During this particular stay in Stockholm, he met Hendrik A Kramers, who, at the time (1917), was a student of Niels Bohr in Copenhagen. Kramers and Klein met several times during the next few years both in Stockholm and in Copenhagen, which was to be Klein's next destination.
Sa panahon na ito partikular na manatili sa Stockholm, siya matugunan Hendrik A Kramers, na, sa panahon (1917), ay isang mag-aaral ng Niels Bohr sa Copenhagen. Kramers at Klein matugunan ng ilang beses sa panahon ng susunod na ilang taon na pareho sa Stockholm at sa Copenhagen, kung saan ay Klein ang susunod na destinasyon.