The need for a separate unit of length for x-ray wavelengths is discussed and compared to the case of electrical units. The introduction and development of the x unit is reviewed. It was effectively defined in terms of calcite crystals and was intended to be very nearly equal to 10−11cm. The numerical value used to specify it was based on Siegbahn's value of the grating constant of calcite measured relative to rock salt. The latter value had been computed by Moseley from the density of rock salt and from Avogadro's number, which in turn rested on the erroneous Millikan oil-drop value of electronic charge. Ruled grating measurements of x rays eventually revealed the discrepancy. Variations observed among calcite crystals indicated the desirability of basing the definition on a standard wavelength rather than a species of crystal. For these reasons, Bearden, while preparing a new x-ray wavelength table, defined a new Å unit in terms of the W Kα1 wavelength w kα1  0.209 010 0 Å). This scale eliminates the major problems associated with the old x unit. The numerical factor was chosen to make the experimental value of the conversion factor to absolute units (angstroms) unity on the basis of the data then available; the estimated probable error was given as 5 ppm.

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