| The evolution of cooking methods has been interesting indeed, from the open fires of primitive man, who must have decided at some point that cooked meat tasted better than raw, on to wood fired stoves, fossil fuel-based cooking, whether coal, propane, or gas, and let’s not forget electric range tops. Standing on its own in the modern kitchen is the microwave oven. What will be next? The space age food pill dispensing stations of the futuristic cartoon family, The Jetsons?
We’ve been talking about resonant cavity magnetrons and the purpose they serve within a microwave radar system. We also learned about Dr. Percy Spencer’s discovery and how microwave radar transmissions emanating from a magnetron can cook food, not to mention melt candy bars.
Figure 1- Microwaves Melt Candy Bars and Cook Food
Although the technologies used in microwave radars and microwave ovens are similar, they do have important differences. It would be both unsafe and impractical to install microwave radar systems into kitchens. Radiation emitted from radar wave guide lacking proper containment would bounce aimlessly around the kitchen, posing a threat to human safety. You see, microwaves don’t know the difference between our bodies and the food we wish to cook. They’ll heat up human tissue just as readily as a bowl of chicken soup. Another issue is that runaway microwaves lose much of their effectiveness through their aimless bouncing about, and much of it would not be directed to the food itself. Dr. Spencer would learn how to corral that energy, making microwave cooking a commercial success.
The biggest problem for Dr. Spencer to overcome was containment of the microwaves. They needed to be directed towards food in order to efficiently heat them. His first microwave oven was a metal box containing an opening at the top into which a magnetron wave guide could be inserted. This would then introduce microwaves into the box, and the metal construction of the box would safely contain them. The safety issue had now been resolved because the waves couldn’t escape, they would simply bounce around inside the box and most of their energy would be transferred into any food placed inside.
Dr. Spencer’s employer, the Raytheon Corporation, produced the first commercial microwave oven in 1954, and it was appropriately named the “Radarange.” It was huge, almost six feet tall, and weighed in at about 750 pounds! Hardly something that could fit into a home kitchen. Despite its massive size, the Radarange wasn’t all that powerful and couldn’t compete against the compact countertop microwave ovens in use today.
It wasn’t until 1967 that technology improved enough to give us the smaller, more efficient units affordable to consumers. This improvement involved using a newly developed semiconductor device called a “diode” within the high voltage electric circuitry that powers the magnetron. We’ll learn more about these technologies in our next post.
Also in our next post, we’ll see how high voltage circuits can pose electrocution hazards in a way you‘d never expect. I discussed one of these instances in my recent appearance on The Discovery Channel program, Curious and Unusual Deaths, soon to be aired.
Archive for July, 2011
| Ever discover a melted candy bar in your pocket? You immediately start to think about the sources for the heat that had caused the mess. Did you stand too close to the stove, were you outside in summer heat too long, or did you simply sit on it? Or was it perhaps caused by being in proximity to a whirring device, something which does not seem to generate any heat at all? If you’ve been reading along with us, you know what device I’m talking about.
Last time we talked about an effect known as cavity resonance and how a sound is created, much like a musical note, when we blow across the top of a glass pop bottle containing some air space. Our breath causes the air molecules to bounce in and out of the bottle’s cavity, producing the sound. Microwave technology works in much the same way, making use of an electronic device called a resonant-cavity magnetron. But instead of generating a musical tone, like our breath does over the bottle, the magnetron produces short wavelength radio waves, known as microwaves, and it was initially developed to generate these microwaves for radar systems. So, how does the magnetron work?
The magnetron contains a series of cavities arranged in a circle, their openings pointed towards the center as shown in Figure 1. Engineers determined that when a high voltage, say 4000 volts, is applied to the magnetron, it begins to boil off electrons through a filament, called a cathode, located at its center. Once free of the cathode, the electrons want to flow to a part of the magnetron called the anode. This is because the cathode is positively charged and the anode is negatively charged, and as we know, electrons like to flow from positive to negative. The anode is also the part of the magnetron containing the cavities, and we’ll see the significance of this in a moment.
