[GreenKeys] Teletype (telephone) Local Loops

[Jim Haynes]

Typically the "battery" supply would come through a resistance lamp
at the telegraph office, then through the wire pair to the customer
location, some current-adjusting resistors there, and back to the
telegraph office on the other wire of the pair.  I've seen several W.U.
teleprinters that were wired that way.  They also included a polarity
sensitive relay to control the motor of the teleprinter...the polarity
was reversed with the telegraph office wanted to communicate with the
customer, otherwise it was there just so the customer could signal the
telegraph office by momentarily breaking the line.

The reason for the resistance lamp is that if the circuit should get
grounded anywhere it would just light the lamp rather than blowing a
fuse.  And the lamp provided some current-stabilizing effect since if
line current increases the lamp filament will get hotter, increasing
its resistance.

The use of two wires of a pair for the telegraph signal helped to
reduce crosstalk to other pairs in the same cable that were carrying
voice.

In Bell System service things got more complicated.  Having a bunch of
teleprinters all drawing 60 ma line current all the time was a significant
waste of power.  So there are various schemes for limiting power, such as
going to 20 ma loops, or having a small current to detect the customer's
call signal before turning on the full line current.  Also there were
wave-shaping measures to get the best performance out of the wire when it
was long.

Bell and W.U. also had at times to put in filtering to prevent 
interference to radios from the telegraph signals.

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