  Perhaps it was possible to have dual citizenship, or as you said, maybe he switched back and forth multiple times.  But he certainly did serve as directory of Physics Institute at the Kaiser Wilhelm Society for the Advancement of Sciences from 1914 to 1933.  If he had no alligiance to Germany all that time, indeed it would seem odd (perhaps even unethical) to accept such a high ranking position for nearly 20 years, and enjoy all the benefits.  He certainly developed the bulk of his scientific achievements there.   03:58 Apr 5, 2003 (UTC)