 I want to show you the most amazing thing. The world's first commercially available chip-scale atomic clock, SymmetraKon's CSAC. That's right, this tiny device, about the size of a quarter, is an atomic clock. The most accurate atomic clocks lose about a second over 138 million years. The way that atomic clocks work amazes me. Let me explain how the very first one worked. I'll start with Jell-O. Tap a block of Jell-O and it wiggles back and forth. Just like the swings of a pendulum and a grandfather clock, the oscillations of this Jell-O keep tying. Now, Jell-O isn't very good for this, but inside an atomic clock there's a chunk of quartz of a similar shape that if we tap it, which we do with a jolt of electricity, it will oscillate some 5 million times per second. It keeps time to about one second in 90,000 years, a fraction of the accuracy needed for an atomic clock. Quartz loses time because it slows down and needs to be nudged to restore its oscillation. That's where the atomic part of an atomic clock comes into play. We use cesium atoms to control these nudges very accurately. Every time the quartz' oscillations slow down, just the tiniest bit, we give it a tap, an electric jolt at just the right time, so essentially its oscillations never decay. Let me show you how we use cesium to do this. The atoms in pure cesium exist mostly in two slightly different forms, a low energy form and one with just a bit more energy. In an atomic clock, these two states have two properties critical to making a clock. One, they can be separated by a magnet, and two, the lower energy atoms can be converted to the higher energy ones if we bombard cesium with the right radiation. Engineers tie the slowing down of the quartz' vibrations to the precise wavelength of the bombarding radiation to create a feedback loop. Let me show you how. In an oven, we heat cesium chloride to create a gaseous stream of cesium ions. The stream contains both the low and high energy ions. We first flow it through a magnet, separating the two types, discarding the high energy ones, allowing the lower energy ions to pass into a chamber. Inside the chamber, we bombard the ions with just the right wavelength radiation to make them jump to higher energy. As these gaseous ions leave the chamber, they pass through another magnet that directs high energy ions toward a detector, this time discarding any lower energy ones. The detector converts the arriving ions to a current. The trick here is to tie that current from the detector to the quartz oscillator. When the quartz' oscillations decay, that is, it slows down a little, then the energy bombarding the cesium ions in the chamber changes and fewer high energy ions exit the chamber so current decreases or stops. This tells the electronics to zap the quartz oscillator and correct the period of oscillation. It does this by applying the proper voltage that, via the piezoelectric effect, taps the quartz and restores its oscillations, thus creating a clock that loses less than a second over many million years. Our world runs off such accuracy. For example, the global positioning system, GPS, requires it. The global positioning system consists of 24 satellites orbiting the Earth. A GPS receiver uses the position of four of these satellites to locate itself, one to correct the time on the receiver and three to locate its position. Here's how it works. A signal is sent to the receiver from the first satellite that contains that satellite's location and the signal's time of departure. The receiver then multiplies the signal's travel time by the speed of light to calculate its distance from the satellite. With one satellite, the receiver knows that it's located on a sphere around that satellite with a radius equal to the calculated distance. So it has the same calculation with a second satellite. The intersection of these two spheres narrows the location to the circumference of a circle. Then with a third satellite, the receiver can reduce the location to a single point. When signals are traveling at the speed of light, being off by even a millisecond means an error of about a million feet or 300 kilometers. But with atomic clock accuracy, the receiver can locate itself to about three feet. I'm Bill Havoc, The Engineer Guy. This video is based on a chapter in the book Eight Amazing Engineering Stories. The chapter features more information about this subject. Learn more about the book at the address below.