[en] The electron density (n/sub e/) and temperature (T/sub e/) of the plasma channel created by the propagation of a relativistic electron beam in air have been measured by a ruby laser Thomson scattering diagnostic. The measurements were made at the MIMI electron beam accelerator (1.6 MV, 21 kA, 70 ns) at various times during the plasma channel development, with 25 ns temporal resolution and 2 mm radial resolution. For example, in 5 Torr air, at the time of maximum electron beam current, the results are n/sub e/ = 1.86 x 10¹⁵ cm^-3 (+- 12%), T/sub e/ = 4.24 eV (+- 20%). These results, as well as those with other timing, are in good agreement with the theoretical results of the Air Propagation Code: n/sub e/ = 1.65 x 10¹⁵ cm^-3, T/sub e/ = 2.59 eV. Signal-to-noise is very good (10:1), limited by x-ray fluorescence of the fiber optics at the spectrometer. In fielding the diagnostic on higher energy accelerators, however, the dominant noise is expected to be the background light from the plasma and hot gas, or the fluorescence of the collecting optics. Improvements to the diagnostic and an ongoing experiment in 80 Torr of air, 4 MV, 50 kA to address these issues is presented