Absolute Values of X-Ray Wave-lengths and the Fundamental Atomic Constants

Abstract

THE wave-lengths of the X-ray spectra are now known with a considerable degree of accuracy (in favourable cases to at least 1 in 100,000). But this means only that the ratio between the wave-lengths and a certain distance within a crystal lattice (for example, the distance between the atomic layers parallel to the cleavage faces of calcite) is measurable with this accuracy. The computation of the dimensions of the crystal lattice from the data involved (electronic charge, density of the crystal, and so on) give the absolute value of the atomic distance with only moderate accuracy (say 3 in 1000). It has therefore been necessary to fix an arbitrary value for the atomic distance in question and thereby for the scale of the X-ray wave-lengths, the X units. The agreement between this scale and the C.G.S. units is for this reason ascertained only within the last-mentioned limit. On the other hand, if we were able to fix the scale of the X units in centimetres, this gives us a possibility of determining in absolute values the atomic distances and, indirectly, the other fundamental constants (for example, the electronic charge).