Frequency standards and measurement

The two time scales furnished by astronomical measurements have differed by up to 30 seconds in time or 7 parts in 108 in frequency. The atomic unit defined in terms of the caesium hyperfine line is accurate to within 1 part in 1011. The rubidium gas-cell standard is a convenient small working standard and the hydrogen maser a potential standard of high accuracy. Modern developments in solid state physics have greatly simplified the techniques of pulse counting and frequency synthesis. The methods used for generating and measuring the frequencies of the caesium and rubidium hyperfine lines are given as illustrations of current techniques.