[GreenKeys] What is a polar relay for?

[kf9nz at juno.com]

       There are many different kinds of polar relays.   Railroad signal
systems use a type of polar relay to control signal indications:   normal
polarity= green, reverse polarity= yellow  no voltage=red  for example
The things we are talking about here are a different thing entirely. 
Wireline circuits have a lot of other characteristics than resistance -
and most weren't iron wire.   Capacitance, inductance and leakage all
affect the wave shape and that affects the  received signal which affects
the way the sounder, uh..sounds.   A teletype machine has an
electromagnet that converts the electrical signal to the mechanical
signal that causes the machine to print.   If the armature does not
follow the signal accurately, the result is obvious.    By using polarity
reversals, the transition from a MARK signal and a SPACE signal is much
sharper.    There are other reasons for using polar relays, such as using
one wire to send messages simultaneously in both directions ("Duplex").  


        The relay most of us are familiar with is the Western Electric
255A.  Those things have several adjustments, and it was very easy to get
them so far out of adjustment, they would drive you crazy trying to get
them right.  They plugged into a socket making them easy to replace   I
was so happy to see  sealed mercury wetted contact polar relays replace
the fussy 255a's

Frankf9nz

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