History
Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. After the turn of the century it was found that negative resistance mercury lamps could amplify, and they were used. The invention of the audion tube in 1906 made transcontinental telephony practical. In the 1930s vacuum tube repeaters using hybrid coils became commonplace, allowing the use of thinner wires. In the 1950s negative impedance gain devices were more popular, and a transistorized version called the E6 repeater was the final major type used in the Bell System before the low cost of digital transmission made all voiceband repeaters obsolete. Frequency frogging repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century.
Read more about this topic: Repeater
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