Some inventions arrive after years of careful planning.

The microwave oven arrived because a man walked past some radar equipment and noticed that his snack had melted.

That's the short version. The longer version is better, because the man in question is one of the most remarkable engineers you've never heard of.

The engineer without a diploma

Percy Spencer was born in Maine in 1894. His father died when he was a baby, and he was raised by an aunt and uncle in poverty. He left grammar school at around 12 to work in a spool mill.

Then he heard that the local paper mill was going to install electricity. Almost nobody in the area understood how it worked. Spencer taught himself — well enough to be hired as one of the people installing it.

He later joined the US Navy, where he learned radio technology, and kept teaching himself everything from calculus to chemistry by reading at night. By the time the Second World War began, he was one of the leading engineers at a Massachusetts company called Raytheon, and one of the world's experts on a device called the magnetron.

The device that won a war

The magnetron generates microwaves — short radio waves — and it was the heart of radar, the technology that let the Allies spot aircraft and ships through darkness and cloud.

The problem was making them. In the early 1940s, magnetrons were painstakingly built by hand. Spencer developed ways to manufacture them far faster, transforming production and helping put radar into Allied ships and planes. The US Navy later honoured him for his wartime contribution.

How a snack became an oven

The story has been retold so many times that it has picked up variations, but the core of it — as told by Raytheon and Spencer's family — goes like this.

What microwaves actually do

There are a lot of myths about microwave ovens, so here's the real picture.

A microwave oven fills its metal box with microwaves at a frequency of about 2.45 gigahertz. Molecules in food that have an uneven distribution of electric charge — water is the main one, though fats and sugars matter too — try to line up with the rapidly flipping electric field. Their jostling produces heat.

Myth: microwaves cook from the inside out. They don't. Microwaves penetrate a few centimetres into food at most. The outer layers heat, and the heat spreads inward by conduction — which is why big dishes have cold middles.

Myth: 2.45 GHz is the "resonant frequency" of water. It isn't. The frequency was chosen partly because it's an internationally allocated band for industrial, scientific and medical use, and because it penetrates food reasonably well.

Myth: microwaving makes food radioactive. Microwaves are non-ionising radiation, like radio waves. They heat food; they don't make it radioactive.