I was at a lunch for ham radio operators yesterday, and the sea of grey hair that I saw has put me in a reflective mood.
I've been searching through various archives this morning, without success, trying to find out if anyone ever figured out the origin of the "E & M" signalling lead designations. When I took the D-18 course at New England Telephone, I read that the designations might have come from old circuit diagrams for telegraph equipment, but that nobody knew for sure.
Bill
I don't remember where I learned it, but I recall it means "Ear and Mouth."
-Gary
***** Moderator's Note *****OK, I guess I didn't write very clearly. I know /what/ the E&M leads do, and the "Ear and Mouth" or "R/e/ceive and Trans/m/it" ways of telling them apart.
What I want to know is /where/ the "E" designation came from; i.e., who/when someone decided to call the "receive" lead the "E" lead, and likewise for the "M" lead. I was taught that these terms /might/ have come from "historical" documents, but that no one knew for sure.
So, let me put the question another way. Why is the receive lead called "E" instead of "Z", or "Y", etc. Why is the transmit lead called the "M" lead, instead of "X" or "W", etc.? Where were these two letters first used in this context?
My thanks to the many respondents who sent in their posts.
Bill Horne Moderator
I *strongly* suspect it was from very early telephones. Which had one lead connected to the 'earpiece', and the other lead connected to the 'mouthpiece'. One side of the local battery was grounded, other side ran to the microphone in the mouthpiece. 2nd side of the microphone went out to the one of the phone wires. other phone wire came in and went to one side of the speaker in the earpiece. 2nd side of the speaker went to ground.
With the 'mouthpiece' and 'earpiece' being -separate- physical objects, unlike the modern 'handset' -- which contains both elements -- and with effectively -no- other components, naming the leads for the object to which they connected ("earpiece"/"mouthpiece"") would seem 'intuitively' obvious'.
***** Moderator's Note *****I'm going to reserve judgement. The "E" and "M" leads have always been for signalling AFAIK, and I suspect that the lead designations came from some early duplex equipment drawings in the telegraph world.
Bill Horne Moderator
FWIW, here is what is posted on Wikipedia at
This is in addition to the "Ear & Mouth" possibility.
Seems pretty unlikely unless either you think that telephones were invented in the UK, or that Bell used British terminology because he was Canadian.
As far as I know, what's called earth in the UK has always been ground in the US, and I believe the thing that the crank turned was usually called a generator or a ringer generator.
I'll leave my 25c on Ear and Mouth.
R's, John
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