The British — under attack by German bombers during the Second World War — desperately needed better radar to identify oncoming planes. Their search led two British scientists back to, ironically, a German scientist.

In 1939, John Russell and Henry Boot studied the experiments of the German Heinrich Hertz. You’ll remember (from part two) that Hertz first found James Clerk Maxwell’s predicted radio waves in 1887. To capture those waves, Hertz used a wire loop.

Russell and Boot realized that Hertz’s loop was a resonator — it didn’t just capture radio waves, it produced its own radio waves. The wavelength of the waves depended on the size and shape of the wire loop. Hmmm, the same idea might work to make microwaves!

The two scientists determined that to make microwaves with a wavelength of about 10 centimetres they would need a loop about 1.2 cm across. Instead of thin loops, however, Russell and Boot used hollow cylinders (sort of like a lot of loops stacked together) to increase their output. They called their device the cavity magnetron.

Electric current flows from the centre of the magnetron toward the outer cavities. But along the way, a magnetic field — made by powerful magnets located above and below — turns the electric current in a circle, past the cavity openings.

It’s a little like air moving over the top of a pop bottle. The electric current moving past the cavities makes the cavities vibrate. But instead of sound, the vibrating cavities make microwaves.

The cavity magnetron produced about 100 times more power than any other microwave-making device. With the microwaves produced by the cavity magnetron, British and American forces defeated the Germans in the air (where radar sets installed inside airplanes helped pilots find their targets) and at sea (where radar helped defeat the deadly German U-boats). Many military historians credit the cavity magnetron with winning the Second World War for the Allies.

But that’s not all it could do. That’s right, we’re getting closer, Grasshopper. Stay tuned…