First Telephone Repeater

On 18 October 1913 a telephone repeater developed by Western Electric was placed in service on a circuit between New York and Philadelphia. It was probably the first high-vacuum tube amplifier in commercial service.

A repeater is an electronic device that receives a signal and retransmits it with greater strength. Before the invention of this technology, long-distance communication by phone was limited and expensive. As the distance between points increased, clarity dropped; only through the installation of thick copper wires and loading coils, which slowed the rate of signal loss, could engineers resolve voice attenuation. But these coils could not add energy to the line.

In 1906, Lee De Forest submitted a patent for the Audion, the first triode vacuum tube, which promised a solution to the problem of voice amplification. An American engineer who would eventually boast of over one hundred patents, De Forest pioneered the development of wireless broadcasting and telephony. The Audion was originally designed as a two-electrode device, but within a year, De Forest added a third electrode, a grid, between the cathode and anode of the diode. This triode could be used to amplify electrical signals, and became the standard electronic switching element of the first half of the twentieth century until the invention of the transistor in 1947.

De Forest was aware of AT&T’s ambitious plan to create a transcontinental telephone line from New York to San Francisco. In 1912, he began work on converting the Audion into a telephone amplifier. Although it initially created a howling sound, AT&T was nevertheless impressed with De Forest’s design, and bought the patent for the Audion for use in telephone repeaters and wireless communications in 1913 and 1914.

Harold D. Arnold and his team of researchers at AT&T’s Western Electric determined that the Audion worked by creating a vacuum in the tube, and not, as De Forest claimed, through the operation of ions created within the gas of the tube. By removing these ionized gases, Arnold determined, the Audion could be a practical amplifier.

AT&T and its national system of laboratories went to work on bridging the continent’s telephone networks using the triode as a repeater. Having successfully demonstrated its use in the short distance between New York and Philadelphia in 1913, it soon installed an integrated system of lines, loading coils, and repeaters across the Western United States. On June 27, 1914, the last pole of this transcontinental line was raised on the Nevada-Utah state line, and commercial service began on January 25, 1915.