World's first wireless telephone.

A Photophone receiver and headset, one half of Bell and Tainter's
optical telecommunication system of 1880

The Photophone, also known as a radiophone, was invented jointly by Alexander Graham Bell and his then-assistant Charles Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C. Both were later to become full associates in
the Volta Laboratory Association, created and financed by Bell.

Bell believed the Photophone was his most important invention. The device allowed for the transmission of both articulated sounds and normal human conversations on a beam of light. On April 1, 1880, and also described by plaque as occurring on June 3, Bell's assistant transmitted the world's first wireless telephone message to him on their newly invented form of telecommunication, the far advanced precursor to fiber-optic communications that came into widespread use during the 1980s. The wireless call was sent from the roof of the Franklin School to the window of Bell's laboratory, some 213 metres (700 ft) away.

Of the eighteen patents granted in Bell's name alone, and the twelve he shared with his collaborators, four were for the Photophone, which Bell referred to as his 'greatest achievement', writing that the Photophone was "the greatest invention [I have] ever made, greater than the telephone".

Bell transferred the Photophone's rights to the American Bell Telephone Company in May 1880. The master patent for the Photophone (U.S. Patent 235,199 Apparatus for Signalling and Communicating, called Photophone), was issued in December 1880, many decades before its principles could be applied to practical applications.

Historical plaque marker on the side of the Franklin School at
13th & K Streets NW in Washington, D.C., marks one of the sites
used with Bell's famous invention.

Design and history

The photophone used crystalline selenium cells at the focal point of its parabolic receiver. This material's electrical resistance varies inversely with the illumination falling upon it, i.e., its resistance is higher when it is in the dark, and lower when it is exposed to light. The idea of the Photophone was thus to modulate a light beam: the resulting varying illumination of the receiver would induce a corresponding varying resistance in the selenium cells, which were then used by a telephone to regenerate the sounds captured at the receiver.

The modulation of the transmitted light beam was done by a mirror made to vibrate by a person's voice: the thin mirror would alternate between concave and convex forms, thus focusing or dispersing the light from the light source. The photophone functioned similarly to the telephone, except the Photophone used light as a means of projecting information, while the telephone relied on a modulated electrical signal carried over a conductive wire circuit.

Illustration of a Photophone transmitter, showing the path of reflected sunlight,
before and after being modulated.

Although the photophone was an important invention, it was many years before the significance of Bell's work was fully recognized. While Bell had hoped his new photophone could be used by ships at sea and to also displace the plethora of telephone lines that were blooming along busy city boulevards, his design failed to protect its transmissions from outdoor interferences such as clouds, fog, rain, snow and such, that could easily disrupt the transmission of light.

Until the development of modern laser and fiber-optic technologies for the secure transport of light, factors such as the weather or a lack of electricity or sunlight inhibited the use of Bell's invention.

World's first wireless telephone communication – April 1880

Illustration of a Photophone receiver, depicting the conversion of
modulated light to sound, as well as its electrical power source (P).

In their Washington, D.C. experiment, Bell and Tainter succeeded in communicating clearly over a distance of some 213 meters (about 700 ft.), using plain sunlight as their light source –practical electrical lighting having only just been invented by Edison. Tainter, who was on the roof of the Franklin School, spoke to Bell, who was in his laboratory listening and who signaled back to Tainter by waving his hat from the window as had been requested.

The receiver was a parabolic mirror with selenium cells at its focal point. Conducted from the roof of the Franklin School to Bell's laboratory at 1325 'L' Street, this was the world's first wireless telephone communication (away from their laboratory), thus making the photophone the world's earliest known radiophone and wireless telephone systems. The selenium cells in the parabolic receiver had an electrical resistance varying between 100 and 300 Ω (ohms).

Public acceptance

Although the Photophone was an important invention, it was many years before the significance of Bell's work was fully recognized. While Bell had hoped his new Photophone could be used by ships at sea and to also displace the plethora of telephone lines that were to bloom along busy city boulevards, his design failed to protect its transmissions from outdoor interferences such as clouds, fog, rain, snow and even dust storms that could easily disrupt the transmission of light.

Until the development of pulse-code modulation in the 1930s, and the modern laser and fiber optic technologies of the 1960s and 1970s for the secure transport of light, factors such as the weather and the lack of sunlight inhibited the use of Bell's invention. In an era where people heated their homes with fireplaces and local transportation was completely horse-drawn (electric utility lighting had just started being developed, and the first battery-powered autos were still a decade away), voices emanating from regular 'wired' telephone lines were sometimes viewed with incredulity, the work of possible supernatural forces. Thus the social reticence to the Photophone's futuristic form of telecommunications was palpable, as could be discerned in an 1880 New York Times commentary:

"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."

Sunlight's laughter

However at the time of their February 1880 breakthrough, Bell was immensely proud of the achievement, to the point that he wanted to name his new second daughter "Photophone", which was subtly discouraged by his wife Mabel Bell (they instead chose Marian, with Daisy as her moniker). He wrote somewhat ecstatically:

“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.”
—Alexander Graham Bell, in a letter to his father Alexander Melville Bell, dated February 26, 1880

A diagram from one of Bell's 1880 papers.

Bell later also speculated on the photophone's possible future applications, writing prior to any perception of the laser, and which also vaguely foretold fiber-optic telecommunications: "Can Imagination picture what the future of this invention is to be!.... 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." He also pondered its possible scientific use in the spectral analysis of artificial light sources, stars and sunspots.

Commemoration

In 1947 on the centenary of Bell's birth, the Telephone Pioneers of America dedicated a historical marker on the side of one of the buildings, the Franklin School, that Bell and Tainter used for their successful trial, which was also the world's first wireless telecommunication. The plaque, which did not acknowledge Tainter's contribution, read:

FROM THE TOP FLOOR OF THIS BUILDING

WAS SENT ON JUNE 3, 1880

OVER A BEAM OF LIGHT TO 1325 'L' STREET

THE FIRST WIRELESS TELEPHONE MESSAGE

IN THE HISTORY OF THE WORLD.

THE APPARATUS USED IN SENDING THE MESSAGE

WAS THE PHOTOPHONE INVENTED BY

ALEXANDER GRAHAM BELL

INVENTOR OF THE TELEPHONE

THIS PLAQUE WAS PLACED HERE BY

ALEXANDER GRAHAM BELL CHAPTER

TELEPHONE PIONEERS OF AMERICA

MARCH 3, 1947

THE CENTENNIAL OF DR. BELL'S BIRTH

Plaque on the exterior of the Franklin School

at 13th & K Streets NW in Washington, D.C.