Today,atomic clocksare the most accurate clocks in existence. They are considerably more accurate thanquartz clocksas they can be accurate to within a few seconds over thousands of years.
Atomic clocks were first theorized byLord Kelvinin 1879.In the 1930s the development ofMagnetic resonancecreated practical method for doing this. A prototypeammoniamaserdevice was built in 1949 at the U.S.National Bureau of Standards(NBS, nowNIST). Although it was less accurate than existingquartz clocks, it served to demonstrate the concept.
The first accurate atomic clock, acaesium standardbased on a certain transition of thecaesium-133atom, was built byLouis Essenin 1955 at theNational Physical Laboratoryin the UK.Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scaleephemeris time(ET).