DC and Quasi DC Fields



Earth's magnetic field
DC or "static" lines of magnetic flux naturally emanate from the earth and provide a compass with the ability to indicate "Magnetic North". The "field strength" of the Earth's magnetic field varies from place to place on the globe, but can be thought of as approximately 350 - 500 milliGauss (mG). Through the characteristic of permeability, all magnetic fields, including the Earth's prefer to accumulate in iron or steel rather than air. Air has a permeability of 1, while steel has a permeability of approximately 1,000. Much as a conductive rod will attract an electric field, (e.g., "lightning"), DC magnetic lines of flux will accumulate inside and near the steel structure of a building and field levels will be elevated, perhaps to 1,000 mG or more.

MRI Instrument
As a measurement instrument is moved away from the concentration of fields in the steel structures, the field levels will be lower, closer to 300 mG. Elevated levels of DC magnetic fields can also occur in a building as a result of steel members in the building becoming magnetized. Welding during construction can cause magnetization or a high DC magnetic field source like an MRI instrument can permanently magnetize the steel around it. Areas of a building with magnetized steel can have DC magnetic field levels in the range of 2,000 mG or more.

Under certain circumstances, static DC fields can be made to act like an AC field. Since magnetic fields prefer to travel inside of iron or steel, a large mass of steel (an elevator, a train, automobile or a truck) moving through a DC field will distort the lines of flux temporarily. When the mass of iron is outside of the influence of the flux lines, they will return to their original position. This movement will resemble, technically and in effect, a slowly oscillating AC field. These temporary moving DC fields are referred to as "quasi" or "time varying" DC fields.

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