Running a microwave oven with nothing it is hard on the magnetron that
produces the microwaves:
"What happens if you start the microwave oven with nothing inside?
The magnetron creates microwaves that travel into the cooking chamber
and should be absorbed there. If there is no food (or rather no
water-containing food), those microwaves will not be absorbed and will
eventually find their way back to the magnetron. Eventually the
magnetron will absorb as many microwaves as it emits. This situation
is hard on the magnetron, which works best when it has very little
radiation returning to it. That is why you should never run a
"Microwave Ovens" (March 20, 1995)
Physics 106N - How Things Work [University of Virginia]
For a more technical discussion of possible negative consequences of
running an empty microwave oven, see:
"Unwise Microwave Oven Experiments," by William J. Beaty [under "Q:
Corona discharges? High voltage? Balderdash! The watts per cm^2 is too
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