TELEPHONE

A telephone transforms sound into electrical signals and back again. When a number is dialled, signals are sent to an exchange, which routes the call. When the caller speaks, a microphone in the handset produces electrical signals. These are sent as electric currents along wires or as radio waves through the air-to the receiver of the other telephone, where a loud-speaker reproduces the sound.

Schematic of a landline telephone installation

EARLY DEVELOPMENT 

• 1844 — Innocenzo Manzetti first mooted the idea of a “speaking telegraph” (telephone).
• 26 August 1854 — Charles Bourseul published an article in a magazine L'Illustration (Paris) : "Transmission électrique de la parole" [electric transmission of speech].
• 26 October 1861 — Johann Philipp Reis (1834–1874) publicly demonstrated the Reis telephonebefore the Physical Society of Frankfurt.
• 22 August 1865, La Feuille d'Aoste reported “It is rumored that English technicians to whom Mr.Manzetti illustrated his method for transmitting spoken words on the telegraph wire intend to apply said invention in England on several private telegraph lines", however telephones were not demonstrated there until after Alexander Graham Bell received his patent in the United States of America in 1876.
• 28 December 1871 — Antonio Meucci files a patent caveat(No. 3335) in the U.S. Patent Office titled "Sound Telegraph", describing communication of voice between two people by wire.
• 1874 — Meucci, after having renewed the caveat for two years does not renew it again, and the caveat lapses.
• 6 April 1875 — Bell's U.S. Patent 161,739 "Transmitters and Receivers for Electric Telegraphs" is granted. This uses multiple vibrating steel reeds in make-break circuits.
• 11 February 1876 — Gray invents a liquid transmitter for use with a telephone but does not build one.
• 14 February 1876 — Elisha Gray files a patent caveat for transmitting the human voice through a telegraphic circuit.
• 14 February 1876 — Alexander Bell applies for the patent "Improvements in Telegraphy", for electromagnetic telephones using undulating currents.
• 19 February 1876 — Gray is notified by the U.S. Patent Office of an interference between his caveat and Bell's patent application. Gray decides to abandon his caveat.
• 7 March 1876 — Bell's U.S. patent 174,465 "Improvement in Telegraphy" is granted, covering "the method of, and apparatus for, transmitting vocal or other sounds telegraphically … by causing electrical undulations, similar in form to the vibrations of the air accompanying the said vocal or other sound."
• 10 March 1876 — The first successful telephone transmission of clear speech using a liquid transmitter when Bell spoke into his device, “Mr. Watson, come here, I want to see you.” and Watson heard each word distinctly.
• 30 January 1877 — Bell's U.S. patent 186,787 is granted for an electromagnetic telephone using permanent magnets, iron diaphragms, and a call bell.
• 27 April 1877 — Edison files for a patent on a carbon (graphite) transmitter. The patent 474,230 was granted 3 May 1892, after a 15 year delay because of litigation. Edison was granted patent 222,390 for a carbon granules transmitter in 1879.