IN order to determine the ratio between the X-unit and the absolute unit of length, I have registered certain X-ray lines in high orders with a concave glass grating (R = 5 m.) and determined their wavelengths by comparing them with known spark lines in the first order, registered on the same plate1. The X-ray line which turned out to be most suitable for such relative measurements was the aluminium K12 line, which has been determined very accurately by Larsson2 with a crystal grating. From nine different plates I have found the values given in the accompanying table. The value found by Larsson is Al K12 = 8322·48 X.U., or, corrected for the refraction in the crystal, 8321·35 X.U. The difference “ between the measured values and the crystal determination is given in the second column. For every value found for the Al K12 line I have computed the corresponding value for the electronic charge e.
See also Siegbahn and Söderman, NATURE, 129, 21, Jan. 2, 1932.
Larsson, Diss. Uppsala, Univ. Årsskr., 1929.
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