The lines of force encvircling a permanent magnet or a moving charged element: The magnetic field is present in and around the Earth.

The intensity of the magnetic field at the Earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the north pole.

A "gauss" is equivalent to 1 maxwell per square centimeter, and a "Maxwell" is equivalent to the flux that produces one abvolt in a one-turn circuit when the flux is reduced to zero at a uniform rate in one second, while a "flux" is the electric or magnetic field lines of force that traverse a given cross-sectional area.

Alternation of the Earth's magnetic polarity or a reversal of the polarity of the Earth's magnetic field which has occurred in the past as shown by the remanent magnetization found in igneous and sedimentary rocks: The Earth's internal geomagnetic field reversal changes, on average, about every 300,000 to 1 million years.

This geomagnetic reversal is very sudden on a geologic time scale, apparently taking about 5,000 years.

The time between reversals is highly variable, sometimes less than 40,000 years and at other times as long as 35 million years and no regularities or period times have been discovered so far.

A long interval of one polarity may be followed by a short interval of an opposite polarity.