This sub-series consists of 13 files relating to Edward Appleton's research on recombination and attachment, dated c.1930-1962,including ‘β -determinations' and 'negative ions'. This material spans a considerable period of time. The earlier work, dated c. 1930-1938, conducted at King's College, London, and at Cambridge, was originally contained in two folders labelled by Appleton, 'Recombination and Attachment' and 'Radio Expt. [Experimental] Recombination'. These folders are preserved at Coll-37/C.288 and C. 289 respectively, but the material has been reassigned as a sequence of smaller items. Appleton's folder labelled 'Recombination Theory' contained correspondence from the 1950s and is preserved at Coll-37/C. 297. Items Coll-37/C.298-C.300 are Appleton's folders, all labelled β -Determinations (β = coefficient of recombination), and containing miscellaneous research material continuing to c.1962.