Any clock in which the basic intervals of time are determined by reference to the resonant frequency of radiation absorbed or emitted by atoms or molecules. If the period is governed by the resonant frequency of molecules, the clock is preferably referred to as a molecular clock. Atomic clocks are commonly classified as active or passive according as they use radiation emitted by or absorbed by atoms. Typical passive atomic clocks are cesium clocks and rubidium clocks. Cesium clocks use cesium atoms for determining the basic intervals; rubidium clocks use rubidium atoms. Because the second is defined by international agreement as the length of time needed for a specified number of cycles of a particular frequency of radiation emitted by the cesium atom, cesium clocks are primary standards of time and frequency. Rubidium clocks have show smaller variations in frequency over short periods of time than do cesium clocks. The first complete and operating atomic clock was developed by H. Lyons and his associates at the U. S. National Bureau of Standards (NBS) in 1948-1949. It was actually a molecular clock; it used ammonia molecules for determining the basic frequency and interval of time. True atomic clocks came later and resulted from the work of L. Essen and his associates at National Physical Laboratory (England) and of Lyon and his associates at NBS from about 1952 onward.