The E6 telephone repeater background, description, and theory of operation

In 1948 the Western Electric Company introduced a 2-way telephone voice repeater which operated in a radically different manner than had previous 2-way telephone repeaters. The new repeater employed the negative impedance principle, meaning that it was essentially a 2-terminal device intended to be inserted in series with a telephone circuit. Previous voice repeaters were 4-terminal units made up of combinations of 1-way amplifiers and hybrid coil circuits connected to provide 2-way transmission. The popularity of the new repeater stemmed from its low cost and from the fact that it could pass d-c supervisory currents and dial pulse signals without need for special bypass equipment.