Caesium atomic clock. Physicists Jack Parry (left) and Louis Essen (right) adjusting their caesium resonator, which they developed in 1955. Atoms of vapourised caesium-133 oscillate between two energy levels as they pass back and forth between magnets at each end of the resonator. Counting these oscillations is the basis of the standard second, where one second is about 9193 million oscillations. Essen and Parry's resonator led to the replacement of the astronomical second with the atomic second as a standard of time, as it was the first apparatus to be used reliably as a clock on a long-term basis. Photographed at the National Physical Laboratory, Teddington, UK, in 1956.