Degaussing: Most current PCs use cathode ray tube monitors that accelerate electrons through a vacuum tube toward a phosphor coated screen. The screen fluoresces when the electrons strike it. The electron beams are deflected in a precise manner by electric fields generated in a large coil called a deflection yoke. However, electron beams are sensitive to magnetic fields as well as electric fields.

Magnetic fields strong enough to affect monitors can be generated by motors, wiring, other monitors, or the Earth itself. Oscillating fields varying at the same rate as the monitor scan or a multiple thereof cause jitter or wavy lines and are more noticeable than static fields.

Static fields or varying fields that abruptly terminate can permanently magnetize metallic components near the CRT causing permanent picture problems

Degaussing at power on is used in many monitors to remove inadvertent magnetization of metallic parts. Degaussing places a strong alternating magnetic field on the CRT components. The field slowly sinks to zero. This randomizes the spin orientation of atoms and thus demagnetizes the materials. Manual degaussers generally use the wall power frequency. The drill is to wave the degausser slowly around the CRT then slowly back off a couple of meters. The point is to randomly magnetize any components of the monitor that have become magnetized.

The Earth has a large, but not especially strong, magnetic field centered on magnetic poles centered in the Canadian Arctic and in Antarctica near Commonwealth Bay. Technically, the strength of the Earth's field probably should be reported in Oersted, But "Gauss" (usually the same number although it measures something slightly different) or nanoTesla are often used instead. Both the orientation and strength of the Earth's field suffer from local anomalies of tens of percent over distances as short as a few tens of kilometers. The strength of today's Earth's field varies from .25 to .65 Oersted. The field orientation varies considerably becoming mostly downward in the high latitudes.

The Earth's magnetic field is strong enough to move images around a CRT a bit -- typically a few mm. The amount of shift is a function of accelerating voltages and length of the CRT electron path. Since the entire image shifts the same small distance, this is not generally a problem except in rare situations such as a CRT with a fixed, touch-screen overlay. Rarely, the field may bend the electron beam enough that color phosphors struck through a shadow mask or aperture grill will overlap onto adjacent "dots" of a different color. This is essentially a color purity problem and can usually be corrected with the purity control.

It is claimed that monitors may be designed specifically for Southern, Northern and Equatorial markets, and that military and exploration CRTs may use active magnetic field compensation instead of degaussing. At least one monitor -- the IBM 8511 -- does list different models for the three markets. It is not clear that this is really necessary. It would appear that unless a compass is built in, only vertical components of the Earth's field can be factory compensated for since the shift due to horizontal components will vary with the direction the monitor is facing.

Return To Index Copyright 1994-2008 by Donald Kenney.