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1. History of dielectric heating
In 1945, Dr. Spencer working for RAYTHEON Co. in the U.S.A found out a pieces of chocolate in his pocket melting suddenly by the
electric wave (ultra high frequency wave using for radar) on his experiment for radar. then began research in heating matters by the
electric wave.
So far, experiments for heating foods by the electric wave has been developed into the household telephone as well as microwave
oven, etc. The heating method for microwave is to give an electric wave to the food and make the food itself generate heat. It is
different from that of the past which was to give heat to the food and make the temperature rise.
So, since the microwave oven does not give any heat to the food, we do not need appliances like the gas oven.
Not only the microwave oven itself but utensil don’t get hot either, when heat is provided, the utensil will gradually got hot by thermal
conduction, however, the temperature of the surface and the inside of the fond get hot simultaneously and the food is well-done rapidly.
2. The principle of dielectric heating
The microwave oven has a vacuum tube called magnetron (M.G.T), it injects the electric wave of 2450MHz (The vibrations of two
billion four hundred and fifty million per second are given to the oven) This electric wave is called the microwave. The wireless electric
wave
(having not more than 1 m of the wave length)
is using for the radio telephone and broad casting of T.V. and radar
(a radio locator)
.
It is similar to an electric wave and its wave length is short. (The length of the electric wave having frequency of 2450MHz is 12.2cm)
The characteristics of this microwave is as follows:
1. When it touches the metal, it is reflected and changed direction.
2. It passes through ceramics, porcelain (glass, etc), plastics, and paper.
3. If it is touches the food or the wood having water in humidity, it is absorbed and generates heat.
The Microwave oven cook food using these characteristics. Generally, the food, whether it is large or small has humidity. So if you put
this food in the microwave oven and it gets the electric wave, so the energy of it is absorbed in the food since it set up a vibration of
the molecules with a frequency two billion four hundred and fifty million times per second. The friction among molecules of food occurs
repeatedly. It then causes the frictional heat, and the temperature of the food rises.
3-1. What’s the microwave? (1/2)