Everybody,
this is a solenoid.
A solenoid actuator uses one of these to move something (the "armature") through its center. Typically the armature is either magnetic or ferrous (so that it will react to the field created by the coil.
Here is a good cross-sectional image of one (in this case it is used as a valve actuator, not unlike the pneumatic solenoids we use).
Most of the solenoids
here are similar to the ones the rule is referring to.
I hope this helps someone.