Figure 1 – Interior View of a Resonant-Cavity Magnetron
Before the electrons can take their desired straight path to the anode, they are deflected by powerful magnets located on either end of the magnetron. These magnets force the electrons to move in a circular pattern over the openings in the cavities. Like the air molecules passing over the top of a pop bottle when you blow across it, the electrons move over each cavity opening in the magnetron, creating not musical tones, but microwaves. The microwaves are then collected from the magnetron using an antenna and directed along a tube called a wave guide. The microwaves leave the wave guide when they are transmitted by radar systems. The radar system then transmits the microwaves towards moving objects they wish to track. These tracked objects are as diverse as airplanes, ships, and weather patterns. See Figure 2.
Figure 2 – Microwave Radar Transmission
So, how does the microwave oven fit into our discussion? In 1946 an engineer by the name of Percy Spencer was working on a radar magnetron for the Raytheon Corporation, a producer of electronic technology for industry and defense. During the course of his work he unexpectedly exposed himself to microwaves from a wave guide, and he couldn’t help but notice that the candy bar in his pocket had melted. Putting two plus two together, he realized the microwaves had caused the candy bar to heat up. Dr. Spencer further experimented and came to the conclusion that microwaves can cook foods far more quickly than conventional ovens, and the modern day kitchen appliance was soon born.
Next time we’ll look at how Dr. Spencer’s microwave cooking discovery was developed into the microwave oven we find in most kitchens. We’ll also see how even an unplugged microwave oven can pose an electrocution hazard, as I explained in the Discovery Channel program I was recently featured on, Curious and Unusual Deaths.
| 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.
| Pop some corn, defrost meat, quickly heat a frozen dinner. What modern day miracle is capable of doing all this and more? The microwave oven, of course. They’ve become an integral part of modern kitchens and our lifestyle. Did you ever wonder how they came to be? We’ll have to turn back the hands of time over 70 years to the dark days of World War II to find out.
WWII began when Adolph Hitler’s Nazi regime conspired with Joseph Stalin’s Soviet Russia to invade Poland in September, 1939. From there, German forces went on to sweep across much of continental Europe, and even as far as North Africa and the Middle East. In the face of these and future war maneuvers, Great Britain remained our ally and a significant force to be dealt with.
Although Hitler was intent on bringing the British Empire to it knees by means of aerial bombardment, he did not succeed. He failed to take into account the fact that the British Royal Air Force Fighter Command had a unique advantage over his German Luftwaffe– or Air Force to those of you not up on your German. The advantage was called RADAR, an acronym for RAdio Detection And Ranging. Thanks to this proprietary technology, the British Fighter Command was able to detect incoming German warplanes well in advance of their arrival over British soil, and this provided British pilots with enough time to scramble their fighter planes and get a jump on their attackers. Although horribly outnumbered, the Royal Air Force planes managed to inflict heavy casualties on the Luftwaffe. In fact, this Battle of Britain, as it came to be known, marked the first defeat of Hitler’s military forces during the war.
So how does this modern marvel of RADAR work? Simply stated, it transmits radio waves from an antenna. When these waves strike an object, say an approaching German airplane, some waves are reflected back to a special receiver antenna. The reflected radio waves are then converted by the radar system into a “blip” on a viewing screen. A radar operator stands at the ready, trained in the art of interpreting radar blips. He or she is quickly able to assess the approaching object’s position, direction of travel, and speed of approach.
Although the ideas that led to the development of radar had been around for some time before WWII, it wasn’t until the late 1930s that engineers began developing it for air defense purposes. They first developed what was called a long wave radar system, referring to the fact that the radio waves transmitted had a long wavelength, about 1.5 meters. The British, at the top of their game, had anticipated the Nazi threat and installed an array of these on their east coast by 1940. Although long wave radar worked well enough during the Battle of Britain, it required massive, land-based antennas to operate, which proved to be cumbersome. It also couldn’t give operators a good idea of how many planes were approaching.
