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See Inside November 2008

How the Microwave Works

The science behind nuking that TV dinner



GEORGE RETSECK

Occasionally it is a treat to remind ourselves how remarkable some of our most common gadgets are. A typical microwave oven ramps up the electricity from a 120-volt wall outlet to an incredible 3,000 volts or more and safely cooks food in just a minute or two, yet it costs less than a pair of good shoes. And we can watch the show through the handy window.

The key component is the magnetron. Although the name conjures up hardware from a questionable science-fiction movie, the sophisticated vacuum tube generates microwaves powerful enough for military radars (for which it was originally developed). Instead of a flame or electric coil generating heat that warms food from the outside, the microwaves penetrate food and create heat from within.

Slide Show: View Inside the Microwave

Some people still seem wary of the technology, however, even though microwave ovens have been sold since the 1950s. The classic fear is: Can’t the microwaves fly through the window and harm our bodies—especially our eyes? No. The waves reflect off a metal screen embedded in the glass. “The holes are so much smaller than the wavelength of the microwaves that the screen acts like a solid metal mirror,” notes Louis A. Bloomfield, a physics professor at the University of Virginia.

Several years ago nutritionists raised concerns that the microwaves depleted nutrients in food. If anything, studies have shown the opposite. All cooking methods can destroy vitamins; the extent of the damage depends on the temperature and the length of cooking time. Most research indicates that microwave ovens result in less extreme temperatures and in fact require less time for cooking than stove-top or oven methods. Boiling food is particularly deleterious.

A recent flap is whether microwave ovens can interfere with Wi-Fi networks. A tightly sealed oven will not do so, because the electromagnetic radiation cannot escape. But tiny leaks could possibly cause problems. “Wireless transmissions are exquisitely sensitive to electromagnetic radiation,” Bloomfield says. “So even if a leak were on the order of one part in a billion, our bodies would never notice, but a Wi-Fi signal could.”

Did You Know ...
WHOOSH: 
The whooshing sound a microwave oven makes has nothing to do with the magnetron, which resonates at a frequency far too high for human hearing. The noise is from the fan that blows air across the magnetron to keep it cool.

HUM: 
Microwave ovens also produce a hum. It comes from the transformer, diode and capacitor, which vibrate as they step up the 60-hertz electric power from a wall outlet.

SPARKS: 
Despite common wisdom, metal does not necessarily cause sparking inside a microwave; indeed, the cooking chamber walls are metal. Shape matters. Sparks are caused by a buildup of charged particles that suddenly arc when they are pushed by a voltage that changes dramatically over a short distance. A flat, round, metal platter will spread charge around it, preventing buildup; the “crisper” tray that lies underneath some microwaveable pizzas and the sleeve that envelops certain foods (such as Hot Pockets sandwiches) have a metal coating that gets very hot and browns the food yet does not spark. But sharper points, such as fork tines or the many tiny edges in aluminum foil, concentrate charge and also cause localized drops in voltage, which together create corona discharge—a spark.

DEFROST: 
For decades, ovens achieved “defrost” or any low-power setting simply by turning the magnetron on and off, so that it would generate full-power microwaves for only part of the total cooking time—a cycle that is clearly audible. Some new units have a pulse-width modulator—a hefty electronic circuit that clips the power to the transformer, which lessens the power of the microwaves.

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