The pioneering experiments of Appleton and Barnett, which measured the height of reflexion of radio waves, is described, and an account given by Barnett is included. It is shown how the method of wavelength change was used in the late 1920s to investigate several ionospheric phenomena. When Appleton based his magnetionic theory on an earlier theory of Lorentz he decided not to include the 'Lorentz term', whereas in another investigation, Hartree decided to include it. Later Darwin showed that Appleton was right, and Hartree wrong. Attention is drawn to the inconsistency by which the expression derived by Appleton is usually called the 'Appleton-Hartree equation'.