Electron motion in the magnetic field of Helmholtz coils

To study the motion of electrons in a homogeneous magnetic field, you usually use a so-called filament beam tube, which you place inside a large pair of Helmholtz coils. Inside the cathode ray tube, an electron gun generates a fine electron beam with variable velocity. The direction of electron motion is perpendicular to the B-field of the Helmholtz coils. To make the electron paths visible, the evacuated glass tube contains, for example, some neon gas. This is excited to glow by the electrons and thus makes the trjectory of the electrons visible.