The History of the Atomic Clock

Man has been obsessed with time-keeping since the earliest societies. Sundials that broke the day down into equal increments were discovered in Egypt dating back to 1300 BC. This ancient civilization divided the year into 36 equal months, or decans, of 10 days each, then added 5 extra days to keep their calendar straight. We can thank the Babylonians for dividing hours and minutes into 60 equal increments. Much later, with the advent of the atomic clock during the 20th century, time can be measured with astonishing accuracy.

History

Isidor Isaac Rabi who was born in the Austrian Empire but immigrated to the United States with his parents while still an infant, is credited with being the father of the atomic clock. A celebrated physicist, Rabi's inquiry into the nature of proton binding in atomic nuclei enabled him to develop his atomic beam magnetic resonance detection method. Rabi received the Nobel Prize for his discovery.

Time Frame

Rabi was the first to theorize that an extremely accurate clock could be created based on atomic beam magnetic resonance. The first so-called atomic clock was built in 1949 by the National Bureau of Standards, the precursor to the contemporary National Institute of Standards and Technology (NIST). The first atomic clocks were based on the vibration of ammonia molecules.

Features

The NBS-1 apparatus, a device which eventually became the calibration source for clocks, was developed in 1952 by the NIST. NBS-1 technology is based on the frequency of the cesium atom. Commercial cesium-based atomic clocks entered the market place in 1958. They cost a mere $20,000 each.

Significance

In 1967 the The 13th General Conference on Weights and Measures re-defined the "second" based on vibrations of the cesium atom, thereby effectively removing astronomical observations as a basis for calculating the passage of time.

Considerations

Advances in cesium beam technology continue to increase the accuracy of succeeding generations of the NBS atomic clocks. NBS-6 boasted the ability to keep time accurately for up to 300,000 years without 1 second variation. NIST-7, which came into service in 1993 claimed to be 20 times more accurate than its predecessor, and NIST-F1, which was introduced just before the turn of the century, claims accuracy of within 1 second every 20 million years. The National Institute of Standards and Technology continues to refine the atomic clock. As of 2005 the agency estimated the cesium fountain atomic clock NIST-F1 to be accurate to within 1 second every 60 million years.

Resources

• Displays time accurately anywhere in the world