 At Berlin , Franck attended lectures by Max Planck and Emil Warburg . On 28 July 1904 he saved a pair of children from drowning in the Spree River . For his Doctor of Philosophy ( Dir . Phil . ) under Warburg 's supervision , Warburg suggested that he study corona discharges . Franck found this topic too complex , so he changed the focus of his thesis . Entitled Über der <unk> <unk> der <unk> ( " On the Mobility of Ions " ) , it would subsequently be published in the Annalen der Physik . 
