Magnetrons
by Chris Woodford. Last updated: May 31, 2011.
Want to cook a dinner in five minutes or make an airplane safer to fly in bad weather? You'll be needing some microwaves, then. Those are the invisible, super-energetic, short-wavelength radio waves that travel at the speed of light, doing the important stuff in microwave ovens and radar-navigation equipment. Making microwaves is easy if you have the right equipment—a handy gadget called a magnetron. What is it and how does it work? Let's take a closer look!
Photo: The CV64 cavity magnetron, developed in Birmingham in 1942, was small enough to fit inside an airplane. Devices like this made it possible for planes to use radar defenses for the first time. An exhibit at Think Tank (the science museum in Birmingham, England). Sorry about the slightly poor quality of the image: the exhibit is inside a glass case and hard to photograph.
How does a magnetron work?
Magnetrons are horribly complicated. No, really—they're horribly complicated! To understand how they work, I find it helps to compare them to two other things that work in similar ways: an old-style TV set and a flute.
A magnetron has quite a lot in common with a cathode-ray (electron) tube, the sealed glass bulb that makes the picture in an old-style television set. The tube is the heart of a TV: it makes the picture you can see by firing beams of electrons at a screen covered in chemicals called phosphors so they glow and give off dots of light. You can read all about that in our main article on television, but here (briefly) is what's happening. Inside the TV, there's a negatively charged electrical terminal called a cathode that's heated to a high temperature so electrons "boil" off it. They accelerate down the glass tube, attracted by a positively charged terminal or anode and reach such high speeds that they race past and crash into the phosphor screen at the tube's end. But a magnetron doesn't have the same purpose in life as a TV. Instead of making a picture, it's designed to generate microwaves—and it does that a little bit like a flute. A flute is an open pipe filled with air. Blow across the top in just the right way and you make it vibrate at a specific musical pitch (called its resonant frequency), generating a sound you can hear that corresponds directly to the length of the pipe.
A magnetron's job is to generate fairly short radio waves. If you could see them, you could easily measure them with a school ruler. They're usually no shorter than about 1mm (0.04 in; the shortest division on a metric ruler) and no longer than about 30cm (12in; the length of a typical school ruler). The magnetron does its stuff by resonating like a flute when you pump electrical energy into it. But, unlike a flute, it produces electromagnetic waves instead of sound waves so you can't hear the resonant energy its making. (You can't see that energy either, because your eyes aren't sensitive to short-wavelength, microwave radiation).
Artwork: Right: One of the drawings of the high-energy magnetron developed in the 1940s by Percy Spencer, who went on to perfect the microwave oven while working at Raytheon. (I've colored it in to match my own artwork below.) You can see a bigger version of this drawing and read the full technical details by searching for patent #2,408,235 at the US Patent and Trademark Office.
