[GreenKeys] FW: [External] Re: Polar Relay ID

[Jones, Douglas W]

From: Chuck Rehor [chuck at rehor.com] -- Tuesday, June 20, 2023 5:49 PM
> ... what is a Polar Relay and what does it do?

Simple relays (invented by Samuel Morse and Joseph Henry) has two states, on and off, and is insensitive to the direction of current flow through the coil.  If your telegraph or teletype signalling system uses current for space and no current for mark (or visa versa) you use simple relays to receive or relay the signal.

A polar relay also has two states, entering one state when the current through the coil flows one way, and the other state when the current flows the other way.  If your telegraph or teletype signalling system uses current flowing one way for mark and the other way for space, you use polar relays to receive or relay the signal.

Both kinds of relays tend to have hysteresis, remaining in one state until there is a significant change in current toward the other state.  In the case of polar relays, there is usually a bias magnet working against an opposing spring.  Sometimes  there is an adjusting screw to move either the magnet or change the spring tension.  These are adjusted to make the hysteresis symmetrical around zero current, so the relay changes state when the current reaches the same threshold in either direction.

Polar relays were very important for the early transatlantic cables, and it is generally more noise resistant.  Some RTTY receivers (RF modems, really) naturally create a polar signal by the nature of their receiver mechanism.

          Doug Jones