After completing his PhD in 1933, Ruska continued to work in the field of electron optics, first at Fernseh Ltd in Berlin-Zehlendorf, and then from 1937 at Siemens-Reiniger-Werke AG. At Siemens, he was involved in developing the first commercially produced electron microscope in 1939. As well as developing the Technology of electron microscopy while at Siemens, Ruska also worked at other scientific institutions, and encouraged Siemens to set up a laboratory for visiting researchers, which was initially headed by Ruska's brother Helmut, a medical Doctor who developed the use of the electron microscope for medical and biological applications.