[GreenKeys] What is a polar relay for?

[Sam Hallas]

amourdutigre wrote:
 > In my readings about electromechanical TTY units, I keep seeing
 > something called a polar relay being referred to. I googled this, and
 > found that it is a device that can be used to revers the polarity on
 > a landline telegraph circuit (not the local loop). Also, most of what
 > little I could find had to do with hand (Morse) telegraphy, nothing
 > with TTY.

Maybe your search reference should have been 'polarized relay' See 
Wikipedia:


      "Polarized relay

"A Polarized Relay placed the armature between the poles of a permanent 
magnet to increase sensitivity. Polarized relays were used in middle 
20th Century telephone exchanges 
<http://en.wikipedia.org/wiki/Crossbar_switch> to detect faint pulses 
and correct telegraphic distortion. The poles were on screws, so a 
technician could first adjust them for maximum sensitivity and then 
apply a bias spring to set the critical current that would operate the 
relay."

They were much more common in 'rest of the world' teletype applications 
where bipolar signalling was employed. +/-80v signalling for teleprinter 
loops works better on longer lines. Of course the old telegraph cables 
also used polar signalling - cable code is like Morse only with current 
reversals instead of long and short pulses. The 'Carpenter relay' is a 
particularly sensitive polarised relay which was commonly used on long 
teleprinter circuits.

All the printers made for the UK Telex market had centre-biased magnets 
and used +/- 80v signalling.

Cheers,
Sam