[GreenKeys] Polar Relays

[Jim Haynes]

Yeah, polar signaling is definitely advantageous where long wire lines are
involved, especially open wire lines with their leakage varying with
weather.  And it's easy to implement for a circuit with only two stations,
or for a circuit with one transmitter and many receivers.  But gets 
awkward for more complicated circuits.

Early on of course there was the single loop telegraphy, especially
applicable to railroads where the wire line ran alongside the tracks and
dipped into each station along the way.  Battery needed only at one or
both ends of the circuit, except for small battery to operate sounders
from relays.

But for other situations what came about was a virtual single loop, where
lines ran to the customers' premises from telegraph or telephone offices
and the long distance transmission might be handled by polar currents
or carrier channels.  In the telegraph office there would be repeaters
connecting the customer premises circuits to the long haul lines such that
the whole system appeared to operate like a single neutral loop, but in
fact it was the repeaters that did the magic of concealing the complexity.

 	---

 	"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