Polarised relay

I believe a "polarised" relay has a permanent magnet in the "magnetic circuit" which makes the armature move in one direction if the net coil current is positive and the other direction if the net coild current is negative. It could be operated to act as a "center off" relay or to simply operate for current only in one direction. There was a significant air gap in the magnetic circuit in all permitted positions of the armature to keep the relay from "latching up."

Edison made his first money in making arrangements for making a single telegraph wire carry traffic in both directions at the same time and for the line to carry more than one message at a time.

| Reading about electrical inventions back when Edison ruled | I often see the polarised relay mentioned. What characterizes | this relay?

Probably the fact that Edison was a DC guy, and the relay used DC and it operated one way for current in one direction, and the other way for current in the othe direction. Reversing the coil energizing polarity with AC doesn't do much.