By allowing the positive ions to pass through an electric field and thus giving them a certain velocity, it is possible to distinguish them from the neutral, stationary atoms.

Die elektrischen Quanten. Career
Stark worked in various positions at the Physics Institute of his alma mater until 1900 when he became an unsalaried lecturer at the University of Göttingen. (Sonderabdruck aus 'Annalen der Physik' 4.