A parallel-plate vacuum chamber is described and the theory of its use as an electron energy spectrometer is given. The spectra of low-energy (up to about 10 eV) electrons both liberated within and emitted from sheets of copper, aluminium and graphite irradiated by 250 kV X rays, are derived under various assumptions of angular distribution. The total emission of low-energy electrons is found to be 1·63, 1·74 and 1·28 coulombs per rad cm2 for copper, aluminium and graphite respectively, when irradiated by X rays having energies, based on HVTs in copper, ranging from 60 to 156 keV. Comparison is made with values of energy dissipation density derived from secondary electronemission studies. An explanation is advanced to account for the negative slope of the current-voltage characteristic of the spectrometer and similar devices at high bias voltages.