Using a convex crystal spectrometer, a numerical method was elaborated
which allows one to determine the linear size of an x-ray source from
the measure of spectral linewidths. This method was applied to the de
termination of the size of a very brief spark, produced by a high-volt
age discharge near tungsten and iron tips, It is shown that, under the
experimental conditions, the broadening geometric effect related to t
he size predominates over all the other broadening effects.