[GreenKeys] Pulling vs Holding selector magnets

[Jim Haynes]

And some holding magnet macbines will have a toggle switch to
make the coils connected series or parallel.

The original reason for the holding magnet selector, and this comes from
the mouth of the inventor, Walt Zenner, was to allow a lower loop voltage
on the cord circuits of TWX switchboards.  Switchboard operators come
in contact with the plugs on the cords, and 120V could be deadly if the
operator had an unusually low resistance path to ground.  Holding magnet
selectors allowed operation on 48V, which was also standard on voice
switchboards and thus assumed to be safe enough.

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man