Degaussing is the process of decreasing or eliminating an unwanted magnetic field. It is named after Carl Friedrich Gauss, an early researcher in the field of magnetism. Due to magnetic hysteresis it is generally not possible to reduce a magnetic field completely to zero, so degaussing typically induces a very small "known" field referred to as bias.
Today the most common use of degaussing is in CRT-based TV sets and computer monitors. For example, many monitors use a metal plate near the front of the tube to focus the electron beams from the back. This plate, the shadow mask, can pick up strong external fields and from that point produce discoloration on the display.
To minimize this, CRTs have a copper coil wrapped around the front of the display, known as the degaussing coil. Tubes without an internal coil can be degaussed using an external hand held version. Internal degaussing coils in CRTs are generally much weaker than external degaussing coils, since a better degaussing coil takes up more space. A degauss causes a magnetic field inside the tube to oscillate rapidly, with decreasing amplitude. This leaves the shadow mask with a small and somewhat randomized field, removing the discoloration.
A degaussing in progress
Many televisions and monitors automatically degauss their picture tube when switched on, before an image has been displayed. The high current surge which takes place during this automatic degauss is the cause of an audible 'thunk' or loud hum which can be heard (and felt) when televisions and CRT computer monitors are switched on. Visually, this causes the image to shake dramatically for a short period of time. A degauss option is also usually available for manual selection in the operations menu in such appliances.
In most commercial equipment the current surge to the degauss coil is regulated by a simple PTC thermistor device which initially has a low resistance but quickly changes to a high resistance due to the heating effect of the current flow. Such devices are designed for a one-off transition from cold to hot at power up, so 'experimenting' with the degauss effect by repeatedly switching the device on and off is not recommended as it may cause this component to fail. The effect will also be weaker, since the PTC won't have had time to cool off