.
3 Amos E. Dolbear was born in Norwich, Connecticut, in 1837. From 1874 until his death in 1910 he was professor of physics at Tufts College, near Boston.
4 Emile Berliner was born in Hanover, Germany, in 1861, and came to this country in 1870. Besides his transmitter he invented the talking-machine known as the Victor. His home in in Washington.