Archive for July 18th, 2011

The Heart of the Microwave Oven

Monday, July 18th, 2011
     In the world of inventions it happens with some regularity that an invention to do one thing unexpectedly leads to a device that does something completely different.  Take for example Edison’s phonograph.  At the time, he was working on an invention to record the dots and dashes of Morse code telegraph messages so they could be sent out repeatedly without an operator having to tap them out each time and possibly making mistakes while doing so.  Little did he know that this would lead to the evolvement of the phonograph and recording industries.

     Another invention “by mistake” took place when the resonant cavity magnetron, originally developed for use with microwave radar, led to the development of the microwave oven.  Last week we talked about how long wave radar, the first type of radar to be developed, was effectively used by the British to repel enemy air attacks during World War II.  But long wave radar was large and cumbersome to employ, and it soon evolved into an improved version, the shorter wave, or, as we know it, microwave radar.  So what is this resonant cavity magnetron that led to its creation?  A pop bottle can give us a clue.

     Blow across the top of an empty glass pop bottle (or soda bottle, depending on the part of the country you’re from) and a familiar resonant sound results.  The sound is created by an effect known as cavity resonance, and other bona fide musical instruments make use of this phenomenon to produce musical sounds.  How this works is that where a cavity exists, when air molecules are introduced into it, the molecules are caused to resonate in and out of the cavity many times per second.  This creates a sound at a certain frequency, that frequency depending on the shape and dimensions of the cavity, as well as the size of its opening.

     The resonant cavity magnetron, or magnetron for short, is actually a high powered vacuum tube that operates in a very similar fashion to a pop bottle, or any other musical device making use of a cavity, but instead of using air molecules to generate sound waves, it uses electrons to generate short wavelength radio waves, called microwaves.  The magnetron contains a series of cavities that are arranged in a circle with the openings pointing inward towards the center, as shown in Figure 1.

Figure 1 – Interior View of a Resonant-Cavity Magnetron

     Next week we’ll see what this interesting looking device has in common with the simple act of blowing air across the top of a pop bottle and what this all has to do with microwaves.

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