Abstract: A method and apparatus centers of an electron beam (3) in a surface coating installation with movable substrates. A rotating magnetic field is superimposed on a main magnetic field which serves to guide and focus the electron beam. A focal spot of the electron beam is thus caused to rotate about a theoretical axis (17) of the electron beam (3) and the beam sweeps the entire surface (32) of a raw material source (4). Changes in the consumption of reactive gas supplied to the installation are measured by a flowmeter (21). Through a control unit (20) connected to the flowmeter, stationary magnetic fields are superimposed on the rotating magnetic field and the electron beam (3) is aimed at the center of the surface (32) of the raw material source (4) as a function of variations in the reactive gas flow.