A magnetron uses the interaction of electrons with copper cavities and a magnetic field to produce microwave radiation. It is used in radar and microwave ovens to cook food. The device consists of a copper cylinder with cavities, a metal cathode, and a waveguide to direct the microwaves. The most common use is in microwave ovens, where the microwaves vibrate water molecules in food to heat it. The magnetron was invented in the 1920s by Albert Hull, but it was not until the late 1930s and early 1940s that it was further developed by Harry Boot and John Randall for use in radar. The first microwave oven was created in 1947 by Dr. Percy Spenser.
A magnetron is a device that uses the interaction of a flow of electrons, driven by a magnetic field, with cavities within a block of copper to produce microwave radiation. The frequency range of the radiation depends on the size of the cavities. The devices are used in radar and microwave ovens, where the radiation vibrates molecules in food, particularly water molecules, leading to a rapid rise in temperature sufficient to cook the food.
How does it work
A magnetron consists of a short copper cylinder with a series of cavities opening into a central vacuum chamber containing a metal cathode. A permanent magnet provides a magnetic field that runs parallel to the cylinder axis. The cathode is heated by a high voltage direct current, which produces electrons which flow towards the cylinder wall, at right angles to the magnetic field. The electrons are deflected by the field in curved paths, causing circular currents to be created within the cavities. These currents produce microwave radiation at frequencies related to the size of the cavities.
The microwaves must therefore be directed to where they are needed. This is achieved by a metal structure known as a waveguide, along which the waves travel. It normally extends out of the main body from one of the cavities, capturing the microwaves and guiding them along its length. In the case of a magnetron used for radar, the waveguide will connect to an antenna that transmits the waves. In a microwave oven, it will direct the waves into the oven chamber so they can be used for cooking.
it is used
Magnetrons are used to generate microwaves for radar, as they can achieve the required power. A drawback with a simple magnetron is that although the range of frequencies produced is determined by the size of the cavities, there is variation within that range, due to current fluctuations and temperature changes. While this is not a problem when the energy produced is used for heating, it does affect the accuracy of the radar images. This can be overcome by using adjustable conducting materials which can be inserted into the cavities to adjust the radiation as needed.
The most familiar use of magnetrons is in microwave ovens. These direct the waves into a small cooking chamber, where the food can be cooked very quickly. Some molecules in food are polar, meaning they have a positive charge on one side and a negative charge on the other. These molecules, when bombarded with electromagnetic radiation in the microwave range, align themselves with the alternating electric and magnetic fields produced by the waves, causing them to vibrate rapidly, which leads to rapid heating. One of these molecules is water, present in significant quantities in most foods.
History
In the 1920s, Albert Hull, an employee of a well-known electric company, was researching vacuum tubes when he created the magnetron. Hull, however, could think of few uses for his invention, and it lay largely unused for some time. In the late 1930s and early 1940s, two engineers named Harry Boot and John Randall decided to explore the device further. Earlier versions consisted of a cathode and anodes inside a glass tube, but Boot and Randall instead used copper, a good electrical conductor, to build an enclosure with cavities that doubled as anode. This resulted in a much more powerful device, producing 400 watts of power in a space of less than four inches (10cm).
When Boot and Randall developed more powerful magnetron tubes, they found they were ideal for radar. During World War II, US submarines began to use them, which allowed radar equipment to detect enemy ships faster. In the late 1940s, Dr. Percy Spenser, an American engineer and inventor, further tested the output of magnetron tubes in his laboratory. He noticed that the candy bar in his pocket had completely melted as he worked with the tubes. He decided to put some popcorn kernels next to the machine to see what would happen and noticed him popping the kernels.
Dr. Spenser called his assistant and the two men agreed to place a whole egg next to the device. When the egg exploded, Dr. Spenser realized he had discovered a fascinating form of cooking. Spenser went on to help create the world’s first microwave oven in 1947. The initial model weighed more than 700 pounds (318 kg), stood more than five feet (1.5 meters) tall, and cost more than $5,000 US dollars (USD). ).
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