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The Most Accurate
Clocks in the World
    How do you know what time it really is?  Have you ever looked at the clock on your wrist, and it says noon ... lunch time! But the clock on the wall says it's five minutes until noon.  Don't you wish there was a way that all time could be the same?  Accurate time affects much more than just getting to lunch on time or five minutes early.  Transportation, communication, manufacturing, electric power, and other technologies are all extremely dependent on time.  As our obsession with time grows, we race to find the most accurate way to measure it.  Scientists have been working on a solution to this problem for decades. And they have found the beginning of an answer.

    R ight now, the ATOMIC CLOCK is the most accurate time measuring device in the world.  While pendulums and crystal quartz clocks are thought to have been very accurate, they are affected by the environment that surrounds them.  For example, humidity affects the functions of a quartz watch.  For these clocks to perform accurately, they must constantly be measured and fixed.  So scientists have been interested in finding things in nature that are regular and dependable.    The Earth goes around the sun in about 365 days, the moon goes through a cycle about every month, and, until recently, the geyser Old Faithful shot out steaming water about every 15 minutes every day.  But scientists want absolute accuracy.  Investigating the natural vibrations of atoms seems to be the best the answer.  An atom vibrates at very regular intervals in nanoseconds.  One nanosecond is a billionth of a second, so it takes one billion nanoseconds to make one second (for our British friends, Americans use billion to refer to one thousand million).

    There are many atomic clocks that vary according to the natural element on which they are based (for example, hydrogen, ammonia, or cesium).  All natural elements absorb and release electromagnetic radiation at a certain fixed frequency.  In simple terms, atomic clocks are electronic devices that can measure time by counting the number of times atoms vibrate.  Because many different types of elements are used for atomic clocks, the clocks work in different ways.  However, they all rely on the fact that atoms "take in" and "shoot out" electromagnetic rays at a stable frequency.  Cesium atomic clocks are one of the most accurate and most commonly used types of  atomic clocks.  The picture above is a very old cesium atomic clock.

   To give you an idea about the size of a laboratory Cesium atomic clock, it is about the size of a railroad flatcar.  The  picture to the left shows examples of laboratory atomic clocks. These are even a bit small compared to others.  A Cesium atomic clock called the NIST-7, or National Institute of Standard Technology, is stationed in Boulder, Colorado.  The cesium clock in Boulder is the most accurate clock on Earth to date; it is used as the official clock of the United States.  If you're tired of the inaccuracy of the clocks around you, just adjust all your clocks to the NIST-7 standard time. The other type of atomic clocks are Commercial Cesium atomic clocks. They are about as large as a suitcase, although the size varies.  Some of these clocks are used in scientific laboratories, but most are for public use.  The picture to the right is an example of a commercial atomic clock.Although the commercial Cesium clocks are cheaper, they're still accurate and precise within their time measurement functions.  Some Cesium clocks are used to synchronize other types of clocks.  To give you an idea about the accuracy of an atomic clock, a Cesium clock only loses two nanoseconds a day or one second in 1,400,000 years.
How They Work
(A Complicated Explanation)
In 1955 after decades of scientific research, the first Cesium atomic clock was built. It is the most accurate time measuring device to date.  The way these clocks work is very complicated.  The first thing that occurs is that liquid Cesium is put into an oven and heated until it changes into a gas.  Cesium atoms escape at very high speeds through a hole in the oven.  The speeding atoms now pass between two electromagnets that split the atoms into two beams: one in a high energy state and one in a low energy state.  The beams in the low energy state go through a U-shaped hole where they are exposed to radiant energy by microwaves of very specific wavelengths.  Once again, the altered atoms are heated up and excited. Many of the lower atoms are excited to a higher energy state.  These beams continue through some more electromagnets and are once again divided.  The atoms that are now in the high energy state pass a hot wire and become electrically charged.  The now-pure atoms are directed onto an electron multiplier.  The measurement of an atom's resonance frequency is then found by adjusting the frequencies of the microwaves, and the electron multiplier is intensified to the highest degree.  This frequency is put into a feedback control circuit, which is connected to a quartz crystal oscillator (like a quartz watch that's set at a certain frequency). This makes a pulse almost exactly once every second.
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