Atomic Length Standards

Although the world's official primary standard of length is still the one‐meter distance between two lines on a metal bar, practically all precise measurements of length requiring accuracies greater than one part in ten million are made and will continue to be made with light waves. Only recently, however, has it become possible for most research organizations to have an ultimate primary standard of length in their own laboratories. In March, the National Bureau of Standards and the Atomic Energy Commission issued an announcement stating the availability to science and industry of primary standards of length in the form of spectroscopic lamps containing a single pure isotope of mercury, thus enabling any laboratory possessing the necessary auxiliary optical equipment to become its own bureau of length standards. The mercury is sealed in a small glass tube and when subjected to high frequency radio waves it emits light waves with very sharply defined wavelengths uniquely characteristic of the mercury, which, with the help of interferometric techniques, can give highly precise length measurements.