Science corner

Sophie Duncan describes a hot topic

Would you find this object in

a a lawn mower?

b a microwave oven?

c a car alarm?

The answer is b because the picture shows a cavity magnetron, a central part of any microwave.

The cavity magnetron was invented in the Second World War to generate high frequency waves for use in radar. J T Randall and H A H Boot, who worked in the physics department at the University of Birmingham, created it, and it was first operated in 1940.

Six years later Percy Spencer was experimenting with a magnetron when he noticed that the sweet bar in his pocket had melted. Percy was intrigued, so he put some popcorn in front of the magnetron to see what would happen. Not surprisingly the popcorn popped. Percy even experimented with an egg, which reputedly exploded in a colleague's face.

The heating effects of the magnetron's radiation had been noticed before, but it was thought to have no useful value. Percy disagreed, and went on to design a metal box to contain the microwave radiation to see how quickly food could be cooked.

The idea was refined and in 1947 the microwave oven was ready to market. The original versions were huge, similar in size to a large fridge freezer, and not surprisingly they were not used domestically. It took a few more years, and a number of changes to the design, before the microwave oven became compact, efficient and popular. So now millions of people have cooked popcorn using a magnetron, just like Percy in 1946.

Sophie Duncan is a physicist and programme manager with Science Year

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