Atomic Clocks Explained

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Is an Atomic Clock Radioactive?

An atomic clock keeps time better than any other clock. They even keep time better than the rotation of the Earth and the movement of the stars. Without the atomic clock, GPS navigation would be impossible, the Internet would not synchronise, and the position of the planets would not be known with enough accuracy for space probes and landers to be launched and monitored.

An atomic clock is not radioactive, it doesn’t rely on atomic decay. Rather, an atomic clock has an oscillating mass and a spring, just like ordinary clocks.

The big difference between a standard clock in your home and an atomic clock is that the oscillation in an atomic clock is between the nucleus of an atom and the surrounding electrons. This oscillation is not exactly a parallel to the balance wheel and hairspring of a clockwork watch, but the fact is that both use oscillations to keep track of passing time. The oscillation frequencies within the atom are determined by the mass of the nucleus and the gravity and electrostatic “spring” between the positive charge on the nucleus and the electron cloud surrounding it.

What Are The Types of Atomic Clock?

Today, though there are different types of atomic clock, the principle behind all of them remains the same. The major difference is associated with the element used and the means of detecting when the energy level changes. The various types of atomic clock include:

The Cesium atomic clock employs a beam of cesium atoms. The clock separates cesium atoms of different energy levels by magnetic field.

The Hydrogen atomic clock maintains hydrogen atoms at the required energy level in a container with walls of a special material so that the atoms don’t lose their higher energy state too quickly.

The Rubidium atomic clock, the simplest and most compact of all, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the surrounding microwave frequency is just right.

The most accurate commercial atomic clock available today uses the cesium atom and the normal magnetic fields and detectors. In addition, the cesium atoms are stopped from zipping back and forth by laser beams, reducing small changes in frequency due to the Doppler effect.

When Was The Atomic Clock Invented? atomic clock

In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NIST) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1.

In 1955, the National Physical Laboratory (NPL) in England built the first cesium-beam atomic clock used as a calibration source. Over the next decade, more advanced forms of the atomic clocks were created. In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom; the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium atomic clock, was completed in 1968, and was used into the 1990s as part of the NPL time system.

In 1999, NPL-F1 began operation with an uncertainty of 1.7 parts in 10 to the 15th power, or accuracy to about one second in 20 million years, making it the most accurate atomic clock ever made (a distinction shared with a similar standard in Paris).

How Is Atomic Clock Time Measured?

The correct frequency for the particular cesium resonance is now defined by international agreement as 9,192,631,770 Hz so that when divided by this number the output is exactly 1 Hz, or 1 cycle per second.

The long-term accuracy achievable by modern cesium atomic clock (the most common type) is better than one second per one million years. The Hydrogen atomic clock shows a better short-term (one week) accuracy, approximately 10 times the accuracy of a cesium atomic clock. Therefore, the atomic clock has increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.

Synchonising to an Atomic Clock

The simplest way to synchonise to an atomic clock is to use a dedicated NTP server. These devices will receive either the GPS ataomic clock signal or radio waves from places like NIST or NPL.


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Richard N Williams is a technical author and a specialist in the NTP Server and Time Synchronisation industry. Richard N Williams on Google+