[GreenKeys] Why 7.42 code

[jhhaynes at earthlink.net]

I'm reading a Bell System historical document which provides the official
answer to why they used a 7.42 code rather than the 7.0 used by Western
Union or the 7.5 used in Europe.  Some of the very early Western Electric
printing telegraph machines used a receiving distributor where an
electromagnet operated the latch to let the distributor start rotating.
If they made the receiving distributor have seven segments of equal
size then they needed to add the operating time of the electromagnet
latch release, and that was found to take 0.42 of pulse time of one
segment length.  So they transmitted 7.42 code to allow extra time
for the latch release.  (Morkrum did it differently, ran the transmit
and receive shafts at different speeds; but the Bell System insisted
on 7.42 code for compatibility with their early Model 5 teleprinters.
And thus it has been ever since, though the Model 5 machines were long
gone long ago.



jhhaynes at earthlink dot net