 This iconic image by Leonardo da Vinci is called Vitruvian Man. It's the Renaissance ideal of bodily perfection, yet its notion of best is nearly the opposite of an engineer's. Of more used to engineers are some drawings developed in the 1930s by industrial designer Henry Dreyfus. These were drawings of ordinary people with their far-from-ideal proportions. Dreyfus used these figures to create John Deere tractors, locomotives, and clocks. He designed the iconic Model 302 telephone and the round Honeywell thermostat. Dreyfus' approach demonstrates that an engineering solution can only be judged based on how it handles the constraints unique to its situation, a balance of cultural forces, societal values, availability of material resources, and even urgency. This means that the notion of best changes with time. A wonderful demonstration of that is the evolution of this device. It's called a Cavity Magatron, which powers mobile radar. It was created by British scientists in early 1940 in response to fears that Nazi planes would bomb London. This device generates microwave radiation of sufficient power and of the right frequency to detect individual planes, to operate an electrode called a cathode is placed in the center. When powered electrons fly from this cathode to the copper magnetron called the anode block. To generate microwave radiation, this powered magnetron is placed in a magnet. As this magnetic field increases, the electrons inside the magnetron begin to spiral to the anode block, and when the magnetic field is strong enough, the electrons don't quite reach the edge of the cavities, and they start to spin. As this spinning electron cloud brushes by the openings to the circular cavity, microwave radiation is generated. In addition, this magnetron had to be cooled so it was housed in this water-cooled tube. The standard of best used by the physicist engineers who developed this magnetron was simply that it worked. Prior to this, nothing had sufficient power. That notion of best changed in the next stage of evolution of this magnetron. The device worked in the laboratory, but now it needed to be mass manufactured to equip thousands of British-American and other allied planes and ships with radar. Because the Nazis had cut the United Kingdom off from the continent, the British needed U.S. technical expertise and production capability. They turned to Raytheon, a small manufacturer of radio and vacuum tubes, and to their vacuum tube engineer, Percy Spencer. For engineer Spencer, the notion of best changed. Now the issue was one of prioritizing speed of production while maintaining reliability. The central problem he faced was this. The slightest deviation in the cavity's diameter along the length of the cylinder would change the frequency of the radiation and blur the results from the radar. The tolerance was remarkable. The diameter of these cylindrical cavities must deviate by at most one ten thousandths of an inch. In SI units, that's a tiny fraction of a millimeter, about 2.54 microns. To create such a cavity, took a master mechanic a week to finish a single magnetron. Yet Britain and the Allied forces needed thousands of radars. Spencer produced this magnetron, likely used on board a ship in the Pacific. If we could see through the top, we would see the cavities. So all of this houses the anode block. Recall that this would sit in a magnet in this way. You would power it here, and microwave radiation would emerge from here. Let's look at how Spencer designed it to be mass manufactured and to operate aboard a plane or ship. Note the side of the magnetron. You see the metal pieces with the space between them. These are cooling fins, so this magnetron was air-cooled instead of water-cooled like the first working prototype. That's a significant improvement. But to manufacture it rapidly, while maintaining tight tolerances on the cavity diameters for Spencer part of best with speed, he used a time-honored engineering rule of thumb. Break complex problems into smaller, more manageable pieces. To show you his solution, I have a replica of the central part of this magnetron with the black coating removed. Instead of a solid block of copper, he assembled the body out of 13 thin sheets of copper. In each of these sheets, a part of the cavity was punched, so that when the sheets were stacked on top of each other to construct the magnetron, they created a cavity throughout. These sheets could be rapidly punched with large presses, which enabled Raytheon to manufacture 2,500 a day. These laminations solved the key problem of manufacturing a tight tolerance on the diameter of the hole along the length of the cavity. If we look at the original magnetron and a lamination from the side, you can see easily how that tolerance need only be held for a short distance, a much easier task. Spencer had taken all the precision off the hands of the humans crafting the magnetrons and given it to the die, which could be used thousands of times. Now, the evolution of the magnetron as an illustration of what engineers mean by best doesn't stop here. A magnetron creates the microwaves in a household microwave oven. But to reversion the magnetron for an