The Accuracy of the Atomic Clock

Apart from the digital clocks and cellphones which tell the time, no clock, as it seems, can be relied upon to keep perfect time. Even today's most advanced digital clocks, whether it's on a cellphone, iPhone, radio or TV etc, are not one hundred percent accurate. As for the analogue forms of time, the best mechanical timepieces gain or lose about four seconds a year, and even the modern quartz crystal clocks are accurate only to about one second in ten years. However, an atomic clock is thought to be accurate and precise to one second in about 1,000 years, or at least in that estimated time.

Every analogue clock measures time by counting the regular vibrations of something. The very early clocks used the swing of pendulum, which are still popular today in many peoples homes, and early watches had the constant oscillation of a balance wheel. Quartz clocks have crystals designed to vibrate at 100,000 cycles a second when electric current is applied.

Atomic clocks keep time by the vibrations of their atoms, which is more than 9 billion vibrations a second in the most commonly used cesium (also spelled as caesium) atomic clock. Compared to mechanic clocks, which are upset and disturbed by such things as temperature and friction or maybe noise, the atomic clocks are virtually unaffected by outside conditions. They have no dial or any sort of display by which you can read the time, in other words, what they do is regulate quartz clocks to keep them extremely accurate.

The general housing of the cesium clock is a vacuum tube with a small electric oven at one end in which a piece of cesium is melted and turned into vapor. Cesium is a silvery-white metal quite similar to sodium with a very low melting point of 28.5 degrees Celsius (83 Fahrenheit) compared to iron's 1535 degrees Celsius (2795 F) melting point. The cesium atomic clock used at Greenwich Observatory until 1962 was accurate to one second in 300 years. This clock is now in London's Science Museum in the UK.

Like all atoms, the cesium atom has a nucleus surrounded by orbiting electrons. These electrons can be manipulated in an electromagnetic field. When this occurs, the atom's energy level is changed to a lower or higher state, and it either absorbs or radiates electromagnetic energy. The radiation frequency is continually and absolutely constant. The vaporized cesium is subjected to an electromagnetic field inside a resonator, and causing sharply defined vibrations with a frequency of 9,192,631,770 hertz which is cycles per second.

This is the precise frequency at which the atoms change their energy state. The quartz clock is used to keep the resonator vibrating at this frequency of 100,000 hertz being multiplied electronically.

As long as the quartz clock is accurate, the cesium atoms will all resonate uniformly.They are focused on a detector that senses any change in their energy concentration. If there is a change, this detector sends an error signal through the circuit to the quartz clock. With this sort of response, the quartz clock is electronically adjusted to rectify the vibration of the resonator.

Atomic clocks are used to calibrate other clocks and for scientific research in observatories and space labs. They are also used on high-speed aircraft for synchronizing with radio-frequency navigation signals. Other materials besides cesium can be used for atomic clocks.

Ammonia was used in the first atomic clock built in the year 1947 at the National Bureau of Standards in Washington DC, United States.The highly accurate atomic clocks at present use hydrogen, which has a radiation frequency of more than 1420 billion hertz. They are known to be accurate to around one second in nearly 2 million years. Plus American scientists are developing a mercury atomic clock which they expect to be accurate to one second in 10 billion years.