Measurements of the attenuation curves of low energy x-ray beams are used in this work as a functional description of such beams. Measurements of the transmission curves through aluminum made with a low volume ionization chamber have been used to predict transmission curves through copper. The results are compared with the data of attenuation in copper measured with the above-mentioned ionization chamber, and also with calculations of attenuation in copper made from reference x-ray spectra and tabulated values of the mass attenuation coefficients of copper.