File:AT&T type 3A mechanical telephone repeater diagram.png

English: Type 3A mechanical carbon repeater element, a crude early telephone repeater used in the early years of the 20th century by American Telephone and Telegraph Co. to amplify signals on long distance telephone lines. It consisted of a "speaker" and carbon microphone coupled together. The weak incoming signal was applied to a coil, which vibrated an iron plunger, creating sound vibrations, which applied varying pressure to a microphone cell containing granulated carbon, with electrodes. The varying resistance of the carbon modulated a strong current through the cell, which passed into the outgoing telephone line, producing a larger amplitude than the incoming signal. The carbon telephone repeater was invented in 1903 by Herbert E. Shreeve of AT&T and the first successful test was in 1904 in a circuit between Amesbury, Massachusetts and Boston. The carbon repeater was very distorting and unsatisfactory, and was replaced by Audion vacuum tube repeaters, the first successful type of repeater, in 1914, making possible the first transcontinental telephone line between New York and Los Angeles in 1915. In this survey article in 1919 the authors say the carbon repeater is still in use in the Bell System, and only about 1000 of the new Audion repeaters have been installed.

One major problem with carbon repeaters was that temperature changes due to the current through them caused thermal expansion of the carbon granules, changing the pressure on them and thus their resistance, causing DC offsets on the line. This repeater uses a feedback loop to apply a current to a compensating winding to maintain a constant pressure on the carbon.