Monitors are very prone to interference from magnetic fields, even the
earth's natural magnetic field. The earth's magnetic field is always
changing, thus it has a constant influence on the monitor. In order to
reduce these effects every monitor has a degauss circuit that helps keep
the colors pure and the geometry correct.
There is a coil of wire that goes around the picture tube just under the
plastic near the front of the monitor. This is the degauss coil. When
the monitor is turned on it will automatically degauss itself by sending
a large current though this coil of wire. The degaussing circuit uses a
thermistor to taper off the degaussing current over a period of a few
seconds. Tapering the current off prevents the degaussing process from
actually creating a residual magnetic field in the metal parts of your
monitor instead of eliminating them.