![]() In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium clock, was completed in 1968, and was used into the 1990s as part of the NIST time system. ![]() Over the next decade, more advanced forms of the clocks were created. They became increasingly complex until, in the seventeenth century, Christiaan Huygens invented the classic pendulum clock. In 1955, the National Physical Laboratory in England built the first cesium-beam clock used as a calibration source. ![]() By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NIST) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1. Norton made an astronomical clock for George III which still stands in Buckingham Palace.In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. Considerable experimental work followed, and it was not until 1906 that the first self-contained battery-driven clock was invented. There are clocks by him in the Royal Collection and many museums worldwide. Invented in 1840, the first battery electric clock was driven by a spring and pendulum and employed an electrical impulse to operate a number of dials.
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