ATOMIC TIME

time scale generated by atomic clocks, which furnish time more accurately than was possible with previous astronomical means (measurements of the rotation of the Earth and its revolution about the Sun). International Atomic Time (abbreviated TAI, from the name in French) is based on a system consisting of about six primary, laboratory-constructed, cesium-beam atomic clocks along with a larger number of secondary, commercially made cesium clocks. Signals from these atomic clocks are transmitted to the International Time Bureau at Paris, which uses them to form TAI. It is estimated that the second of TAI reproduces the SI second to within about 1 part in 1013. Two clocks that differ in rate by this amount would drift apart by only three milliseconds in 1,000 years. See also atomic clock; Coordinated Universal Time.