[GreenKeys] What is a polar relay for?

[Lee Mushel]

Jeeze!   Sam's experience certainly demonstrates why you shouldn't get your
information from the internet and I might add, CNN.   Sam, keep a watch on
ebay or go to the used book people and get a couple of the old RTTY
manuals---not a League publication.

73

Lee   K9WRU
----- Original Message ----- 
From: "Sam Hallas" <s.hallas at ntlworld.com>
To: "Green Keys" <greenkeys at mailman.qth.net>
Sent: Tuesday, December 30, 2008 6:29 AM
Subject: Re: [GreenKeys] What is a polar relay for?


> 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
>
> _______________________________________________
> GreenKeys mailing list
> GreenKeys at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/greenkeys
>
>