How Atomic Clocks Work

How Is Atomic 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 clocks (the most common type) is better than one second per one million years. Hydrogen atomic clocks show a better short-term (one week) accuracy, approximately 10 times the accuracy of cesium atomic clocks. Therefore, the atomic clocks have increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.


The National Company in Massachusetts produced the first commercial atomic clocks using cesium. Today, they are produced by various manufacturers, including Hewlett Packard, Frequency Electronics, and FTS. New technology continues to improve performance. The most accurate laboratory cesium atomic clocks are thousands of times better than commercially produced units.

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About the Author

Douglas Dwyer is the founder of Frequency Precision Ltd. in the UK. He provides consulting and design services to the world-wide electronics industry. Mr. Dwyer has been involved in frequency control since the mid-1960s and has published articles on crystal oscillators, temperature-compensated crystal oscillators, oven-controlled crystal oscillators, surface acoustic wave oscillators, and industrial quartz crystal fabrication technologies. The history of clocks and watches has been an interest of his since he got involved in the design of quartz watches.