How an atomic clock works, and its use in the global positioning system (GPS)

Bill shows the world's smallest atomic clock and then describes how the first one made in the 1950s worked. He describes in detail the use of cesium vapor to create a feedback or control loop to control a quartz oscillator. He highlights the importance of atomic team by describing briefly how a GPS receiver uses four satellites to find its position. You can learn more about atomic clocks and the GPS system in the EngineerGuy team's new book Eight Amazing Engineering Stories http://www.engineerguy.com/elements

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  Science & Technology

[Steve55599]

This is a good introduction to atomic clocks, but it fails to clearly point out that it's not just the quartz that vibrates or resonates at a certain frequency. The cesium atom has it's own resonance frequency (9,192,631,770 cycles per second. This is actually THE definition of a second). When a cesium atom receives microwave energy at exactly this frequency, it changes its energy state. "A cesium atom always resonates at the same known frequency -- that is what makes atomic clocks so precise."

[engineerguyvideo]

The 9,192,631,770 cycles per second gives the impression that it is a cesium atom vibrating. Remember that number is the ENERGY gap between the two lowest energy levels of cesium. (E=h*nu, where h is planck's constant and nu is the frequency; we report it instead in joules or wavenumbers ... I used to be a spectroscopist!) So, it is not correct to think of cesium vibrating like a pendulum. What gives a clock the great accuracy is that this energy gap is the same for every atom. (cont -->)

[engineerguyvideo]

(cont) in fact Cesium was chosen because its lower energy states aren't "messy" (i.e., a number of transitions) and it can be easily prepare as a gas. I recommend Essen's early paper on it from the1950s.

[bembry3000]

I'll start with Jell-O

[Matthew Burt]

@vinchanzo espionage: its like triangulation. If you know the position of x and y you can calculate the position of z.

[Jeroen Peeters]

For regular use, you are standing upon the earth, which is a sphere itself. Only when you are up in the air the fourth one really matters. Well that is how I think it works

[chriscon22]

Yes and it has to be corrected every so slightly for relativity.

[Daniel Walsh]

Why are three satellites sufficient beyond the fourth time-keeping one? As you say, the first two narrow the position down to a circle. But the third will narrow the position down to the intersection of this circle with a sphere, which is actually two points, not one.

[kazamatsri]

If the oscillations essentially never decay...where does the decay come from if we keep jolting the quartz?

[Vladimir Zharkov]

Those tricky bitches, thats brilliant.

[Ritt Momney]

never stop making these