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.

15 Comments

1. 1. wzp 10:06 AM 10/30/08

   Wow

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2. 2. kurtisnelson 10:40 AM 10/30/08

   My microwave makes my wifi connection die completely anywhere in the house.

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3. 3. benny10468 04:38 PM 10/30/08

   I thought the cooking from within was a myth. That it cooks from the outside like everything else.

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4. 4. wzp 05:44 PM 10/30/08

   Well maybe if the article said something about water molecules and resonance then we would all know. I think it is ironic that the title is "How a microwave oven works". It should be : "We think we heard in Prevention that microwaes don't deplete nutrients from food".

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5. 5. Jofez 09:19 PM 10/30/08

   Of course it depletes nutrients and vitamins. Our living conditions and our environment are filled with health problems. You're reheating food here. Once you cook it, you reheat it! It doesn't taste exactly the same. There must be alot of people out there that must know that rehating food causes cancer.

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6. 6. normanicus 12:00 PM 10/31/08

   A couple of years back I was experimenting with Wifi. I wanted to find a way to reliably attenuate the signal at close distance. To try this out I put a cordless phone inside a one litre aluminium container and that inside a bigger one. I then put this lot inside the microwave oven and closed the door. I called myself on that phone and the phone still rang, in spite of two layers of metal plus the microwave door. Why?

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7. 7. scohn in reply to Jofez 02:38 PM 10/31/08

   Cooking/heating in any form will tend to alter the original state of any food and thus possibly deplete some of the nutrients. But CANCER? Oh really - By what mechanism will simply heating make a food cancerous?
   Does that mean that eating in a warm house is more dangerous than eating outside (if its cool out)???
   
   Granted, supplying enough energy with the right substances present, can induce chemical reactions. But that is the very reason that this form of heating food can be the safest. The most energy is applied INside, where no foreign substances should be. Furthermore, the amount of energy used can be much less producing fewer chemical reactions.

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8. 8. ve2sxa in reply to normanicus 05:46 PM 10/31/08

   I believe that at a short distance the effect was due to the near-field condition. Close to the transmitting antenna, the magnetic field is decoupled to the electric field, and would get to the receiver. The microwave shielding would only block an electromagnetic far-field wave.

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9. 9. John Spraggs in reply to normanicus 04:55 PM 11/1/08

   First, the microwave door has a 1/4 shorted stub built in where it overlaps the door frame which blocks leakage near the magnetron's output frequency. The tuned gap is not effective at most other frequencies, leaving the door to act as a slotted antenna and let the cordless signal through. There are 4 widely different bands in use for cordless phones and there are several different wifi bands. Careful measurements would be affected by the bands chosen for the experiment.
   
   Second, did the aluminum container have a tightly fitting aluminum lid? Aluminum always has an insulating oxide layer that must be broken through each time the necessary electrical contact is to be made. A good connection all around the lid should have blocked the signal completely in your test.
   
   Finally every implementation of shielding I've ever tested has resulted in large error bars. There are so many confounding factors that influence the results. Reflections, impedance mismatches, non-ideal test antennas, all make this a frustrating field to gather repeatable results.

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10. 10. bucketofsquid 10:33 AM 11/6/08

    Jofez, I generally don't hate my food at all. Even if I hated my food I doubt I would change my mind, not once but twice and "rehate" my food.
    
    Oddly enough, while reheating (notice the spelling) does have a negative affect on nutrition, I have never seen anything indicating a link to cancer although cooking fats increased saturated fat levels. Hate, however, stresses the body and weakens the immune system so maybe that actually would matter.
    

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11. 11. stew-dent 03:49 PM 11/11/08

    Contrary to what the article claims, it is possible to heat both dry ceramics and dry glass with microwaves. This is a standard way of taking the chill off cold plates taken from an uninsulated cabinet in wintertime. Apparantly the shape of silica and alumina molecules are similar enough to water that they oscillate at microwave frequencies.

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12. 12. stew-dent 04:01 PM 11/11/08

    While we're on the subject, why does the oven need a magnetron to generate 2.45 ghz, when my cell phone produces the same frequency without one? Why not just scale up a cell phone oscillator to the desired output power level and do away with the magnetron magnet and related parts?

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13. 13. factfinder 04:47 PM 6/5/09

    Is there moving parts in the magnetron, or all is created by only electric current passing through?

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14. 14. Alistarz in reply to kurtisnelson 11:10 AM 7/29/11

    Then get a new microwave.

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15. 15. ash_ef in reply to Jofez 09:51 AM 10/7/12

    I can understand that if nutrients are removed (by the way, it is very easy to lose water-soluble nutrients) that you lose many chemo-protective mechanisms for protection against cancer. But reheating having a causal link - I am not so sure. Possibility, of course. But do you have anything scientific?

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