To study the motion of electrons in an electric field perpendicular to their direction of motion, you use a so-called electron deflection tube. In the electron deflection tube, an electron gun
first generates a fine electron beam. This then enters the E-field of a plate capacitor, which is perpendicular to the direction of electron motion. There, the electrons are then deflected. The path of the electrons in the E-field of the plate capacitor is made visible to the eye with the aid of a fluorescent screen.