On the basis of the earlier measurements of Hess and Kolhoerster, H. Benndorf ~ considered the effect of the cosmic radiation in producing electrical conductivity in the upper atmosphere. In his theory ions were produced at a rate proportional to the product of the cosmic-ray intensity by the density of the atmosphere and they disappeared by recombination, the coefficient for which was determined by the theory of J. J. Thomson. 2 Benndorf concluded that a conductivity Io ~ยฐ times that at the Earth's surface could be produced by the cosmic radiation at Ioo km. The increase was due to a hundred-fold increase in the cosmic-ray intensity, accounting for a tenfold increase in the number of free ions and a tog-fold increase in ionic mobility.
In recent times cosmic-ray intensities have been measured at elevations greater than those reached by Kolhoerster and considerable evidence has been gained regarding the production of secondaries, giving some rational basis for extrapolation of the ionizing effects beyond the levels attained in balloon-flights. Knowledge of the actual state of ionization in the higher layers has also been greatly enriched by the investigations of radio-wave reflections, following the methods introduced by M. A. Tuve and G.