Carbon Transmitter

From GHN

Carbon Transmitter

A telephone using a carbon button. Courtesy: National Park Service, Edison National Historic Site.

Menlo Park, New Jersey. On 12 March 1877 Edison discovers that mere pressure, not vibration, is sufficient to capture voice for transmission to electrical signals. He received three patents in 1892 for the carbon transmitter he designed for the American Speaking Telephone Company based on this idea, making the telephone a viable commercial technology. 

Although Thomas Edison is most well-known for high-profile inventions such as the phonograph and lighting system, he made lesser-known contributions to a lot of other devices too. One of these is the telephone. Although Alexander Graham Bell gets the credit for inventing the telephone, it was Edison who came up with most widely used transmitter today: we call it a microphone and it’s the part of the phone you speak into. Carbon transmitters for telephones were among Edison’s smallest inventions, but they made the telephone a commercial technology and proved to be one of his most successful ideas.

In 1876 Bell invented the first telephone capable of sending and receiving the human voice along a wire. At first, he used the same device as transmitter and receiver (the receiver is the part you listen to), but soon decided that he should use a separate transmitter and receiver to make the telephone work better. 

Bell’s work inspired a lot of other inventors and businesses to experiment with telephones. One of those was Edison, who became interested in the telephone shortly after Bell unveiled it. Bell tried to use patents to protect his invention, but the telephone shared so many features with earlier telegraph technology that not every aspect of it could be protected that way. So similar was the early telephone to the telegraph that it was originally called a “speaking telegraph.”

In 1877 Western Union hired Thomas Edison to improve the telephone without violating any of Bell’s patents. Edison focused his energy on the telephone transmitter. Bell’s transmitter of 1876 transmitted sound weakly, so many inventors believed they could find something more sensitive that would work better.

Edison had begun experimenting on the telephone in late 1876 and had settled on a transmitter that used a little button of compressed carbon, which he placed in a circuit between metal contacts. Carbon is unusual because its electrical resistance changes under pressure, even very small amounts of pressure. Even though the pressure of a sound wave is tiny, it is enough to cause a significant change in the resistance of the carbon button. If a battery is attached to the contacts and to a telephone line, the carbon button’s changing resistance has the effect of converting the incoming sound waves into a wavering electric signal, which can then be transmitted down the wire. This is exactly what Edison needed to make an improved telephone transmitter different enough from Bell’s to avoid a lawsuit. Even more exciting was that it also worked better than Bell’s technology. Western Union used carbon button transmitters in its telephones to provide service in New York. Although Western Union soon left the telephone business to refocus on telegraphy (with dire long-term results), the carbon-button microphone lived on.

In 1879 the Bell Telephone company was using a transmitter invented by Emile Berliner and modified by Bell employee William Blake. They were dissatisfied with its poor sound quality. Eventually, Bell Telephone obtained the rights to the carbon button transmitter from Western Union and began including it in their telephones. Around 1881 they purchased the rights to a British invention that substituted ground carbon for the compressed carbon button and began using that instead. The ground carbon gave better volume at the receiving end, so it was especially good for long-distance calls. But still they were not satisfied because the transmitter was unreliable.

It was Edison to the rescue again. In 1886 he found an unusual way of improving the carbon granules—roasting them. Transmitters made with these roasted carbon granules were far superior to any that came before them. In 1890, after five years of development, the Bell Company introduced a transmitter using Edison’s carbon transmitter. The granular carbon transmitter became the standard technology in telephones almost immediately, and with a few modifications was used for almost 100 years. Only in the 1980s was the carbon granule transmitter replaced by smaller microphones suitable for new technologies such as cordless and cellular telephones.