Engineering Expert Witness Blog

      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.

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