 With his thesis completed , Franck had to perform his deferred military service . He was called up on 1 October 1906 and joined the 1st Telegraph Battalion . He suffered a minor horse riding accident in December and was discharged as unfit for duty . He took up an assistantship at the Physikalische Verein in Frankfurt in 1907 , but did not enjoy it , and soon returned to Frederick William University . At a concert Franck met Ingrid Josephson , a Swedish pianist . They were married in a Swedish ceremony in Göteborg on 23 December 1907 . They had two daughters , Dagmar ( Daggie ) , who was born in 1909 , and Elisabeth ( Lisa ) , who was born in 1912 . 
