[SML] Solenoids

[Nigel Worsley]

> My idea to throw in the ring would be the door-holder electromagnets used in so many
> buildings nowadays.

These can be exceedingly powerful, but there is usually a significant
residual magnetism after the current is turned off. A sutably strong
release spring should deal with this, but may end up having to be
strong enough that it has to be manually overpowered to reset the
mechanism. Even more so than with solenoids, they have very little
force until fully activated.

Nigel Worsley