It's working is based on the detail that if a beam of electron is subjected to the crossed electric field   and magnetic field , it experiences a force because of each field. In case the forces on the electrons in the electron beam because of these fields are equal and reverse, the beam acts undeflected.

C = Cathode, A = Anode, F = Filament, LT = Battery to heat the filament, V = potential difference to accelerate the electrons, SS' = ZnS coated screen, XY = metallic plates (Electric field produced among them)