In 1868,
Swedish physicist
Anders Jonas Ångström created a chart of the spectrum of
sunlight, in which he expressed the wavelengths of electromagnetic radiation in the
electromagnetic spectrum in multiples of one ten-millionth of a millimetre (or .) Ångström's chart and table of wavelengths in the solar spectrum became widely used in the
solar physics community, which adopted the unit and named it after him. It subsequently spread to the fields of
astronomical spectroscopy,
atomic spectroscopy, and then to other sciences that deal with atomic-scale structures.
Early connection to the metre Although intended to correspond to metres, that definition was not accurate enough for spectroscopy work. Until 1960 the metre was defined as the distance between two scratches on a bar of
platinum–
iridium alloy, kept at the
BIPM in Paris in a carefully controlled environment. Reliance on that material standard had led to an early error of about one part in 6000 in the tabulated wavelengths. Ångström took the precaution of having the standard bar he used checked against a standard in Paris, but the
metrologist Henri Tresca reported it to be so incorrect that Ångström's corrected results were more in error than the uncorrected ones.
Cadmium line definition In 1892–1895,
Albert A. Michelson and
Jean-René Benoît, working at the BIPM with specially developed equipment, determined that the length of the international metre standard was equal to times the wavelength of the red line of the
emission spectrum of electrically excited
cadmium vapor. In 1907, the International Union for Cooperation in Solar Research (which later became the
International Astronomical Union) defined the international angstrom as precisely 1/6438.4696 of the wavelength of that line (in dry air at 15 °C (hydrogen scale) and 760
mmHg under a gravity of 9.8067 m/s2). This definition was endorsed at the 7th
General Conference on Weights and Measures (CGPM) in 1927, but the material definition of the metre was retained until 1960. From 1927 to 1960, the angstrom remained a secondary unit of length for use in spectroscopy, defined separately from the metre.
Redefinition in terms of the metre In 1960, the metre itself was redefined in spectroscopic terms, which allowed the angstrom to be redefined as being exactly 0.1 nanometres.
Angstrom star After the redefinition of the metre in spectroscopic terms, the angstrom was formally redefined to be 0.1 nanometres. However, there was briefly thought to be a need for a separate unit of comparable size defined directly in terms of spectroscopy. In 1965, J.A. Bearden defined the
Angstrom Star (symbol: Å*) as 0.202901 times the wavelength of the tungsten \kappa_{\alpha 1} line. This auxiliary unit was intended to be accurate to within 5
parts per million of the version derived from the new metre. Within ten years, the unit had been deemed both insufficiently accurate (with accuracies closer to 15 parts per million) and obsolete due to higher precision measuring equipment.
Current status Although still widely used in physics and chemistry, the angstrom is not officially a part of the
International System of Units (SI). Up to 2019, it was listed as a compatible unit by both the
International Bureau of Weights and Measures (BIPM) and the US
National Institute of Standards and Technology (NIST). However, it is not mentioned in the 9th edition of the official SI standard, the "BIPM Brochure" (2019) or in the NIST version of the same, and BIPM officially discourages its use. The angstrom is also not included in the
European Union's catalogue of units of measure that may be used within its internal market. ==Symbol==