Photophone: How Alexander Graham Bell Transmitted Sound by Sunlight

Aug 3, 2022 0 comments

Alexander Graham Bell’s greatest invention was the telephone. But if you told that to Mr. Bell, he would have disagreed.

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

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

Illustration of the photophone's transmitter.

What is the photophone?

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

A strong beam of light was reflected off this vibrating mirror. When observed from the location of the receiver, the light beam seemed to pulsate corresponding to the audio frequency of the sound waves acting upon the mirror. Converting the pulsating light beam back to sound proved to be harder. In one experiment, Bell built a receiver using a deposit of lampblack. It produced a tone that Bell described as "painfully loud" to an ear pressed close to the device.

Illustration of a photophone receiver.

In 1878, while honeymooning in Europe with his bride, Bell read a paper by Robert Sabine, published in Nature, where he discussed the newly discovered property of selenium having a variable resistance when acted upon by light. In his experiments, Sabine used a meter to see the effects of light acting on a selenium bar 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 couple and a 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 would be able to hear what Sabine could only see.

Bell hired Charles Sumner Tainter, an instrument maker, and together the pair managed to make a functional photophone in their laboratory by attaching a set of metallic gratings to a diaphragm, with a beam of light being interrupted by the gratings movement in response to spoken sounds. When the modulated light beam fell upon their selenium receiver Bell, on his headphones, was able to clearly hear Tainter singing Auld Lang Syne.

On April 1, 1880, Bell and Tainter successfully communicated over a distance some 79 meters. A few months later on June 21 they succeeded in communicating clearly over a distance of some 213 meters using plain sunlight as their light source. Tainter stationed himself on the roof of the Franklin School and spoke to Bell, who was in his laboratory listening. Bell then signaled back to Tainter by waving his hat vigorously from the window.

Franklin School where the first wireless communication took place. Photo: Difference engine/Wikimedia

Bell and Tainter eventually devised more than fifty means for voice modulating a light wave before moving on to the development of improved sound recording methods and other projects. The popular press of the period was not overtly impressed. The New York Times was openly hostile to the idea. In August 1880, the paper wrote:

The ordinary man ... will find a little difficulty in comprehending how sunbeams are to be used. Does Prof. Bell intend to connect Boston and Cambridge ... with a line of sunbeams hung on telegraph posts, and, if so, what diameter are the sunbeams to be ....[and] will it 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 on his shoulder, and suspending them from pole to pole, there will be a general feeling that there is something about Professor Bell's photophone which places a tremendous strain on human credulity.

Bell, however, was very enthusiastic over his new invention. He dashed off a letter to his father where he announced:

I have heard articulate speech by sunlight! I have heard a ray of the sun 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 sun's disk. You are the grandfather of the Photophone and I want to share my delight at my success.

Bell hoped his new photophone could be used by ships at sea. He could also see wireless communication displacing the tangle of telephone lines that were blooming along busy city boulevards. “Can imagination picture what the future of this invention is to be,” Bell asked. “We may talk by light to any visible distance without any conduction wire.... In general science, discoveries will be make by the Photophone that are undreamed of just now.”

Unfortunately, Bell failed to protect its transmissions from outdoor interferences such as clouds, fog, rain, snow and such, that could easily disrupt the transmission of light. Before long, Marconi's radio transmissions started to far surpass the maximum range of the photophone.

Photophone saw a brief revival and adoption in the German Navy at the turn of the 20th century, thanks to German-Austrian experiments. The German physicist Ernst Ruhmer created a superior receiver using which he achieved sending distances as far as 15 kilometers using high-powered searchlights provided by the Navy. The German Siemens & Halske Company produced units commercially for the German Navy, which offered communications up to 11 kilometers using voice-modulated ship searchlights. The British and the United States governments also worked on technical improvements to Bell's system during World War 1.

Today, light beams are the major carrier of information globally, although not in the form Bell had envisaged. Instead of wirelessly transmitting light signals, these are now carried by optical fibers across continents.


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