The importance of Mr. Gray’s, as of La Cour’s inventions, depends, however, far more on the way in which they increase the message-bearing capacity of telegraphy than on their power of conveying airs to a distance. At the Philadelphia Exhibition, Sir W. Thomson heard four messages sounded simultaneously by the Gray telephone. The Morse alphabet was used. I have mentioned that in that alphabet various combinations of dots and dashes are used to represent different letters; it is only necessary to substitute the short and long duration of a note for dots and dashes to have a similar sound alphabet. Suppose, now, four tuning-forks at the transmitting station, whose notes are Do
, Mi, Sol, and Do
, or say C, E, G, and C´, then by each of these forks a separate message may be transmitted, all the messages being carried simultaneously by the same line to separate sounding reeds (or forks, if preferred), and received by different clerks. With a suitable key-board, a single clerk could send the four messages simultaneously, striking chords instead of single notes, though considerable practice would be necessary to transform four verbal messages at once into the proper telephonic music, and some skill in fingering to give the proper duration to each note.
Lastly, we come to the greatest achievement of all, Professor Graham Bell’s vocal telephone. In the autumn of 1875 I had the pleasure of hearing from Professor Bell in the course of a ride—all too short—from Boston to Salem, Mass., an account of his instrument as then devised, and of his hopes as to future developments. These hopes have since been in great part fulfilled, but I venture to predict that we do not yet know all, or nearly all, that the vocal telephone, in Bell’s hands, is to achieve.
It ought to be mentioned at the outset that Bell claims to have demonstrated in 1873 (a year before La Cour) the possibility of transmitting several messages simultaneously by means of the Morse alphabet.
Bell’s original arrangement for vocal telephony was as follows:—At one station a drumhead of goldbeaters’ skin, about 2¾ inches in diameter, was placed in front of an electro-magnet. To the middle of the drumhead, on the side towards the magnet, was glued a circular piece of clockspring. A similar electro-magnet, drumhead, etc., were placed at the other station. When notes were sung or words spoken before one drumhead, the vibrations of the goldbeaters’ skin carried the small piece of clockspring vibratingly towards and from the electro-magnet, without producing actual contact. Now, the current which was passing along the coil round the electro-magnet changed in strength with each change of position of this small piece of metal. The more rapid the vibrations, and the greater their amplitude, the more rapid and the more intense were the changes in the power of the electric current. Thus, the electro-magnet at the other station underwent changes of power which were synchronous with, and proportionate to, those changes of power in the current which were produced by the changes of position of the vibrating piece of clockspring. Accordingly, the piece of clockspring at the receiving station, and with it the drumhead there, was caused by the electro-magnet to vibrate with the same rapidity and energy as the piece at the transmitting station. Therefore, as the drumhead at one station varied its vibrations in response to the sounds uttered in its neighbourhood, so the drumhead at the other station, varying its vibrations, emitted similar sounds. Later, the receiving drumhead was made unlike the transmitting one. Instead of a membrane carrying a small piece of metal, a thin and very flexible disc of sheet-iron, held in position by a screw, was used. This disc, set in vibration by the varying action of an electro-magnet, as in the older arrangement, uttered articulate sounds corresponding to those which, setting in motion the membrane at the transmitting station, caused the changes in the power of the electric current and in the action of the electro-magnet.
At the meeting of the British Association in 1876 Sir W. Thomson gave the following account of the performance of this instrument at the Philadelphia Exhibition:—“In the Canadian department” (for Professor Bell was not at the time an American citizen) “I heard ‘To be or not to be—there’s the rub,’ through the electric wire; but, scorning monosyllables, the electric articulation rose to higher flights, and gave me passages taken at random from the New York newspapers:—‘S. S. Cox has arrived’ (I failed to make out the ‘S. S. Cox’), ‘the City of New York,’ ‘Senator Morton,’ ‘the Senate has resolved to print a thousand extra copies,’ ‘the Americans in London have resolved to celebrate the coming Fourth of July.’ All this my own ears heard spoken to me with unmistakable distinctness by the thin circular disc armature of just such another little electro-magnet as this which I hold in my hand. The words were shouted with a clear and loud voice by my colleague judge, Professor Watson, at the far end of the line, holding his mouth close to a stretched membrane, carrying a piece of soft iron, which was thus made to perform in the neighbourhood of an electro-magnet, in circuit with the line, motions proportional to the sonorific motions of the air. This, the greatest by far of all the marvels of the electric telegraph, is due to a young countryman of our own, Mr. Graham Bell, of Edinburgh, and Montreal, and Boston, now about to become a naturalized citizen of the United States. Who can but admire the hardihood of invention which devised such very slight means to realize the mathematical conception that, if electricity is to convey all the delicacies of quality which distinguish articulate speech, the strength of its current must vary continuously, and as nearly as may be in simple proportion to the velocity of a particle of air engaged in constituting the sound?”
Since these words were spoken by one of the highest authorities in matters telegraphic, Professor Bell has introduced some important modifications in his apparatus. He now employs, not an electro-magnet, but a permanent magnet. That is to say, instead of using at each station such a bar of soft iron as is shown in [Fig. 6], which becomes a magnet while the electric current is passing through the coil surrounding it, he uses at each station a bar of iron permanently magnetized (or preferably a powerful magnet made of several horse-shoe bars—that is, a compound magnet), surrounded similarly by coils of wire. No battery is needed. Instead of a current through the coils magnetizing the iron, the iron already magnetized causes a current to traverse the coils whenever it acts, or rather whenever its action changes. If an armature were placed across its ends or poles, at the moment when it drew that armature to the poles by virtue of its magnetic power, a current would traverse the coils; but afterwards, so long as the armature remained there, there would be no current. If an armature placed near the poles were shifted rapidly in front of the poles, currents would traverse the coils, or be induced, their intensity depending on the strength of the magnet, the length of the coil, and the rapidity and range of the motions. In front of the poles of the magnet is a diaphragm of very flexible iron (or else some other flexible material bearing a small piece of iron on the surface nearest the poles). A mouthpiece to converge the sound upon this diaphragm substantially completes the apparatus at each station. Professor Bell thus describes the operation of the instrument:—“The motion of steel or iron in front of the poles of a magnet creates a current of electricity in coils surrounding the poles of the magnet, and the duration of this current of electricity coincides with the duration of the motion of the steel or iron moved or vibrated in the proximity of the magnet. When the human voice causes the diaphragm to vibrate, electrical undulations are induced in the coils around the magnets precisely similar to the undulations of the air produced by the voice. The coils are connected with the line wire, and the undulations induced in them travel through the wire, and, passing through the coils of another instrument of similar construction at the other end of the line, are again resolved into air undulations by the diaphragm of this (other) instrument.”