A magnetron is a device that uses the interaction of a stream of electrons, guided 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 causes the molecules in food — particularly water molecules — to vibrate, leading to a rapid rise in temperature that is sufficient to cook the food.
How It Works
A magnetron consists of a short copper cylinder with a number of cavities that open into a central vacuum chamber containing a metal cathode. A permanent magnet provides a magnetic field that runs parallel to the axis of the cylinder. The cathode is heated by a high voltage direct current, causing it to produce electrons that stream out toward the cylinder wall, at right angles to the magnetic field. The electrons are deflected by the field into curved paths, causing them to set up circular currents within the cavities. These currents produce microwave radiation at frequencies that are related to the size of the cavities.
The microwaves must then 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 outside 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 that they can be used for cooking.
Magnetrons are used to generate microwaves for radar, as they can achieve the required power output. 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 fluctuations in the current, and changes in temperature. While this is not a problem when the energy produced is used for heating, it affects the accuracy of radar images. This can be overcome using adjustable conducting materials that can be inserted into the cavities to tune the radiation as required.
The most familiar use of magnetrons is in microwave ovens. These direct the waves into a small cooking chamber, where food can be cooked very rapidly. Some molecules in food are polar, which means that they have a positive charge at one side and a negative charge at the other. These molecules, when bombarded with electromagnetic radiation in the microwave range, align themselves to the alternating electric and magnetic fields produced by the waves, causing them to vibrate rapidly, which leads to fast heating. One such molecule is water, which is present in significant amounts in most foods.
In the 1920s, Albert Hull, an employee at 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 remained 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 had consisted of a cathode and anodes inside a glass tube, but Boot and Randall instead used copper, a good electrical conductor, to build a casing with cavities that also acted as an anode. This resulted in a device that was much more powerful, producing an output of 400 watts in a space smaller than four inches (10 cm).
As Boot and Randall developed stronger magnetron tubes, they discovered that these were ideal for radar. During World War II, United States submarines began using them, which allowed the radar equipment to detect enemy ships more quickly. In the late 1940s, Dr. Percy Spenser, an American engineer and inventor, further tested the output of magnetron tubes in his lab. He noted that the candy bar in his pocket had completely melted while he was working with the tubes. He decided to place a few kernels of popcorn near the machinery to see what would happen, and noticed that it made the kernels pop.
Dr. Spenser called for his assistant and the two men decided to place a whole egg near 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 over 700 pounds (318 kg), was more than five feet (1.5 meters) tall, and cost more than $5,000 US Dollars (USD).