How Alexander Graham Bell invented a device to transmit sound by means of sunlight

The greatest invention of Alexander Graham Bell was the telephone (although today it is not very clear if it was just his invention). But if Mr. Bell were told that, he wouldn’t agree.

On June 3, 1880, Alexander Graham Bell transmitted the first wireless telephone message on his newly invented “photophone,” a device that allowed sound to be transmitted in a beam of light. This event took place 19 years before the first wireless radio transmission.

Bell described the invention of the photophone as the greatest achievement of his life, but the importance of Bell’s work was not fully recognized at the time. It would be another century before his work was fully appreciated.

Illustration of the photophone transmitter | public domain photo on Wikimedia Commons

The photophone was a device similar to the contemporary telephone, except that it used modulated light as the means of wireless transmission instead of modulated electricity carried by wires. In its simplest form, the apparatus consisted of a flat mirror of flexible material, against the back of which the speaker’s voice was directed. The air pressure generated by the voice caused the flexible mirror to vibrate, becoming alternately convex and concave, thus scattering and condensing the light.

A strong beam of light was reflected from this vibrant mirror. When viewed from the receiver location, the light beam appeared to pulse in correspondence with the audio frequency of the sound waves acting on the mirror.

Converting the pulsing beam of light into sound proved more difficult. In an experiment, Bell built a receiver using a reservoir of lampblack (coal dust). It produced a tone that Bell described as “painfully loud” to an ear very close to the device.

In 1878, while honeymooning in Europe with his girlfriend, Bell read an article by Robert Sabine, published in Nature, in which he discussed the newly discovered property that selenium had variable resistance when acted on by light. In her experiments, Sabine used a meter to see the effects of light acting on a selenium rod connected in a circuit to a battery. Sabine wrote:the slightest shadow or other variation in the intensity of the light caused a considerable variation in the electromotive force of the torque and the consequent indication”.

Sabine suggested that selenium could be used as one of the elements in a wet galvanic cell, but Bell found a more practical application for the discovery. Bell reasoned that by adding a telephone receiver to the same circuit he could hear what Sabine could only see.

Bell hired Charles Sumner Tainter, an instrument maker, and together they managed to make a working photophone in his lab by attaching a set of metal grids to a diaphragm, with a beam of light interrupted by the movement of the grids in response to spoken sounds. . As the modulated beam of light fell on his selenium receiver, Bell, through his headset, could clearly hear Tainter singing. Auld Lang Syne.

On April 1, 1880, Bell and Tainter successfully communicated at a distance of about 250 feet. A few months later, on June 21, they managed to communicate clearly at a distance of about 213 meters using sunlight as a source. Tainter stood on the roof of the Franklin School and spoke to Bell, who was in his lab listening. Bell signaled back to Tainter by waving his hat briskly from the window.

Another illustration of a photophone receiver | public domain photo on Wikimedia Commons

Bell and Tainter eventually devised more than fifty means of modulating the voice of a light wave before going on to develop improved sound recording methods and other projects. The popular press at the time was not very impressed. The New York Times he was openly hostile to the idea. In August 1880, the newspaper wrote:

The ordinary man… will find a little difficulty in understanding how the sun’s rays are to be used. Does Professor Bell intend to connect Boston and Cambridge… with a line of sunbeams strung on telegraph poles, and, if so, what diameter will be the sunbeams…[y] it will be necessary to insulate them against the weather… until (the public) sees a man going through the streets with a coil of No. 12 sunbeams over his shoulder, and suspending them from pole to pole, there will be a general feeling that there is something on Professor Bell’s photophone which puts a tremendous strain on human credulity.

However, Bell was very excited about his new invention. He hastened to write a letter to his father in which he announced:

I have heard articulately in the sunlight! I have heard a ray of sunshine laugh and cough and sing! …I have been able to hear a shadow and I have even perceived by ear the passage of a cloud across the disk of the sun. You are the grandfather of the photophone and I want to share my joy at my success.

Bell hoped that his new photophone could be used by ships at sea. He also saw wireless communication displacing the tangle of telephone line cables that flourished along city boulevards. “Can the imagination think what will be the future of this invention?Bell asked. “We will be able to speak by light at any visible distance without any conduction wire…. In science in general, the photophone will make hitherto unimaginable discoveries”.

The Franklin School in Washington where the photophone tests took place | photo Difference Engine on Wikimedia Commons

Unfortunately, Bell did not shield its transmissions from outside interference, such as clouds, fog, rain, snow, and others, which could easily disrupt light transmission. Before long, Marconi’s radio transmissions began to far exceed the maximum range of the photophone.

The photophone had a brief revival and adoption in the German navy in the early 20th century, thanks to German-Austrian experiments. German physicist Ernst Ruhmer created a superior receiver with which he achieved sending distances of up to 15 kilometers using high-powered searchlights provided by the Navy. The German company Siemens & Halske produced commercial units for the German Navy, offering communications up to 11 kilometers using voice-modulated ship searchlights. The British and American governments also worked on the technical improvement of the Bell system during World War I.

Today, beams of light are the world’s primary carrier of information, though not in the way Bell had envisioned. Instead of transmitting light signals wirelessly, they are now transported by fiber optics from one continent to another.


This article was published on Amusing Planet. Translated and republished with permission.

Sources

alexander graham bell, On the production and reproduction of sound by light, American Journal of Science October 1880, s3–20 (118) 305–324; DOI: doi.org/10.2475/ajs.s3–20.118.305 | Arturo Gallardo, Forrest M. Mims IIIFiber-optic communication began 130 years ago | Jose Antonio Martin Pereda, History of telecommunications | Wikipedia