With the power supply unplugged, connect the terminal posts to the two ends of the Crooke's tube with the cables. The post on the right should go to the cathode (which is inexplicably marked with a "+"). Plug in the supply and turn it on. The switch is at the end of one of the cords coming out of the supply.
This works best with the lights dimmed. If you are lecturing in 101 or 103 Churchill, be warned that it takes about 15 minutes for the room lights to come back on after they are turned off.
The electron beam makes a nice green stripe where it strikes the phosphor plate. Now take the horseshoe magnet and deflect the beam. Show that reversing the direction of the magnetic field reverses the direction of deflection. Show that the direction of the force is perpendicular to both the magnetic field direction and the direction of motion of the electrons. If you are brave, you can use a compass to find the polarity of the magnetic field and verify the right-hand rule. Don't forget that electrons are negatively charged.
Applications: Some oscilloscopes and other CRT's actually use magnetic coils to deflect the beam rather than electric plates. Circular accelerators used in particle physics (cyclotrons, betatrons, synchrotrons) all use exactly the same deflection method to keep the particle beams moving in a circle. Particle spectrometers also use the curvature in a magnetic field to determine the momentum of particles. Mass spectrometers use this magnetic curvature separate different type of particles (molecules, atoms, isotopes) with the same charge and kinetic energy but different masses.