Absolute triply differential ( $e,2e$) cross section measurements are presented for H (for incident energies, ${E}_{0}$, of 15.6 and 17.6 eV) for equal energy sharing and the ${\ensuremath{\theta}}_{12}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\pi}$ configuration. Results of distorted partial wave calculations agree with the measurements; those of convergent close coupling calculations agree with the relative angular distributions, but are lower than experiment by factors of from 2 to 7. Relative experimental results for H for ${E}_{0}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}14.6\mathrm{eV}$ show a qualitative change in shape, which agrees with theory. Implications of the absolute experimental results for the range of validity of the Wannier threshold law are discussed.