High-vacuum electron tube with high-accuracy electron gun and ruled target phosphorescent screen. Determine the fundamental electron charge-to-mass ratio (e/m), study mean electron velocities using field cancellation, and explore electron optics using this tube. The electron beam trajectory can be manipulated using two methods: 1) deflection induced by the electric field of the built-in plate capacitor, or 2) magnetically using external Helmholtz Coils S (see Recommended).