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In the exchange telephone plant, speech is transmitted largely at voice frequencies over a single pair of wires that carries conversation in both directions. Until recently, adequate transmission was assured through a suitable choice of coil loading and conductor size. The need for voice frequency amplifiers had long been recognized, but wide use of the conventional hybrid type of repeater with separate amplifiers for conversation in the two directions had not been economic. In 1948, however, a new type of repeater was introduced that used the same source of gain for transmission in both directions, and did not require costly filters. This repeater is known as the E1 repeater. It operates as a negative impedance in the line. This paper describes the wide field of usefulness of two new types of negative impedance repeaters, the E2 and E3. Sufficient theory is‐given to show the advantages of using two types of negative impedance in a line, the series type and the shunt type, and how, by the addition of the shunt type to the series type, it is possible to insert gain without serious reactions on line impedance. The new equipment is described and maintenance features, together with methods of testing this relatively new type of repeater, are discussed.

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