E and M Signaling - Origin of "E&M"

Origin of "E&M"

Various alternative explanations for the term "E&M" have arisen:

1. "E" stands for "ear," i.e., when the near-end "E" lead was grounded, the far end was calling and "wanted your ear." Whereas "M" is commonly called "mouth," because when the near-end wanted to call (i.e., speak to) the far end, -48 vdc was applied to that lead. Probably this is a mnemonic turned folk etymology.
2. In another story, the proposed labels were R and T for reception and transmission of signaling. However R and T were already used as labels for the tip and ring wire pair carrying voice signals. Thus a letter within each of the word recEive and transMit was chosen. This explanation is probably a backronym.
3. Yet another explanation is that these were sequential designations on the wiring list of the original design. The original design, however, has not been offered in evidence.
4. The most likely explanation is that E&M comes from "earth" and "magneto" from the very earliest days of telephony. An actual magneto (coil) was used to apply -48 volts to the M lead through mechanical relay switches, while the E lead is normally held to ground (earth) unless acknowledging the signaling from the M lead.