E & M

(Ear & Mouth) Also known as "rEceive & transMit" and "earth & magneto," it is an early telephony term for signaling. For example, PBX operators would signal the long distance operator using E & M. A voltage would be applied to the "mouth" lead of the circuit, which signals the "ear" lead at the other end. There are a variety of two-wire and four-wire E & M signaling methods used between PBXs and PBX to network connections throughout the world.