Description of a velocity analyzer (with diagrams) is given. It is a device for measuring the velocities of charged particles which comprises pairs of spaced electrodes, a collector for charged particles, and means for propelling charged particles in a direction so as to pass between the pairs of electrodes. The pairs of electrodes and the collector are so arranged to permit passage of the charged particles between them to the collector plate. Means are provided for applying an alternating substantially square wave potential of known frequency to the pairs of electrodes so that adjacent pairs are opposite in phase and so that one half-cycle of the wave appears as zero potential between the pairs of electrodes.