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What is a Fax?

Updated on November 17, 2009

A fax, or facsimile, is the transmission of still images by wire or radio. A fax scans a document and produces electrical signals that are then sent to another fax machine, which produces a copy of the document.

Faxes are used when a copy of a document has to be transmitted quickly from one place to another. It is used to send bank documents, order forms, weather maps, charts, X-ray photos, newspaper proof pages, fingerprints, telegrams, identification photos, and other documents and papers.

The document is scanned by a photoelectric cell, whose output is amplified and transmitted either as the amplitude modulation or the frequency modulation of a carrier wave. The modulated wave is sent over telephone lines or by radio to a receiver that is synchronized with the transmitter.

At the receiving end, dark and light regions of an original are directly or indirectly reproduced as corresponding black and gray regions on the copy. Various reproduction methods are used. They include the exposure of photosensitive paper inside a light-tight box; passage of current through paper dampened by a conducting solution; burn-off of a white coating deposited on conducting carbon paper; and mechanical striking against carbon paper in contact with white paper.

For reproduction of printed messages or weather maps, a resolution of 96 scan lines per inch is used in the direction of the paper feed and approximately equal resolution in elements per inch is used in the direction of the scan. For reproduction of newspaper and magazine proof sheets, about 500 scan lines per inch are used.

Nowadays email can do all that, and more. But the fax is still an important piece of office equipment.


The first facsimile system was patented by the Scottish inventor Alexander Bain in 1843. The signal originated in an electrical contact that "scanned" grounded metal letters. The receiver used electrochemical recording paper, and the transmitter and receiver were synchronized by using pendulums. In 1869 Ludovic d'Arlincourt, a Parisian, obtained a British patent that achieved synchronization of the transmitter and receiver by using tuning forks. In 1902 the German physicist Arthur Korn developed the first practical system for transmitting photographs based on the use of a selenium photoelectric cell. He started a commercial service in Germany in 1907, and this service soon spread to other countries. The American Telephone and Telegraph Company introduced a wirephoto service in the United States in 1925, and RCA opened the first transatlantic radio-photo circuit for commercial use in 1926.


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