The electron gun fires a beam of electrons that spread out
as they travel across through the vacuum towards the right hand side of the tube. In the path
of the beam there is a metal cross. The electrons that hit the cross are stopped by the metal,
but those that get past it hit a fluorescent screen at the far side of the tube which glows green
when the electrons collide with it (See Figure 2).
A sharp silhouette of the cross can be seen on the screen
and this shows that the electrons travel in straight lines.
The anode voltage is a few
thousand volts and the same voltage is also connected to the maltese cross. If this voltage is
increased the electrons gain more energy and so are moving faster when they hit the screen.
This makes it glow more brightly. Turning up the heater voltage also makes the screen
brighter because the electron gun is producing more electrons due to the hotter
heater.
If you touch the front of the screen the silhouette will move because you are
earthing it. Electrons that had collected on the glass flow through you to the
ground.
Holding a magnet near the tube (especially near the electron gun or near
the silhouette of the cross) will distort the picture showing that electron beams are affected
by magnetic fields.