This subseries consists of 28 files containing correspondence between Balthasar van der Pol and Edward Appleton. Van der Pol was the first of Appleton's close collaborators, their friendship beginning at the Cavendish laboratory in 1919. Later that year, van der Pol returned to the Netherlands to work in Haarlem under H. A. Lorentz; in 1922, after exploring the possibilities of posts at Delft and (via Appleton) at Cambridge, he accepted (June 1922) a position in the Research Department of N.V. Philips Gloeilampenfabrieken, Eindhoven. The correspondence is particularly intensive during the years 1921-1924, and it is primarily concerned with work on oscillations and non­linearity. From the surviving evidence, Appleton was the more assiduous correspondent, writing letters and postcards from his laboratory, his home and from various holiday addresses, sometimes more than once a day when a thought, or a second thought, struck him. Although the letters are almost wholly concerned with research problems, there are a few personal references to family matters, occasional visits, Cambridge affairs and personalities, etc. The correspondence tails off after about 1925 but resumes in a more desultory, though friendly, fashion after the Second World War. There was a somewhat sharp exchange in 1927 (Coll-37/E.132 and Coll-37/E.144) when Appleton thought that his proposed election to the Royal Society might be jeopardised by references to his work in an article published by Van der Pol, but all was well and the friendly tone of the correspondence resumed undiminished. This close exchange is an interesting example of the development of a collaborative research project conducted by correspondence before telephone communication and air travel became sufficiently cheap to be the accepted mode. It has been referred to in several published works and was the subject of a paper by F.L.H.M. Stumpers, 'Some notes on the correspondence between Sir Edward Appleton and Balthasar van der Pol',Philips Research Reports, Vol. 30, 1975, a copy of which is included in Coll-37/E.144. No doubt as a result of previous interest in the letters, some degree of sorting and numbering had already been carried out, which has imposed constraints on the present listing. Appleton’s and van der Pol's letters were kept in separate sequences and this division has been preserved. Appleton's letters down to October 1924 were numbered 1-110 in red pencil with additional documents sometimes added as, e.g., 6a, 7A, 7B, and some numbers, e.g., 3, 8, are used more than once. Some letters also have words ringed in the same red pencil, presumably because they were thought uncertain or illegible. The numbering stops after 110 (1924). It should be noted that Appleton often left letters undated or with a simple note such as ‘Sunday', and the red-pencil numbering must have had to rely on content alone; there are in consequence inaccuracies and omissions from the sequence. These have been noted where possible in the present listing. Van der Pol's letters are less numerous and only one undated set of notes occurs, which are included in Coll-37/E.144. Both sets are presented chronologically as follows: Coll-37/E.117-E.138 from Appleton to Van der Pol, dated 1920-1954 and Coll-37/E.139-E.144 from Van der Pol to Appleton, dated 1920-1948.