Microwave ovens for home use took off in the early '70s and since have become an essential part of the American kitchen. Unfortunately, microwave-cooking technology was in its infancy then. Significant improvements in convenience and safety have been made in the last ten years.

A microwave oven uses radio waves which pass through matter causing friction and, in turn, heat. This can be compared to heat that is created by rapidly rubbing hands together.

If your microwave oven is more than fifteen years old it should be checked for output efficiency and radiation leakage. You can make a simple output test by placing an 8-ounce cup of water in the microwave and turning it on high for three minutes. By then the water should reach a rolling boil. If not, the unit should be checked out. This is a general test that is used for domestic ovens with an average of 600 to 1000 watts. Lower wattage units will require more time to reach a rolling boil.

Testing for radiation leakage is something that is best left to a professional appliance repair technician. Although there are do-it-yourself testing kits available in electronic stores, the instruments are not nearly as reliable as the equipment that a professional repair technician uses. It's all right to save a few bucks here and there when playing Mr. Fix-it with other household appliances, but under no circumstances should a do-it-yourselfer try to repair a microwave oven. In addition, repairs attempted by anyone other than an authorized service technician will usually void the warranty.

The leakage-testing device that the pros use is certified, has a serial number and has a calibration date. Also, the pro will investigate other aspects of operation to determine if a unit is safe, and whether it should be repaired or discarded. The investment of about $40 for a professional test is worth the peace of mind to know the device isn't a health hazard. The fee can be as little as $10 if you bring the appliance into an authorized service center.

There are a some tell-tale signs of an ailing microwave oven: A flashing LED display is a sure sign that the unit needs attention. Unusual noises during operation, poor heating and electrical arcing in the oven cavity are other conditions that signal the need for a service check.

Poor heating could be the result of an overworked electrical circuit. Operating a microwave on a circuit that is serving other appliances will not only diminish its effectiveness, but could ultimately result in an electrical fire. If the lights dim when the microwave is in use, an electrician should be consulted. When remodeling, a separate electrical circuit should be provided for the microwave oven.

Electrical arcing within the oven cavity often is the result of a dirty oven interior. Food particles left in the oven over long periods of time will eventually turn to carbon and cause arcing which in turn can etch the interior surface of the oven and even compromise the seal around the door. Always wipe up spills promptly and keep the interior of the oven and the area surrounding the door clean. To avoid messy explosions, puncture tomatoes, potatoes, apples and plastic bags containing food, among other things, before cooking.

Another cause for arcing is metal in the oven. Microwaves cannot penetrate metal. Do not place anything made partly or entirely of metal in a microwave oven. This includes pots and pans, metal handles on glass cookware, aluminum foil, meat thermometers and even wire ties.

One of the biggest problems with microwave ovens is hot and cold spots the result of uneven cooking. This deficiency is the rule, not the exception, in older models. The problem has been offset in part by turntable models that provide more even heating. The problem with a turntable version, however, is that it reduces the effective cooking area by about 40 percent. In contrast to the older units, modern units have a "stir system" which consists of a fan-like device called an "antenna" that causes energy to be more evenly distributed throughout the oven and results in more even heating.

Never use a microwave oven for anything other that what it was designed for cooking and heating. Using it to dry clothing or pets could yield devastating results. Also, never run the appliance when empty, and always be sure there is plenty of ventilation around the unit to avoid overheating that can cause serious damage.