Selecton

Meet Selecton, an ion who sails straight through a tunnel that bends all of his friends.

In a velocity selector, an electric field and a magnetic field cross at right angles. The electric force on a charge is qE and does not depend on speed. The magnetic force is qvB, and it grows with speed. The two forces point in opposite directions.

Only ions with exactly the right speed feel balanced forces and fly straight. Setting qE = qvB gives v = E/B. With E = 1.0 × 10⁵ V/m and B = 0.10 T, only ions at 1.0 × 10⁶ m/s get through the slit at the end.

Slower ions are pushed by the electric force, and faster ones by the magnetic force, so both hit the plates. The rule works whatever the charge or mass.

Mass spectrometers often use a selector first. Once every ion has the same speed, a magnetic field can sort them neatly by mass.