Space-charge electrical conductivity data for 0.1% tritium in deuterium gas is compared to the predictions made by the J. J. Thomson theory of 1899. While this theory is in no way competitive with a full computer model, it is simple and physically illustrates the problem. Thomson’s theory fails at high potentials where the electric charges are swept out. At the lowest potential of 0.1 V, the recombination coefficient from Thomson’s theory agrees with the correct value given by the computer model’s fit of the data.