The development of the experimental technique of radio measurements of upper atmospheric ionization during the past few years has greatly enhanced our knowledge of this region of the Earth's atmosphere. This region is now commonly termed the ionosphere. The radio methods of observation are based upon the phenomenon of refraction of electromagnetic waves in a medium containing free charges. The index of refraction of such a medium to the electromagnetic wave is dependent upon the number of charges in the medium, their mass, and the frequency of the propagated wave. The medium is therefore dispersive, and the higher frequencies, which are less deflected, are more penetrating. If the angle of incidence at the medium and the total deflection of the wave is known, the numbers of charges of any given mass required to give this deviation can be calculated, and a numerical value of ionization can be thus obtained. This number is usually expressed in equivalent numbers of electrons, although it must be understood that if particles of greater mass are present, a greater number of such particles will give, in general, similar effects. If the motions of the particles in the Earth's magnetic field are considered, however, a limiting ratio of electrons to heavier particles can be determined. In this way it is also possible to ascertain information concerning the strength of the Earth's magnetic field in the ionized region.