[Jeffhs]

The first attempt at cooking food using radio waves (not microwaves) goes back to the 1930s. The unit was called the "Radio Sandwich Machine" (RSM); there is a thread here on VK describing this unit, which was an early forerunner of today's smaller and much more efficient microwave ovens. The RSM used four radio transmitting tubes, a separate power supply, and was used mostly at exhibitions, the most notable of which was the New York World's Fair in 1939. Most of these machines were eventually seized by the government and destroyed, probably because of the high level of RF radiation (there were few or no standards for RF leakage at the time). One of the few (and I do mean few) such machines still in existence was on eBay some time ago; whether it sold or not, and if so to whom, I don't know.

BTW, I shudder to think of the arcs a machine like the RSM could generate if someone were to operate it without food in the long chamber atop the cabinet, or with anything metal in there as well. Today's microwaves will smoke and catch fire if abused in this manner; I imagine the RSM would set an entire kitchen afire in two seconds (!) under the same conditions, since the four tubes in the unit probably generated 500+ watts of RF energy. I would think that under such a severe overload, the house fuses would blow in an instant, but with 500-plus watts of RF sparking inside the oven, the thing would probably catch fire long before the fuses let go.