Velocity Selector: crossed electric and magnetic fields

Positive charges enter from the left. The electric field points upward and the magnetic field points out of the screen, so the electric force is upward while the magnetic force is downward. Only particles with v = E / B pass through the field region without bending.

slower beam
selected beam
faster beam
crossed-field region
detector screen

Fields

Beam setup

In the selected beam the two forces cancel inside the plates, so the trajectory stays straight. Slower particles feel too much electric deflection, while faster particles feel too much magnetic deflection.