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The ``ultimate'' electron density in the negative glow plasma of a cold‐cathode discharge in helium at gas pressures 3–12 mm Hg has been determined by simultaneous Langmuir probe and microwave transmission methods. A correlation is obtained at densities above ∼4×1011/cm3 when the influence of positive ion scattering of the electrons diffusing to the probe is considered. The ultimate electron temperature in the helium negative glow was measured by the metallic probe to be quite low, ∼400°K, and the ``secondary'' electron temperature was found to be ∼50 000°K. These probe indications are substantiated by measurement of the microwave noise power radiated from the plasma which is found to correspond to an effective temperature varying from 300 to ∼1000°K.