In the 1880s Bell and his collaborator Tainter performed some experiments using light and heat to generate and carry sound. They wrote up the trials in a little monograph Upon the production of sound by radiant energy.
Bell was the consummate speech scientist.
He thought like a larynx and experimented like a larynx.
A larynx interrupts airflow, creating resonant pulses in the mouth and pharynx. Bell’s electrical work focused on interrupted positive current (which he called undulating current) while other experimenters were starting with mathematically ideal sine waves. These experiments focused on interrupted light waves stirring up a resonant response in various materials and tubes, an exact light analog to the airflow pulses of the larynx. The goal was to mobilize the known light response of selenium, but Tainter wanted to understand the overall phenomenon first.
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It was therefore supposed that, if we could lead to the ear air that was directly in contact with the illuminated surface, louder sounds might be obtained, and solid masses be found to be as sonorous as thin diaphragms. The first experiments made to verify this hypothesis pointed towards success. A beam of sunlight was focused into one end of an open tube, the ear being placed at the other end. Upon interrupting the beam, a clear, musical tone was heard, the pitch of which depended upon the frequency of the interruption of the light and the loudness upon the material composing the tube.
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Some of the responsive materials seem surprising, but the experiments steered Tainter toward a more promising material, which we now know to be a semiconductor.
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A brass tube leading into the cavity served for connection with the hearing-tube. When this conical cavity was stuffed with worsted or other fibrous materials the sounds produced were much louder than when a test-tube was employed. Mr Tainter next collected silks and worsteds of different colors, and speedily found that the darkest shades produced the best effects. Black worsted especially gave an extremely loud sound. As white cotton-wool had proved itself equal, if not superior, to any other white fibrous material before tried, he was anxious to obtain colored specimens for comparison. Not having any at hand, however, he tried the effect of darkening some cottonwool with lamp-black . Such a marked reinforcement of the sound resulted that he was induced to try lamp-black alone. About a teaspoonful of lamp-black was placed in a test-tube and exposed to an intermittent beam of sunlight. The sound produced was much louder than any heard before.
Upon smoking the interior of the conical cavity and then exposing it to the intermittent beam, the effect was perfectly startling. The sound was so loud as to be actually painful to an ear placed closely against the end of the hearing-tube.
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A thin layer of carbon was used as a detector in early radios, and a thin layer of carbon also assisted in many industrial processes including electrotyping. Clearly something more than heat absorption was happening in this trial, but without the later understanding of semiconductors the path wasn’t pursued.
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Tainter’s light larynx took the form of a siren disk, already used for acoustics in such devices as foghorns.
A light source is focused by a mirror into the siren disk, and the various substances are placed in a sort of test tube surrounded by an ear trumpet.
It’s clear that Tainter started with a sewing machine, so I did the same. Many centuries ago I had made a Poser model of a sewing machine, and brought it back for this purpose. Amortize, amortize, amortize!
Here’s the light larynx in operation:
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The next version was fancier, using a pendulum to gate the light into two different substances so the two could be compared immediately and perceptually. Better this way or this way?
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The graphite layer was tried in a multipurpose experimental setup to test the purely mechanical effect as well as the electrical effect. A glass substrate was silvered, then a zigzag path was etched out and covered with graphite. The result looks a lot like a solid-state diode to modern eyes.
This device was used with ordinary sound instead of the siren. Sound moved a foil diaphragm slightly:
The moving reflection from the bending diaphragm was focused into the graphite zigzag, and the resulting sound was audible. Tainter also tried applying an interrupted voltage across the carbon, connecting each of the separated silver parts to one side of the battery. Again the sound was audible. It’s not clear if this effect was due to heat. It could have been an electrostrictive effect like a capacitor speaker.
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Finally they switched the graphite zigzag for a layer of selenium, already established as a light-sensitive resistor. This is the final Photophone, where the modulated light changes the resistance of selenium in series with a battery and a conventional telephone receiver.
This method was reinvented 100 years later as fiberoptics, after long experience and experiments with semiconductor MATERIALS in early radio enabled Bell’s successors to form up photosensitive diodes and light-generating diodes for both ends. The heat-based mechanical response was too slow for speech, and selenium is also a slow responder.
Invention comes from development in materials. Ideas are a dime a dozen. Experience and skill in handling materials can only develop when a variety of REAL industries are working with the material in a variety of ways. Invention has moved to China now because the REAL industries have moved to China.
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