The filter employs perpendicular electric and magnetic fields. Particles enter the filter with a speed perpendicular to both fields, such that they will deviate from a linear trajectory if their speed does not match the ratio E/B. The charge-mass ratio increases the deviation of the particle, but is unaffected for the appropriate matching speed. Also, for particles of negative charge, the forces act in the opposite direction.

In this Demonstration, we show the plates creating the electric and magnetic fields and the vector field representing the direction and intensity of each field. The trajectory of the particles, shown as a blue curve, is determined by these conditions.