Magnets are drawn along the magnetic field gradient. Most are electrostatic or planar magnetic designs. There are several types of electrical currents used in magnetic particle inspection. Corrosion-detecting pigs use either magnetic or ultrasonic sensors. Eddy currents are electric currents that occur in conductors exposed to changing magnetic fields. Eddy currents flow in closed loops in planes perpendicular to the magnetic field. There are four basic coupling mechanisms&colon; conductive, capacitive, magnetic or inductive, and radiative. This process results in parallel sections of oceanic crust of alternating magnetic polarity. Sunspots usually appear in pairs of opposite magnetic polarity. Magnetic and gravitational forces are generally negligible. The information is read optically and written by combining magnetic and optical methods. H is related to the magnetic dipole density. I fitted with degaussing coils to trigger magnetic mines. Magnetometers are devices used to measure the local magnetic field. Governments sometimes operate units that specialize in measurement of the Earth's magnetic field.