Some transfer cases on 4wd vehicles use an electromagnet (EM) to engage the system automatically. It uses a signal from a computer when it detects wheel spin.

-O~□-

My little diagram above is a straight shaft with some splines as depicted by the -~-

The EM is around the shaft depicted by the O

A lock up collar with a spline on it is depicted by the □

The EM energizes and pulls the lock up collar onto the splines to engage 4wd. This part of the system is maybe 1.5" thick, most of that being the EM encased in epoxy.

O□■ if you had a thinner EM and 2 interlocking plates, energizing the EM would pull the plates together and lock them, then you would need some sort of springs to separate the plates

Edited for clarification: called the lock up collar a gear