A cathode ray tube has an electron gun. The cathode is an electrode (a metal that can send out electrons when heated). The cathode is inside a glass tube. Also inside the glass tube is an anode that attracts electrons. This is used to pull the electrons toward the front of the glass tube, so the electrons shoot out in one direction, making a cathode ray. To better control the direction of the ray, the air is taken out of the tube, making a vacuum.