Alexander Graham BELL, Ph.D., 
"On the Production and Reproduction of Sound by Light", 
American Journal of Sciences

Third Series, vol. XX, n°118, Oct. 1880, pp. 305- 324.

[Read before the American Association for the 
Advancement of Science, in Boston, August 27, 1880]


    Without dwelling further upon the researches of others I  may say that all observations concerning the effect of light upon the conductivity of selenium have been made by means of the galvanometer, but it occurred to me that the telephone, from its extreme sensitiveness to electrical influences, might be substituted with advantage. Upon consideration of the subject, however, I saw that the experiments could not be conducted in the ordinary way, for the following reasons: The law of audibility of the telephone is precisely analogous to the law of electric induction. No effect is produced during the passage of a continuous and steady current. It is only at the moment of change from a stronger to a weaker state, or, vice versa, that any audible effect is produced; and the amount of effect is exactly proportional to the amount of variation in the current.

    It was, therefore, evident that the telephone could only respond to the effect produced in selenium at the moment of change from light towards darkness, or, vice versa, and that it would be advisable to intermit the light with great rapidity so as to produce a succession of changes in the conductivity of the selenium, corresponding in frequency to musical vibrations within the limits of the sense of hearing. For I had often noticed that currents of electricity, so feeble as hardly to produce any audible effects from a telephone when the circuit was simply opened and closed, caused very perceptible musical sounds when the circuit was rapidly interrupted ; and that the higher the pitch of the sound the more audible was the effect. I was much struck by the idea of in this way producing sound by the action of light.

    I proposed to pass a bright light through one of the orifices in a perforated screen consisting of a circular disc or wheel with holes near the circumference. Upon rapidly rotating the disc  an intermittent beam of light would fall upon the selenium and a musical tone should be produced from the telephone, the pitch of which would depend upon the rapidity of the rotation of the disc.

    Upon further consideration it appeared to me that all the audible effects obtained from variations of electricity could also be produced by variations of light, acting upon selenium. I saw that the effect could not only be produced at the extreme distance at which selenium would normally respond to the action of a luminous body, but that this distance could be indefinitely increased by the use of a parallel beam of light, so that we might telephone from one place to another without the necessity of a conducting wire between the transmitter and receiver.

    It was evidently necessary in order to reduce this idea to practice, to devise an apparatus to be operated by the voice of a speaker, by which variations could be produced in a parallel beam of light, corresponding to the variations in the air produced by the voice.

    I proposed to pass light through a perforated plate containing an immense number of small orifices.

    Two similarly perforated plates were to be employed. One was to be fixed and the other to be attached to the center of a diaphragm actuated by the voice; so that the vibration of the diaphragm would cause the movable plate to slide to and fro[m] over the surface of the fixed plate, thus alternately enlarging and contracting the free orifices for the passage of light. In this way the voice of a speaker could control the amount of light passed through the perforated plates without completely obstructing its passage. This apparatus was to be placed in  the path of a. parallel beam of light, and the undulatory beam emerging from the apparatus could be received at some distant place upon a lens, or other apparatus by means of which it could be condensed upon a sensitive piece of selenium placed in a local circuit, with a telephone and galvanic battery.

    The variations in the light produced by the voice of the speaker should cause corresponding variations in the electrical resistance of the selenium at the distant place, and the telephone in circuit with the selenium should reproduce audibly the tones and articulations of the speaker's voice.

    I obtained some selenium for the purpose of trying the apparatus described; but found upon experiment that its resistance was almost infinitely greater than that of any telephone that had been constructed; and I was therefore unable at that time to obtain audible effects in the way desired. I believed, however, that this obstacle could be overcome by devising mechanical arrangements for reducing the resistance  of the selenium, and by constructing special telephones for the purpose.

    I felt so much confidence in this that in a lecture delivered before the Royal Institution of Great Britain, on the 17th of May, 1878, I announced the possibility of hearing a shadow by means of interrupting the action of light upon selenium. A few days afterwards my ideas upon this subject received a fresh impetus by the announcement made by Mr. Willoughby Smith,(1) before the Society of Telegraph Engineers, that he had heard the action of a ray of light falling upon a bar of crystalline selenium by listening to a telephone in circuit with it.

    It is not unlikely that the publicity given to the speaking telephone during the last few years, may have suggested to many minds, in different parts of the world, somewhat similar ideas to my own; indeed, it has recently come to my knowledge that a writer (J. F. W., (2) of Kew) on the 13th of June 1878, asked the readers of "Nature" through the columns of that periodical, whether any experiments had been made with a telephone in circuit with a selenium galvanic element arranged as in Sabine's selenium battery; (3) and suggested that it  was not unlikely that sounds would be produced in a telephone by the action of light of variable intensity upon a selenium element in circuit with it.

    In September or October, 1878, Mr. A. C. Brown, of London, submitted to me, confidentially, the details of a most ingenious invention of his, of which we may yet hear more. This invention, although entirely different from my own, involved the use of selenium in circuit with a battery and telephone, and the production of articulate speech by the action of a variable light.   I am also aware that Mr. W. D. Sargent, of Philadelphia, has had some ideas of a similar nature, the details of which I do not know. I understood from Mr. Sargent that he proposed submitting selenium to the influence of an oscillating beam of light which should be sent on and off the selenium by the action of the voice. If this is so the effect, produced would be, only of an intermittent character and a musical tone, not speech, would be heard from the telephone in circuit with the selenium.

    Although the idea of producing and reproducing sound by the action of light, as described above, was an entirely original and independent conception of my own, I recognize the fact that the knowledge necessary for its conception has been disseminated throughout the civilized world, and that the idea may therefore have occurred, independently, to many other minds.

    I have stated above the few facts that have come under my observation bearing upon the subject.

    The fundamental idea, an which rests the possibility of producing speech by the action of light, is the conception of what may be termed an undulatory beam of light in contra-distinction to a merely intermittent one.

    By an undulatory beam of light I mean a beam that shines continuously upon the receiver, but the intensity of which upon that receiver is subject to rapid changes corresponding to the changes in the vibratory movement of a particle of air during the transmission of a sound of definite quality through the atmosphere. The curve that would graphically represent the changes of light would be similar in shape to that representing the movements of the air. I do not know whether this conception had been clearly realized by J.F. W., of Kew, or by Mr. Sargent, of Philadelphia, but to Mr. A. C. Brown, of London is undoubtedly due the honor of having distincly and independently formulated the conception and having devised apparatus, though of a crude nature, for carrying it into execution.

tainter.gif (31811 octets)

Charles Summer Tainter (1854-1940)

 

    It is greatly due to the genius and perseverances of my friend, Mr. Summer Tainter, of Watertown, Mass., that the problem of producing and reproducing sound by agency of light has at last been successfully solved. For many months past we have been devoting ourselves to the solution of this problem and I have great pleasure in presenting to you to-night the results of our labors.

(1) See Journ. of Teleg. Engin., May 23, 1878, vii, 284.
(2) Nature, xviii, 169.
(3) Nature, xvii, 512, Apr. 25, 1878.

 

ban3.jpg (8020 octets)

Histoire de la télévision      © André Lange
Dernière mise à jour : 10 janvier 2002