oven, the idea of best had to change. At first, the goal was not a household microwave oven, not like today is in addition to a conventional oven. The goal was to replace the conventional oven and to cook fast. The earliest patents show a whole roast, a complete turkey, and a lobster being cooked. And the first microwave oven, which appeared in 1946, was aimed at restaurants, which is a commercial, not a consumer use. This oven was filled with tubes and other electronics and weighed three or four times as much as today's conventional stoves. It was five feet tall with a comparatively tiny cooking chamber and was water-cooled and so had to be plumbed into its location. This cooling was needed because it consumed 3,000 watts of power, four times a typical microwave oven today. Because of this high power, it had to be wired into a 220-volt line, like an oven or dryer. The high power was a design choice of the engineers. They wanted the ovens to cook quickly because they marketed the ovens to chefs and restaurant owners as a way to reduce the time restaurants spent preparing meals. To become a household item, a consumer oven needed a new notion of best. We can see this in the ads of the first successful consumer oven. They still promoted fast cooking, but look now at this specification that mentions a standard 115-volt outlet. And further down in the ad copy, they extoll how it fits onto a kitchen counter. Implicit here is also that the oven was inexpensive, at least compared to those large commercial ovens. Let's look at how this new definition of best changed this wartime magnetron to this. This is a magnetron from one of today's ovens. First, the Raytheon tube had many separate metal parts, which had to be carefully put together. Contrast that to the consumer oven's magnetron, which is a tidy compact unit, produced for about $7 on an automated production line. Here's the magnetron, surrounding it with the cooling fins. Instead of robust pieces of metal, there are thin sheets of punched stainless steel. Next, look at the magnets. These are small ring magnets. Compare them to the magnets used in a wartime magnetron. Here's the magnetron, and this is the magnet. It's a large, expensive El Ninko magnet. In the consumer oven, the embedded magnets are cheap ceramic magnets. The magnetic field of these cheaper magnets is not as stable as an El Ninko magnet. The field dropped as the magnetron operated and increased in temperature, so that after the first minute or two of operation, the magnetic field dropped and the tube's power output to the oven decreased. This wouldn't do for Raytheon's military grade radar, but was good enough for cooking a hot dog. Next, let's look inside the magnetron to see a cross-section. The precise circular cavities of the wartime magnetron are now slightly irregular trapezoidal shapes in the microwave magnetron. The cavities are punched from a single slug of metal by a die. The cavities were not as precise as those of the wartime magnetron, but this was not for a radar system that needed precision to resolve individual aircraft in the sky. More tolerance is allowed in the frequency of the radiation used inside a microwave oven. And lastly, this tube draws only 650 watts, which means it fits into a standard household circuit. The reduced power meant, of course, that the oven wouldn't be able to cook nearly as fast as Raytheon's original model that could blast a beefside into a well-done steak in minutes. But that's not how the oven fit into the world. The microwave oven's niche was not in commercial use, but in the home as essentially a reheater that fit into a busy lifestyle. I've told this more detailed story of the magnetron in the microwave oven rather than repeat a likely apocryphal story, which first appeared in the late 1950s in a popular magazine. It claims that Percy Spencer walked by a bank of World War II magnetron tubes that were operating and that radiation from those tubes melted a candy bar in his pocket, and boom, he invented the household, the consumer microwave oven. This likely mythical story obscures the engineering method and strips away all the richness of how engineers create. It overshadows the extensive and detailed engineering that evolved that early commercial oven of the mid-1940s to the consumer oven. This is just a small sample of the many shapes and sizes of ovens created by engineers over three or four decades until it arrived at the final form to become an eventually ubiquitous invention. To hide the details of engineering hides the creativity of engineering, a creativity we've seen in the series and the mysterious structures of Gobekli Tepa, the majestic cathedrals of the Middle Ages, clever methods to overcome uncertainty and the complexity of fluid flow or the directed evolution of proteins. The astonishing development of the steam turbine and now the evolution of the magnetron to power a microwave oven. Hiding that creativity dissuades our best and brightest from recognizing engineering as a supremely creative endeavor which robs us of the next generation of mental firepower, the new wave of engineers who can help solve the problems our world faces. I'm Bill Hammack, The Engineer Guy.