[GreenKeys] do the length of stop bits affect clutch wear?

[Jim Haynes]

Here's a little-known bit of trivia that I have posted before but
worth posting again I guess.

The question is, why 7.42 unit code (1.42 unit STOP pulse)?

Western Electric developed some teleprinters which used faceplate
receiving and transmitting distributors, and they had both distributors
running on the same shaft.  That obviously prevents stop-start from
working, since the receiver is ready to begin the start segment as soon
as the transmitter finishes the stop segment.  So they put a clutch on
the transmitting distributor, and used a relay to operate the clutch.
This added a little time after the transmitter had finished its rotation
before it could start off again.  Empirically they found that the relay
added 0.42 pulse duration to the process, so it effectively had a 1.42
unit stop pulse.

Now Morkrum (ancestor of Teletype) had solved the problem a different way,
by running the receiving distributor shaft faster than the transmitting
shaft.  So that when the transmitting shaft had completed a rotation the
receiving shaft had finished early and was held back by the clutch.  This
continued through the whole life of the Teletype product line and meant 
that a Teletype printer could work satisfactorily on 7.00 unit code.

The Bell System early on wanted interoperability between the Morkrum and
the Western Electric printers, so they demanded that the Morkrum
transmitters should generate 7.42 unit code.  And this they continued
ever after, even though the Western Electric printers were long extinct.
And for some reason was applied to the machines sold to the military and
other non-Bell customers.  Only Western Union got 7.00 unit code 
keyboards.

So the 7.42 code is ultimately the result of a design short-sightedness
at Western Electric.