Why Does Metal Spark in a Microwave Oven?

History of the Microwave Oven

• The microwave oven was invented in 1946 by Dr. Percy Spencer when he was testing another invention, a vacuum tube called a magnetron. After accidentally realizing that his magnetron had the ability to heat up food, he created a metal box to hold the magnetron and began further experiments. In 1947, the first commercial microwave oven was released. Early microwaves were very large and expensive, but soon after smaller and lesser-priced microwave ovens were released. By 1975, microwave ovens were outselling traditional gas ovens, new styles were being created and microwaves had taken their place in many kitchens across the world.

Electromagnetic Waves

• Microwaves work by shooting tiny electromagnetic waves through food. Electromagnetic waves are used for televisions, cell phones, radio and more. These electromagnetic waves are also called short waves or micro waves, due to their frequency. Low-frequency waves require wire to travel, but higher-frequency waves may travel without wire. Microwave ovens use electromagnetic waves of a high enough frequency that they don't need wire, but rather are contained in the metal microwave oven itself, where they bounce through and around obstacles in their path.

Microwaves and Water

• Microwave ovens use electromagnetic waves of the correct frequency to excite water molecules. The food we eat contains a large percentage of water, so microwave ovens work by heating up the water within the food item placed in the oven. This causes the food to feel warm or hot when taken out of the microwave.

Microwaves and Metal

• Electromagnetic micro waves pass through food, plastic and glass, but reflect off of metal. The electrons on the surface of the metal will jump off of the metal object and into the air, which causes a spark in the microwave oven. Metal molecules are less stable than water molecules, which are unable to move freely. Metal also reflects the electromagnetic waves, so considerable damage may be done to the inside of the microwave oven.

Are Microwaves Harmful?

• Microwave ovens are engineered in a way that keeps most of the electromagnetic waves inside the oven. They have been monitored carefully by the U.S. Food and Drug Administration since 1971 to make sure leakage levels are much lower than could be harmful. Microwave ovens are dangerous if metal is put inside of them, as sparks, fire or melting may occur.