a mass spectrograph combining one or more magnetic deflections with the action of electrostatic fields , characterized by the fact that the system is circularly symmetrical ; by the fact that the ions are injected radially from a source constituted by a heated circular filament and a pair of small ring - shaped magnets which confine the ionizing electrons to the interior or exterior periphery of the analyzer system per se ; by the fact that the first pair of deflection magnets of the analyzer system is ring shaped ; and by the fact that the selective action of the other fields , in particular the electrostatic fields , is due to the prior dispersion obtained by the first magnetic induction .