FWIW, smooth, rounded things tend not to arc. The microwaves will make the electrons in a material start moving faster, and electrons repel each other. If an object is smooth, it's sort of like cars going in a circle... they can go almost infinitely fast and keep going. But if there's a pointy spot, like a surface defect or a large-scale corner, you get them cornered with no way to "flow" out past the ones pushing in, and arcing happens.
Microwaves are not ionizing, either, so they don't break molecules. It's one reason they're used in micro- and molecular biology labs... hot plates can get super fucking hot and scorch shit on the bottom of a vial, and water baths function at one temperature and take a long time to change. Microwaves are fucking awesome.