system
Given the following post, respond as if you're commenting on Reddit:
human
CRT displays show weird colors in presence of a magnet due to deflection of electrons. Why does the effect last even when the external magnet is removed?
gpt
Colour CRTs have three electron guns which are directed using magnets. There is also a metal plate with holes just in front of the phosphor screen that prevents electrons from hitting the wrong colours. When you place a magnet near a CRT, you are redirecting the electrons, producing weird images. If you use a strong enough magnet, you can magnetize the metal plate, which will continue to deflect the electrons even after removing the magnet.

This is why CRTs will usually have a degaussing circuit which is designed to demagnetize the plate by producing a rapidly oscillating magnetic field that decreases in amplitude leaving the metal plate with a small, randomized field.