History of Science: The first two-way telephone over an outdoor line by Alexander Graham Bell — October 9, 1876

On the afternoon of October 9, 1876, Alexander Graham Bell was in Boston and spoke for three hours with his assistant and fellow inventor, Thomas Watson. It would not have been noteworthy except that Watson was in Cambridgeport, across the Charles River.

It was the first two-way telephone transmitted over outdoor wires, paving the way for a global network that would eventually change the way people communicated.

Bell and Watson were communicating via their device in March of the same year, shortly after Bell patented his improved “telegraph” device. But the crackling command, “Mr. Watson, please come here. I want to see you,” was issued within a short distance.

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In contrast, calls on October days lasted several hours and were transmitted over long telegraph lines.

Bell was in a field crowded with inventors who were dreaming up new ways to transmit sound through electricity. Decades ago, Antonio Meucci created a “telephone” to communicate with his bedridden wife in another room, and took preliminary steps to patent it. In 1861, German inventor Johann Philipp Riess coined the term “telephone” to describe a unique device that converted sound waves into electrical signals and vice versa. His transmission faithfully reproduced the melody, but the words were unintelligibly garbled. And around the same time as Bell, Elisha Gray developed a similar water microphone-based design.

The key to Bell’s voice transmission ability was the concept of transmitting multiple frequencies simultaneously, and he did this using what he called “wavy” or variable current, rather than the intermittent pulses of current that Samuel Morse used for telegraphy.

“The rate of oscillation of the current corresponds to the rate of oscillation of the dielectric, and thus the pitch of the sound produced,” Bell’s patent states. “The strength of the current varies with the amplitude of the vibrations, and therefore with the loudness of the sound.” In other words, the current encoding feature is the sound feature.

Bell’s first prototype used a diaphragm, an inductor (an iron core surrounded by a coil of wire), a permanent magnet, and connecting wires. When a sound wave hits a diaphragm, the pressure waves cause the diaphragm to vibrate. These vibrations moved the inductor, changing the magnetic field it produced, which caused current to flow in the coiled wire. That current was then sent through a wire to a receiver that had the same elements reversed.

Although those first long-distance conversations were exciting, the first telephone line, installed in April 1877, only connected merchants’ shops and homes. Telephone lines were originally rented in pairs and used to connect distant locations. However, the opening of “central exchanges” and switchboards about a year later, allowing calls to be routed between locations, dramatically increased the usefulness of the invention.

It took several decades for the first transcontinental and transcontinental telephone calls to be made, with the first undersea transatlantic telephone cable being laid in 1956.

The telephone industry helped fuel many other modern innovations, including switches, transistors that ushered in the computing age, fiber-optic cables for data transmission, and communications satellites.