Then, just months before the beginning of the Battle of Britain, British researchers developed an ultra top secret device they dubbed a “resonant-cavity magnetron,” or magnetron for short. This magnetron was capable of producing short radio waves, in the range of 1 to 10 millimeters, and these came to be known as microwaves.
The technology mastered in the magnetron opened the door to an improved radar system, one which became mobile. These systems were more easily installed on aircraft, ships, and land vehicles. Microwave radar was also found to be better at detecting small objects than its massive predecessor technology. Unfortunately, microwave radar couldn’t be put to use in time for the Battle of Britain, but it did play a pivotal role in the defeat of Axis, that is the opposing side’s, forces during the remainder of WWII.
Next time we’ll take a closer look at the resonant cavity magnetron and find out how microwave radar led to the development of the famous oven that bears its name.
| Most people aren’t aware of just how important those strange looking wall outlets in our kitchens and bathrooms are, you know, the ones with the little buttons that say Test and Reset. They’re known as GFCI outlets, that is Ground Fault Circuit Interrupters, and given the right set of circumstances they could save your life.
The GFCI equipped wall outlet, like a mighty robot, continuously watches the flow of electrons (electrical current) passing through, always on the lookout for incongruities between the hot and neutral wires, and ready to jump into action when necessary. Say, for example, that one of these GFCI equipped outlets has an appliance plugged into it. While the appliance is in safe use there is nothing for the GFCI robot to do. It simply takes note of the balance of electrons flowing between the hot and neutral conductors, notes that they are equal, and continues to watch for inequalities.
But suppose that there is a problem with the appliance, something that causes a ground fault where the user’s body provides an unintended path to errant electrons flowing from the hot side of the wall outlet. Those errant electrons are supposed to traverse the neutral wire back through the wall outlet from whence they came, but they have become unruly. Not to worry, if you are up to code and have an ever vigilant GFCI on that outlet, the robot will immediately notice the anomaly.
Figure 2 – If a Ground Fault Develops in the Hand Mixer and Some Electric Current Flows Through the User’s Body, Then the Robot Notices a Difference In Current Flowing Through the Hot and Neutral Wires in the GFCI Outlet.
The Mighty Robot of the GFCI doesn’t like the fact that the electrons are out of balance, that there are more of them flowing through the hot wire than returning through the outlet via the neutral wire, so within a fraction of a second it will jump into action to correct things. It hits a lever on a spring loaded mechanism that snaps open an electrical switch connecting the appliance to the hot and neutral sides of the outlet, effectively cutting off the flow of electrons to the appliance. Cut off from power, the appliance ceases to function, but more importantly, the flow of electrons through the user’s body has been stopped before their body incurs injury, or death.
Figure 3- In Response to the Ground Fault, the Robot Opens a Switch in the GFCI Outlet to Cut Off The Flow of Electricity to the Hand Mixer. The Person Operating the Hand Mixer is Saved.
The GFCI robot, having done its job, now goes into a sleep mode. It will be reactivated, ready again for its vigilant watch of errant electrons, when the faulty appliance is unplugged and the Reset button is pressed. This button does what it says, it resets the spring loaded mechanism in the wall outlet, closing the electrical switch, and making the outlet functional again. The GFCI robot immediately goes back into active monitoring mode.
Now it should be noted that as dependable as GFCI outlets are, they can become defective. That’s why they have a Test button. This button should be pressed periodically to see if the robot is still on the job. If all is in order, the Reset button pops out of the outlet, and anything plugged into that outlet will not operate. When you press the Reset button back in, everything should operate again if there are no fault conditions.
Could the GFCI’s Mighty Robot have prevented the unfortunate incidents discussed during my tenure on The Discovery Channel? Stay tuned to find out…
That’s it for GFCI outlets. Next time we’ll take a look at how an invention developed to defend the allies during World War II later morphed into a space age device that cooks our food.