Caesium beam atomic clocks. Also known as a frequency standard, these atomic clocks uses caesium to measure the exact length of one second. This is a beam type of atomic clock, with radiofrequency fields and magnets and mass spectrometers used to measure the rate of transition between energy levels. Counting these oscillations is the basis for the standard second, the international definition of time. One second is about 9193 million oscillations of caesium-133 atoms. These atomic clocks are being used at the US Naval Observatory, Washington DC, USA, to provide a basis for standard US time and time standards worldwide. Photographed in 1984.