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.