Experiments are described, the aim of which is the determination of the energy spent in producing one pair of ions when electrons of velocity corresponding to 50–270 volts are totally absorbed in air. It is found that this energy varies with the initial speed of the electron, but that it asymptotes to the value 37 ± 2 electron-volts when the energy of the electron is very great.

This value of the “volts per ion pair” is compared with those found by other experimenters. A correlation of the three electronic “constants,” the range of the electron, the ionization per unit path, and the total ionization is attempted. It is concluded that, unless in the case of very fast cathode rays or β-particles, the “ionization per unit path,” as measured by ionization chamber experiments, has no meaning.