[GreenKeys] Model 15 polarity

[Jim Haynes]

So a clever thing is to put a solid state bridge rectifier between the
input to the machine and the loop it is used in, making it not care
about loop polarity.

In a serious telegraph system the loops running the machines and the
method of transmitting signals over distances are different matters 
altogether.  Consider a railroad where there may be 30 or so stations
all wired in series to the main line.  Simple.  Every station can copy
all the traffic and can send to all the other stations.  But in more
modern systems loops to the customer stations run to the telegraph
offices, where there are repeaters that cause the system to act like
the simple railroad hookup, yet the actual circuits may be neutral
or polar, half duplex or full duplex, perhaps channels of a carrier
system.

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 	"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