This might be a more scientific question, but it relates to cooking and I thought it was interesting.
I just made my lunch which was a microwavable bowl of chunky soup.
The directions said:
Remove metal lid, remaining metal rim is microwavable.
How can this be?
Metal on its own doesn't necessarily cause electric discharge in a microwave.
What causes the sparking that you see when you put a fork in a microwave is due to the "sharp" edges of the fork. These edges concentrate the voltage at their tips which will cause a spark when it exceeds the dielectric breakdown of air.
Things like sheet pans (with rounded edges), or rounded metal racks are used frequently in microwaves with no ill effect. The absence of any pointed edges allows this. The rim of your bowl fits this requirement.
I think it also has to do with arcing. If the metal is close enough to other metal that electricity can leap the gap, it'll spark. I once stuck a metal bowl--very round on the bottom--into a microwave with a metal turn-table. The bowl got arc-welded to the turn table.
