E & M signaling

In telephony, an arrangement that uses separate leads, called the “E” lead and “M” lead, for signaling and supervisory purposes. Note 1: The near end signals the far end by applying -48 Vdc to the “M” lead, which results in a ground being applied to the far end’s “E” lead. When -48 Vdc is applied to the far end “M” lead, the near-end “E” lead is grounded. Note 2: The “E” originally stood for “ear,” i.e., when the near-end “E” lead was grounded, the far end was calling and “wanted your ear.” The “M” originally stood for “mouth,” because when the near-end wanted to call (i.e., speak to) the far end, -48 Vdc was applied to that lead.