Some of history's most significant inventions were made by people with little or no formal education. Consider the invention of stone projectile points, printing, pulleys, sewing needles, sailing vessels, and the wheel and axle, to name just a few.

Percy L. Spencer never graduated from high school, but in 1945 he invented the microwave oven.

Today most kitchens in the U.S. are equipped with at least one of Spencer's microwave ovens.

One heated the cup of tea resting near the keyboard into which these words are being typed. It did this in 90 seconds with very little wasted heat.

The heating efficiency of a microwave oven is due to the fact that water strongly absorbs microwaves. That's why satellite TV signals can be difficult to receive during wet conditions. Most food contains considerable liquid water, and that explains why the microwaves bouncing around inside a microwave oven are quickly absorbed by any food inside.

The energy in the microwaves is then converted into heat. In a conventional oven, considerable heat is wasted in heating the oven itself and the surrounding air when the goal is to heat only the food inside.

Spencer learned about the heating ability of microwaves after a candy bar in his pocket melted while he was standing near a microwave-emitting electronic device called a magnetron. This led to a series of simple experiments that proved food could be heated and even cooked using microwaves. On Oct. 8, 1945, Spencer applied for a patent that he titled "Method of Treating Foodstuffs."

Spencer's patent began with, "My present invention relates to the treatment of foodstuffs, and more particularly to the cooking thereof through the use of electromagnetic energy."

Spencer's patent application stated others also had used radio waves to heat food.

But his method was far more efficient and faster, since it employed microwaves. His patent was granted on Jan. 24, 1950, the delay possibly being attributed to the earlier discoveries he cited.

The nearby photograph shows a cup of water that reached the boiling point after being heated in a microwave oven for about 2 1/2 minutes. The boiling stopped within seconds after the microwave door was opened, so my wife, Minnie, offered to open the door while I quickly took photos.

We soon learned that boiling too little water in the cup resulted in exploding bubbles of steam that sprayed hot water all over the inside of Spencer's invention.

Spencer's career involved far more than inventing the microwave oven.

You can find out much more about this inventive genius in articles on the Web, including

