Correspondence with Dr WF Giauque, University of California, Berkeley; Dr W Meissner, Physics Technical Institute, Berlin and Prof P Debye, Kaiser Wilhelm Institute of Physics, Berlin, Correspondence with Dr WF Giauque, on low temperature research work and Simon's visits to the Magnet Laboratory in Bellevue, near Paris, to carry out adiabatic demagnetization work, 1933-1937. Correspondence with Dr Walther Meissner, on Meissner's low temperature work, including a draft of a paper by Meissner, 'Erzeugung tiefster Temperaturen durch Kompression von flüssigem Helium (Production of low temperatures by compression of liquid helium)', 1926-1936. Correspondence with Prof P Debye, on low temperature work including a draft of a paper by Debye, 'Die magnetische Methode zur Erzeugung tiefster Temperaturen (The magnetic method for the production of low temperatures)', 1934-1938. In 1926 Debye and Giauque had proposed the magnetic cooling method to reach temperatures down to 0.001 °K 1926-1938