[GreenKeys] Tip on loops

[Jim Haynes]

In Real telegraph offices of course they tended to have one DC power
supply for all the loops served by the office.  I can remember when these
were motor-generator sets, later replaced by tube rectifiers in a smallish
office.  See "Rectifiers as Dividend Sources" in W.U. Technical Review
15:3 July 1961 p. 112.

It was common practice in these offices to feed the rectifier output into
a resistance lamp or ballast lamp before it went to the various apparatus
in the loop and the wire line to the destination.  One good reason for 
this is that if the loop should accidentally be grounded, on its way 
through the apparatus and the outside world, the lamp would light rather
than blowing a fuse.  A secondary benefit is that a lamp operates somewhat
as a current regulator, helping to stabilize the loop current against
variations in loop resistance.  The reason for this is the temperature
coefficient of resistance in the metal filament.  As current increases
the filament gets hotter, increasing its resistance and tending to
reduce the current.

In some of my pseudo-telegraph-office circuits I have been using 25 watt
cylindrical lamps, sometimes called "showcase" lamps.  The 25 watt bulb
has a cold resistance of about 60 ohms (measured) and a hot resistance
of about 576 ohms (calculated).  Hence on a dead short with a 120V supply
the current is limited to about 208 ma.  Since the showcase bulbs are
thin and tubular it is easy to mount several of them side by side.

I've attached a couple of photos of homemade telegraph office artifacts.
One a strip of ten resistance lamps, and one of a small telegraph 
switchboard inspired by Western Union style but made by me.

 	---

 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man
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