Alexander Graham BELL, Ph.D.,
[Read before the American Association for the Advancement
Non-Electric Photophonic Receivers.
It is a well known fact that the molecular disturbance, produced in a mass of iron by the magnetizing influence of an intermittent electrical current, can be observed as sound by placing the ear in close contact with the iron, and it occurred to us that the molecular disturbance produced in crystalline selenium by the action of an intermittent beam of light should be audible in a similar manner without the aid of a telephone or battery. Many. experiments were made to verify this theory, but at first without definite results.
The anomalous behavior of the hard rubber screen alluded to above suggested the thought of listening to it also.
This experiment was tried with extraordinary success. I held the sheet in close contact with my ear while a beam of intermittent light was focussed upon it by means of a lens. A distinct musical note was immediately heard. We found the effect intensified by arranging the sheet of hard rubber as a diaphragm, and listening through a hearing tube, as shown in fig.10.
We then tried crystalline selenium in the form of a thin disc and obtained a similar but less intense effect.
The other substances, which I enumerated at the commencement of my address, were now successively tried in the form of thin discs, and sounds were obtained from all but carbon and thin glass.(1)
In our experiments, one interesting and suggestive feature was the different intensities of the sounds produced from different substances under similar conditions. We found hard rubber to produce a louder sound than any other substance we tried, excepting antimony and zinc ; and paper and mica to produce the weakest sounds.
On the whole, we feel warranted in announcing as our conclusions that sounds can be produced by the action of a variable light from substances of all kinds when in the form of thin diaphragms. The reason why thin diaphragms of the various materials are more effective than masses of the same substances, appears to be that the molecular disturbance produced by light is chiefly a surface action, and that the vibration has to be transmitted through the mass of the substance in order to affect the ear.
On this account we have endeavored to lead to the ear air that is directly in contact with the illuminated surface, by throwing the beam of light upon the interior of a tube; and very promising results have been obtained. Fig. 11 shows the arrangement we have tried. We have heard from interrupted sunlight very perceptible musical tones through tubes of ordinary vulcanized rubber, of brass, and of wood. These were all the materials at hand in tubular form, and we have had no opportunity since of extending the observations to other substances. (2)
I am extremely glad that I have the opportunity of making the first publication of these researches before a scientific society, for it is from scientific, men that my work of the last six years has received its earliest and kindest recognition. I gratefully remember the encouragement which I received from the late Professor Henry at a time when the speaking telephone existed only in theory. Indeed, it is greatly due to the stimulus of his appreciation that the telephone became an accomplished fact.
I cannot state too highly also the advantage I derived in preliminary experiments on sound vibrations in this building from Professor Cross, and near here from my valued friend Dr. Clarence J. Blake. When the public were incredulous of the possibility of electrical speech, the American Academy of Arts and Sciences the Philosophical Society of Washington and the Essex Institute of Salem recognized the reality of the results and honored me by their congratulations. The public interest, I think, was first awakened by the judgment of the very eminent scientific men before whom the telephone was exhibited in Philadelphia, and by the address of Sir William Thomson before the British Association for the Advancement of Science. At a later period, when even practical telegraphers considered the telephone as a mere toy, several scientific gentlemen, Professor John Pierce, Professor Eli W. Blake, Dr. Channing, Mr. Clark and Mr. Jones, of Providence, R. I., devoted themselves to a series of experiments for the purpose of assisting me in making the telephone of practical utility ; and they communicated to me, from time to time, the results of their experiment with a kindness and generosity I can never forget. It is not only pleasant to remember these things and to speak of them, but it is a duty to repeat them, as they give a practical refutation to the often repeated stories of the blindness of scientific men to unaccredited novelties, and of their jealousy of unknown inventors who dare to enter the charmed circle of science.
I trust that the scientific favor which was so readily accorded to the Telephone may be extended by you to this new claimant- "The Photophone."
(1) We have since obtained perfectly distinct tones from carbon and thin glass.
(2) A musical tone can be heard by throwing the intermittent beam of light into the ear itself. This experiment was at first unsuccessful on account of the position in which the ear was held.