MAGNETS AND MAGNETIC FIELDS
The theory of magnetic fields indicates that an object is magnetized when part or all of its atomic elements have their north and south poles aligned. Earth’s magnetic field is produced by the rotation and movement of the planet’s molten iron core. This produces a magnetic field that is relatively stable in direction.
Because of this stable magnetic field, early explorers could reliably use compasses built with lodestone, charged iron needles and other naturally magnetic materials. The Earth itself can be considered a bar magnet because of its two poles. Quite frequently, the magnetic field surrounding the Earth strongly magnetizes large ferromagnetic objects that lie aligned with the Earth’s poles for periods of time.
The directions of the Earth’s magnetic field may be changed by major geologic events over many thousands of years. There are many local magnetic anomalies having higher levels of magnetic attraction near the Earth’s surface. It is thought that these may be caused by iron or nickel deposits.
Magnetic Fields
A magnetic field exists within and around a permanent magnet or within and around a conductor carrying an electric current. The magnetic field surrounding a permanent bar magnet has polarity, but the magnetic field surrounding a conductor does not. Some types of magnetic fields may be contained entirely within a ferromagnetic object. A magnetic field surrounding a bar magnet is seen in Figure 1.1.
The theory of magnetism and magnetic particle testing is based on a material property that has its atomic domains oriented. This material is called a magnet and will attract iron, nickel and cobalt, as well as many alloys. Magnets may be permanent (retaining their magnetism more or less permanently) or temporary (retaining their magnetism only as long as a magnetizing force is being applied).
The ability of the magnet to attract or repel iron is not uniform over its surface, but is concentrated at local areas called poles. Each magnet has at least two opposite poles, much like the Earth’s magnetic poles; hence, the poles are respectively called the north and south poles. The attraction and repulsion of poles are illustrated in Figure 1.2.
Because magnetization of certain metals is possible, small particles that are attracted to magnetic fields are applied to the surface of the test object after or during induction of a magnetic field, thereby allowing the detection of certain specific discontinuities that are present in the material. Magnetic particle
testing is capable of revealing discontinuities (imperfections that interfere with the usefulness of an article or exceeds acceptability limits established by applicable specifications) economically; it is one of the most widely used nondestructive test methods. However, this test method is limited to disclosing only those discontinuities
that are at, or very near, the surface and only in magnetizable materials.
Magnetic poles may be formed in ferromagnetic material by several means, including contact with other magnetic materials, passage of suitable electrical currents through the test object and certain types of vibrations or even single impacts. All of these forces act to align atomically small zones called domains in one direction.
This direction is arbitrarily termed north to south.