Editing Degaussing (section)

==Color cathode-ray tubes==
{{unreferenced section|date=November 2014}}
Color [[CRT display]]s, the technology underlying many television and [[computer monitor|computer monitors]] before the early 2010s, require degaussing. Many CRT displays use a [[shadow mask]] (a perforated metal screen) near the front of the tube to ensure that each electron beam hits the corresponding [[phosphor#Color_television_CRTs|phosphors]] of the correct color. If this plate becomes magnetized (e.g. if someone sweeps a magnet on the screen or places loudspeakers nearby), it imparts an undesired deflection to the electron beams and the displayed image becomes distorted and discolored.

To minimize this, CRTs have a copper or aluminum coil wrapped around the front of the display, known as the degaussing coil. Monitors without an internal coil can be degaussed using an external handheld 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 circuit induces an [[oscillating]] [[magnetic field]] with a decreasing [[amplitude]] which leaves the shadow mask with a reduced residual magnetization.

[[Image:Degauss-in-progress at Dell-Trinitron-monitor.jpg|thumbnail|A degaussing in progress]]
Many televisions and monitors automatically degauss their picture tube when switched on, before an image is displayed. The high current surge that takes place during this automatic degauss is the cause of an audible "thunk", a loud hum or some clicking noises, which can be heard (and felt) when televisions and CRT computer monitors are switched on, due to the capacitors discharging and injecting current into the coil. 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 AC current surge to the degaussing coil is regulated by a simple [[positive temperature coefficient]] (PTC) [[thermistor]] device, which initially has a low resistance, allowing a high current, but quickly changes to a high resistance, allowing minimal current, due to self-heating of the thermistor.  Such devices are designed for a one-off transition from cold to hot at power up; "experimenting" with the degauss effect by repeatedly switching the device on and off may cause this component to fail.  The effect will also be weaker, since the PTC will not have had time to cool off.