Fork made sparks in a microwave

[oneamp]

I understand that the electromagnetic waves inside a microwave can induce a current (terminology?) on a metal object within it. What I don't understand is why, when my fork touched the interior casing of the microwave, it made a lot of sparks and an arc-welder like black mark on the wall. If the wall is conductive, why don't the waves induce current on them, as well?

 

[Baluncore]

The current induced in the conductive fork can only consume power if resistance is present. W = I²R
The fork has very low resistance. The walls of the microwave are also very low resistance.

Standing waves with very high voltage can exist on the fork if it's length is close to resonance at the microwave frequency. Those high RF voltages may cause corona discharge from the ends of the fork. When one end of the fork gets close enough to the wall it will arc over. The corona voltage accelerates electrons between the wall and fork which creates a cloud of metal particles in a plasma arc. That is why it looks like an arc welder burn.

Have you never been told not to put conductive metal objects in a microwave?

 

[davenn]

...
Have you never been told not to put conductive metal objects in a microwave?

Apparently not

Dave