How exactly does a plunger relay work?

[elegysix]

Hi, I'm new to relays.
As I understand it, there is a coil which creates a magnetic field to cause the armature to move (such as a plunger) and closes the contacts of another circuit. My question is this - how can the AC cause the plunger to move? isn't the AC producing sinusoidal B field, which would cause the plunger to vibrate, not extend?
Or is the armature made of say aluminum where the changing B field always causes repulsion?

thanks

 

[Wee-Lamm]

The magnetic flux is not acting on the shaft of the plunger, but on the larger end at the end of the shaft.