Tuesday, December 13, 2011

Anders Jonas Ångström and the ångström

Anders Jonas Ångström
As any good chemist knows, an ångström is equal to 1x10-10 meters and is designated by the symbol Å, complete with the little circle on top.  I’ve often wondered where the circle came from,1 but surprisingly until starting this blog had not wondered if the angstrom was named after a person, especially since it does not follow the metric system prefixes.  Anders Jonas Ångström was born in Sweden in 1814 and studied physics at the University of Upsala.  He didn't leave except for brief sojourns to further science.  He was interested in astronomical work, and studied at the Stockholm Observatory before becoming the observer at the Upsala Observatory.  The Stockholm Academy of Sciences gave him the job of analyzing the magnetic data obtained by the “Eugéne,” a ship which had travelled around the world from 1851-1853.  In 1858 he became chair of the department of physics at Upsala University (I told you he didn’t leave).

Ångström is most known, when he is thought of at all, for his work in optics which led him to be considered one of the founders of spectroscopy.  His work was primarily with things that gave off light, such as electric sparks. His greatest work was with the solar spectrum, and it is from this work that his fame as the namesake of the ångström comes.  By studying the wavelengths of light emitted by the sun using diffraction gratings, he determined in 1862 that the sun’s atmosphere contained hydrogen.  In 1868 he published a book containing a map of the entire visible solar spectrum, consisting of 1000 lines: Recherches sur le spectre solaire.  Others doing similar work used arbitrary units, but Ångström used units of ten-billionths of a meter, or at least he thought he did.  He discovered that the meter from which he measured the gratings was too short, and thus all of his calculations were off.2  He began the work to correct it, but died in 1874, before it was finished, leaving his assistant Thalén to finish the job.  A contemporary said that his “work [was] characterized by such accuracy and completeness as to render it worthy of the highest admiration, to be regarded as a pattern to all investigators.”3  Although others had attempted to make such maps, Ångström’s was the most complete and accurate, and so those who came after him used his units to describe future measurements, calling them at first Ångström units and then ångströms.

Ångströms, however, are not the best units for describing visible light, since they result in numbers in the thousands, such as 6534.  The wavelengths Ångström described are commonly now referred to in nanometers, so the values are only in the hundreds.  Chemists, however, have a great affinity for the ångström, since it is the perfect unit for describing the length of chemical bonds, which are on the order of an ångström.  So, although the angstrom is no longer used in the field in which it originated and has been relegated to the status of a “non-SI unit,” it still finds its uses in chemistry although Ångström and his work have been long forgotten.

[1] Upon further investigation, the ring is not a diacritical mark, but an integral part of the letter in Sweden.
[2] Some idea of the complication of the creation and recalculation of this map can be gathered by looking at the exercises in Robert Alexander Houstoun's  A Treatise on Light, p. 254.
[3] Heinrich Schellen, Spectrum analysis in its application to terrestrial substances, and the physical constitution of the heavenly bodies (Longmans, 1872), p. 237.  The figure is also from this book.
As a reward for reading the footnotes, here is a link to an argument that Ångström had with a fellow physicist in the Philosophical Magazine: "Observations on Certain Lines of the Solar Spectrum".  Note that the fellow who argues against Ångström, Pierre Janssen, doesn't have a unit named after him, but they both gave their names to (separate) lunar craters, and only Janssen has a crater on Mars.

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