The angstrom outgrew the metre, then quietly fell out of the official system
One angstrom is a ten-billionth of a metre, about the radius of a phosphorus atom. For half a century spectroscopists defined it by a glowing line of cadmium light rather than trust the metal bar that defined the metre. It remains everywhere in chemistry, yet the international standards body now discourages its use.
The unit honours the Swedish physicist Anders Jonas Ångström, who in 1868 charted the spectrum of sunlight and expressed wavelengths in ten-millionths of a millimetre. Solar physicists adopted his tables and named the unit after him, and it spread into astronomy, atomic spectroscopy and every field dealing with atomic scales. Its symbol is still the Swedish letter Å. The scale is handy: hydrogen's atomic radius is about half an angstrom, and visible light spans roughly 4,000 to 7,000.
Precision was the problem. Until 1960 the metre was the distance between two scratches on a platinum-iridium bar kept in Paris, and relying on such a physical object introduced errors of around one part in 6,000 into wavelength tables. Ångström himself had his reference bar checked against a Paris standard; the metrologist Henri Tresca's report was so wrong that the corrected results were worse than the originals. Between 1892 and 1895, Albert Michelson and Jean-René Benoît measured the standard metre against the red light of excited cadmium vapour, and in 1907 a solar research union fixed the angstrom as an exact fraction of that wavelength. From 1927 to 1960, the angstrom lived as a separate spectroscopic unit, independent of the metre.
When the metre itself was redefined using spectroscopy in 1960, the angstrom became simply 0.1 nanometres again. A short-lived rival, the Angstrom Star of 1965, was defined from a tungsten line and aimed for accuracy within 5 parts per million; within a decade it had proved both less precise than hoped and obsolete.
The angstrom was never part of the International System of Units. Until 2019 the standards bodies listed it as compatible, but the 2019 edition of the SI brochure drops it entirely, and the European Union excludes it from its approved units. Older papers sometimes wrote it as A.U., inviting confusion with both the astronomical unit and the atomic unit of length.
Source: Angstrom
