Angstrom - History

History

Anders Jonas Ångström was one of the pioneers in the field of spectroscopy, and is known also for studies of astrophysics, heat transfer, terrestrial magnetism, and the aurora borealis.

In 1868, Ångström created a chart of the spectrum of solar radiation that expressed the wavelengths of electromagnetic radiation in the electromagnetic spectrum in multiples of one ten-millionth of a millimetre (or 10−7 mm.) Since the human eye is sensitive to wavelengths from about 4,000 to 7,000 ångströms, that choice of unit allowed sufficiently accurate measurements of visible wavelengths without resorting to fractional numbers. The unit then spread to other sciences that deal with atomic-scale structures.

Although intended to correspond to 10−10 meters, for precise spectral analysis the ångström needed to be defined more accurately than the meter which until 1960 was still defined based on the length of a bar of metal held in Paris. In 1907 the International Astronomical Union defined the international ångström by declaring the wavelength of the red line of cadmium in air equal to 6438.46963 international ångströms, and this definition was endorsed by the International Bureau of Weights and Measures in 1927. From 1927 to 1960, the ångström remained a secondary unit of length for use in spectroscopy, defined separately from the metre. In 1960 the meter itself was redefined in spectroscopic terms, and then the ångström was redefined as being exactly 0.1 nanometers.

Although internationally recognized, the ångström is not formally a part of the International System of Units (SI); the closest SI unit is the nanometre (10−9 m). Its use is officially discouraged by the International Committee for Weights and Measures and is not included in the European Union's catalogue of units of measure that may be used within its Internal Market.