This paper provides new elements for reconstructing Heinrich Hertz's conversion from an electrodynamic concept to a concept of field theory, a process that took place between September 1887 and February 1888. First of all, it is argued that, contrary to what one could deduce from Hertz's own presentation in his Untersuchungen (or Electric Waves), paper N∘ 5 was actually written after the first publication of papers N∘ 6 and N∘ 7; this fact is illuminated by Hertz's unpublished correspondence (Section 1). (...) Following this reordering, an approximative process of Hertz's conversion based only on his published papers is described (Section 2). Second, by complementing Hertz's published Erinnerungen (or Memoirs, Letters, Diaries) with his unpublished Versuchsprotokolle (Laboratory notes), a much finer description of this conversion process is discussed (Section 3). It is finally concluded that Hertz's idea of “air waves” belongs at the very end of this process, and was mainly attained through experimental hints found within Helmholtz's theoretical framework. (shrink)