Correspondence with Edward Appleton, dated 1952 and 1958. It relates to Appleton's research at Edinburgh on recombination and negative ions. The material consists of correspondence from D.R. Bates, including research notes; K.G. Emeleus; H.S.W. Massey; W.R. Piggott; J.A. Ratcliffe; J. Sayers, including a research communication; and one letter from K. Bibi, dated 1958. These items are in Appleton's original folder labelled 'Recombination. Theory'.

The material consists of notes and research ideas by Edward Appleton, mostly from the 1930s. It includes collaborative papers by Appleton and James Sayers 'Recombination in the ionosphere', dated 1938, and by Appleton and R. Naismith, 'Solar radiation changes during the sunspot cycle', also dated 1938.