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Microwaves, Metal and Arcing Excerpts from the book The Complete Microwave Oven Service Handbook --NOW available on CD-ROM (CLICK HERE) and from the video You Can Fix Microwave Ovens Copyright ©1998-2008 by J. Carlton Gallawa

Metal Reflects Microwaves

Microwave Cooking Cavity

The characteristics of microwaves make it possible to contain them within a specially constructed metal enclosure called a cavity . The microwaves bounce around this  oven cavity until they are either absorbed by food or dissipate into the air. The interior metal walls of a microwave oven cooking cavity are proportioned with the energy output from the magnetron tube. In other words, the physical proportions of the cavity are tuned to the frequency of the energy, thus producing a uniform electromagnetic wave pattern. (The sealing properties of the door, which essentially appear as a metal wall to the microwaves, and the resonant characteristics of the oven cavity, will be considered in more depth in future additions). Therefore, under normal circumstances the energy reflects from wall to wall like beams of light in a mirrored room. However, if something were to disrupt that energy pattern, it would produce undesirable results, such as arcing.

Like a Small Bolt of Lightning

When metallic objects are indiscriminately placed in a microwave oven cooking cavity, the energy pattern becomes disrupted and distorted. This condition produces arcing. When two metal objects, such as a metal bowl and the [metal] cavity wall, are placed in close proximity and subjected to an intense field of microwave energy, arcing will result.  Arcing occurs because the air between the two metallic objects becomes electrically charged, just as the air between a thundercloud and the earth becomes charged or ionized. This ionized air becomes an electrical conductor, and electric current then leaps the gap like a small bolt of lightning.

However, lightning only lasts for an instant because it discharges or neutralizes the ionized air, but an arc in a microwave oven will continue, to a greater or lesser degree, as long as the microwave energy is applied. At the very least, this can cause marring or pitting of involved surfaces, and at worst, can burn a hole right through the cavity wall.

Why do some cookbooks suggest the use of tin foil? How can TV dinners be microwaved? What about the use of metal racks? These questions and others will be answered soon...

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