[GreenKeys] GreenKeys Digest, Vol 104, Issue 46

[Jim Haynes]

On Sun, 30 Sep 2012, John Nagle wrote:
>
>   What, pulling magnet and holding magnet selectors?
>
>   To reduce the current required, some Teletype machines
> had a "holding magnet" selector, where a cam pushed the
> plate against the selector magnet poles once per bit
> time.  If the magnet was on (MARK), the plate would stay
> there, but if it it was off (SPACE), a spring would pull
> it away.  It takes much less current to hold a plate
> against an electromagnet than to pull it there from
> a distance.
>
And that's the only explanation I have ever seen in print.  However
I was told by the inventor of the thing, Walt Zenner, that the reason
for it was so that they could run TWX switchboards at lower than
120V because it was dangerous to have such a high voltage exposed on
the switchboard cord plugs.