Created by Sir William Crookes during the late nineteenth century, this sealed glass tube was used to demonstrate the path travelled by cathode rays. When an electric current is applied to the tube, a patch of fluorescent light would appear on the walls of the tube due to the interaction of electrons with residual gas in the tube.

Crookes used a Maltese Cross-shaped electrode placed near the end of the tube to prove that this invisible radiation did indeed travel in straight lines. This tutorial can be operated by adjusting the Voltage slider bar to vary the electrical current within the tube. As the current level is increased, the electrons begin to ionize gases trapped within the tube causing them to begin glowing with a fluorescent blue color. As the ionizing electrons pass over the cross, a shadow appears on the one end of the vacuum tube.