[Mondial]

I always remember this effect from a college Physics demonstration many years ago.

The class was located in an old building supplied with 110 VDC power. The instructor had an enormous iron core inductor, connected to a foot long knife switch fed from the DC supply. We all waited to see what would happen. When he closed the switch, absolutely nothing. Then he opened the switch slowly, which produced an arc that lit up the room and got longer and longer until it extended fully across the open contacts.

It certainly got our attention, and to this day I can still visualize the release of the energy stored in an inductor as its magnetic field collapses.