Frequency and Time Standards

The gradual development of quartz and atomic clocks has resulted in an improvement in the accuracy of frequency measurement by a factor of at least 104 in the past 30 years. The best quartz clocks drift upwards in frequency at an average rate of about 1 part in 109 per month and operate with a day to day stability of a few parts in 1011. A caesium standard based on the magnetic deflexion of an atomic beam has been in operation since 1955 and recent models of this type of standard enable frequency and time to be defined in terms of atomic constants with an accuracy of a few parts in 1011. It has revealed irregular and seasonal variations in the length of the day of about 1 msec. The techniques of optical pumping and detection, of the use of buffer gases, and of wall coatings which enable atoms to be reflected without undergoing transitions have made it possible to make working atomic standards which are already of great stability and may well be capable of still further improvement.