This paper examines the development of the magneto-ionic theory for radio-wave propagation in the context of wireless engineering and atmospheric science during the interwar period. Among physicists and engineers, why radio waves can propagate over long distances was a curious phenomenon arising from the new radio technology. In the 1910s, William Henry Eccles at the University of London and Joseph Larmor at Cambridge University suggested that long-range radio waves bounce between the earth and an ionic layer in the upper atmosphere and the apparent wave reflection from the upper layer is actually an effect of ionic refraction. Throughout the 1920s, the explanatory power of this theory expanded significantly: Albert Hoyt Taylor and E.O. Hulburt at the U.S. Naval Research Laboratory used it to deduce the notable “skip effect” in short-wave communication. In addition, Edward Appleton and a few other researchers sponsored by the British Radio Research Board proposed that double ionic refraction due to geomagnetism could account for certain data on the polarization and range of propagating waves.

Dubbed as the magneto-ionic theory, this mathematical model nonetheless played a broader role than an explanatory device in the 1920s-30s, thanks to the introduction of a new experimental technique. In 1924-26, Appleton and Miles Barnett at Cavendish Laboratory and Gregory Breit and Merle Tuve at Carnegie Institution of Washington independently produced “direct” evidence for the upper ionic layer (the ionosphere) as they sent specially modulated radio signals upward and detected their return from the sky. This “radio sounding” scheme soon became the most important method to probe the characteristics of the ionosphere. With the efforts of Appleton and a few other British and American researchers, moreover, the magneto-ionic theory was integrated into the operation of the radio ionospheric sounder and the interpretation of its data and thus served as a “paper tool” for the active remote sensing of atmospheric science.

In the 1930s, Appleton’s group in the U.K. and the U.S. National Bureau of Standards further extended the use of the magneto-ionic theory to short-wave radio engineering, as they came up with means of predicting optimum operating frequencies from the theory’s interpretation of ionospheric sounding data. The scientific work inspired by technological issues was reapplied to the technology. Like other papers in this session, the changing roles of the magneto-ionic theory thus witness the close interaction between physics and technology in the interwar years.