[GreenKeys] polar loops

[jhhaynes at earthlink.net]

I suppose the traditional way to convert from neutral to polar would be to
use a polar relay, with a bias current, in the neutral circuit and have
the contacts generate a polar signal.  As is discussed in lots of places,
you put a constant 30ma current through one winding of the polar relay
in a direction opposite to the 60ma line current in the other winding.

Long ago I had a W.U. 401A tape printer, a design derived from the stock
ticker.  I'm pretty sure it worked on a neutral circuit, but I don't
remember how.  Perhaps there was a spring attached to the armature so
that it biased the armature to spacing and the neutral current moved
it to marking.

The polar circuit as you suggest is used because it gives a cleaner
signal - variations in the strength of current don't matter much, because
the polar selector operates over a wide range of currents.  And it
is symmetrical about the zero axis.  The armature is pulled in one 
direction by mark and in the opposite direction by space, so in a true
polar selector you don't have the complication that the magnetic pull
increases with decreasing air gap as the magnet moves in one direction
and decreases with increasing air gap in the other direction.  If the
positive and negative currents are equal, as they should be, the magnet
behavior is symmetrical in the two directions.

I suppose nowadays it is passe to use a polar relay and you'd like to
do something electronic to generate a polar signal.