Theoretical predecessors: Watson, Larmor and waveguide theory

Even before Schumann, two British researchers laid the mathematical foundations for describing the propagation of electromagnetic waves around the globe. G. N. Watson in 1918 worked out the mathematics of wave propagation around a conductive sphere (the Earth), and J. Larmor contributed to the theory of wave propagation in a space bounded by conductive layers.

Their work created the theoretical framework for the concept of the Earth-ionosphere waveguide — a space in which electromagnetic waves can propagate and reflect between the conductive Earth's surface and the conductive ionosphere. This waveguide concept is a necessary precondition for the formation of resonances: only in a well-defined cavity with conductive walls can standing waves, and therefore resonance modes, arise. Schumann (#9) built on these foundations when he derived the specific resonance condition for the Earth-ionosphere cavity.