He received a doctorate in physics from the University of Berlin, having studied under Max Planck, Walther Nernst, Erwin Schrödinger, and Albert Einstein. Frederick Lindemann visited Berlin in 1930 to buy an improved hydrogen liquefier designed by Francis Simon. Mendelssohn was given the role of demonstrating the equipment. During these trials Lindemann tried to recruit Mendelssohn to join him in Oxford University. However, as Mendelssohn had just accepted a job in Breslau, he felt obliged to refuse. He was joining his cousin, Franci Simon, who had appointed Professor of Physical Chemistry at the University of Breslau. Nevertheless, Lindemann contacted him again in 1932 inviting him to the Clarendon Laboratory in Oxford to install a helium liquifier. This he did, and by the time he returned to Breslau in January 1933, he had a grant application to the Rockefeller Foundation to allow him to join the Clarendon Laboratory. In the end both he and Simon ended up receiving funding from Imperial Chemical Industries for research work at Oxford. Whilst the rising tide of German antisemitism provided the context for his reasons for moving, he was to discover that Lindemann's rivalry with Lord Rutherford and the Cavendish Laboratory, based at the University of Cambridge was influential in England.