# 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.

For more information on atomic clocks and related topics, check out the links on the next page!