Direct microwave exposure is not generally possible, as microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which the oven is constructed. Furthermore, ovens are equipped with redundant safety interlocks, which remove power from the magnetron if the door is opened. This safety mechanism is required by United States federal regulations.[60] Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary.[61] According to the United States Food and Drug Administration's Center for Devices and Radiological Health, a U.S. Federal Standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts of microwave radiation per square centimeter at approximately 5 cm (2 in) from the surface of the oven.[62] This is far below the exposure level currently considered to be harmful to human health.[63]
To find out what makes a great microwave, we spoke to Sharon Franke, former director of the Kitchen Appliances and Technology Lab at Good Housekeeping; appliance expert Chris Zeisler of; cookbook author Leslie Bilderback, who wrote Mug Cakes: 100 Speedy Microwave Treats to Satisfy Your Sweet Tooth; and product managers at both Panasonic and GE. We also read reviews from Good Housekeeping and CNET.
Water and other homogeneous liquids can superheat[54][55] when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start explosively when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven or while adding solid ingredients such as powdered creamer or sugar. This can result in spontaneous boiling (nucleation) which may be violent enough to eject the boiling liquid from the container and cause severe scalding.[56]
The keypad on the Panasonic NN-SE785S is a confusing jungle of icons, which makes it difficult to clearly identify functions. (Below, the icon that looks like weeping asterisks turns out to mean defrost, for example.) There’s not even a numerical pad to set the time for cooking; to do that, you have to go through an icon-based menu, then press an up or down arrow half a dozen times or more to set the time. Though it has a sleek design, our testers felt this model was unnecessarily complicated.
Microwave ovens produce heat directly within the food, but despite the common misconception that microwaved food cooks from the inside out, 2.45 GHz microwaves can only penetrate approximately 1 centimeter (0.39 in) into most foods. The inside portions of thicker foods are mainly heated by heat conducted from the outer 1 centimeter (0.39 in).[38][39]
In our heat map test, the Toshiba produced the most even heating pattern of all the microwaves we tested, perfectly browning the layer of marshmallows from edge to edge. It expertly cooked a baked potato in about 6 minutes, which was average for the microwaves we tried. It also reheated beverages well and perfectly cooked frozen macaroni and cheese. And it made tasty popcorn, with only 5 grams of kernels left unpopped, which was on a par with most of the models we tested. This model allows you to defrost by time or weight, but like most of the microwaves we tested, it didn’t defrost meat well.
The defrost options (varying by type or weight) are ideal if you want to warm something up with the microwave before cooking with the convection setting. If you have a busy kitchen and want multiple ways to cook foods (instead of just nuking the occasional popcorn bag), Cuisinart delivers. About the only thing that it’s missing is the ability to set power levels, but with all these other features, that’s a minor quibble to be had.
Microwave ovens are frequently used for reheating leftover food, and bacterial contamination may not be repressed if the safe temperature is not reached, resulting in foodborne illness, as with all inadequate reheating methods. While microwaves can destroy bacteria as well as conventional ovens, they do not cook as evenly, leading to an increased risk that parts of the food will not reach recommended temperatures.[72]
Countertop microwaves are some of the most popular models, and there are many options to choose from. They are quick and easy to install—just plug them in—and you can move them around as needed, making them a top choice for renters. The main downside of countertop microwaves is that they take up valuable counter space, which can be frustrating if you have a small kitchen. 
Modern microwave ovens use either an analog dial-type timer or a digital control panel for operation. Control panels feature an LED, liquid crystal or vacuum fluorescent display, in the 90s brands such as Panasonic and GE began offering models with a scrolling-text display showing cooking instructions, numeric buttons for entering the cook time, a power level selection feature and other possible functions such as a defrost setting and pre-programmed settings for different food types, such as meat, fish, poultry, vegetables, frozen vegetables, frozen dinners, and popcorn.
In the 1960s,[specify] Litton bought Studebaker's Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton then developed a new configuration of the microwave: the short, wide shape that is now common. The magnetron feed was also unique. This resulted in an oven that could survive a no-load condition: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a trade show in Chicago,[citation needed] and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was faster in Japan, due to a re-engineered magnetron allowing for less expensive units. Several other companies joined in the market, and for a time most systems were built by defense contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business.
Updates writer Eleanor Ford has used a secondhand Toshiba EM925A5A-BS for about seven months now. Even with the previous life of this microwave, our pick has lived up to its reputation. “It’s never failed to do what I expect a microwave to do, heats evenly, doesn't scorch, and is significantly more quiet than any other microwave I’ve used (including my parents’),” Eleanor said. They also noted that the small size allows the microwave to sit on a shelf rather than take up valuable counter space. Eleanor also said that the only notable downside is that they find it difficult to locate the kitchen timer button.
For our 2018 update, we ran a series of tests on 12 microwaves. First, we created our own version of a heat map by cutting a piece of parchment paper to fit the turntable of each model and completely covering it with a layer of plain mini marshmallows. Then we nuked it on high for 2 minutes until the marshmallows began to brown. By looking at the underside of each piece of parchment paper, we could see the pattern of browning and determine how evenly the microwaves generated heat (for more on how microwaves work, see above).

This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field.