Atomic clock. Physicists adjusting the settings to an atomic clock. This atomic clock, also known as a frequency standard, uses an atomic 'fountain' to measure the exact length of one second. The fountain contains atoms that oscillate between two energy levels at high speed. Counting these oscillations is the basis for the standard second. The current basis for the international definition of time is the caesium atomic clock, where one second is about 9193 million oscillations of caesium-133 atoms. Physicists are investigating whether alternative time measurements can be gained by using other elements, such as rubidium. Photographed at the National Physical Laboratory in Teddington, UK.