Magnetism

I

Introduction

Magnetism, one of the fundamental forces of nature. Although familiar to the ancients, the true nature of magnetism has only been understood in the 20th century, and modern views of it are discussed later in this article.

II

Elementary Study

Ordinary magnets are made of iron or steel, and can attract iron or steel. Magnets made of steel are “permanent” magnets in that they can retain their magnetism for a long time; magnets made of soft iron can easily lose their magnetism, and are called “temporary” magnets, but this makes them particularly useful in electromagnets. The two ends of a magnet are called its poles, and are referred to as north and south; when freely suspended, the north pole of the magnet turns to point to magnetic north, which is close to, but not exactly the same as, true north. If two magnets are close to one another, their “like” poles (north-north, etc.) repel one another, and their 'unlike' poles (north-south) attract one another.

A piece of iron or steel can become magnetized in three ways: by stroking it gently, from end to end, with one pole of an existing magnet; by tapping gently while it is lying along a magnetic field (this is how screwdrivers and other tools can become accidentally magnetized); and by placing it inside a long cylindrical coil of many turns of wire (a solenoid), and passing a strong direct current through the coil for a short period. A magnet can become demagnetized by treating it roughly, causing it to receive a lot of knocks, while it is at random directions in relation to a magnetic field, such as the Earth's magnetic field. Placing it in a solenoid with a strong alternating current through it, and slowly reducing the current to zero, or slowly removing the magnet from the coil, can also demagnetize it.

Magnets behave as if they were themselves made up of many tiny magnets, referred to as domains. When these domains are randomly oriented, the metal is not magnetized, but when the domains are lined up along the length of the piece of metal, it becomes a magnet.

The shape of the magnetic field round a magnet can be established by sprinkling iron filings on to a piece of paper placed on top of the magnet. Upon tapping it gently, the filings line themselves up along the 'lines of force' of the field (also known as magnetic flux lines). A more accurate way, which establishes the direction of the field as well as its shape, is by using a plotting compass, itself a small, magnetized needle, free to rotate in a small plastic case with a glass top.

Magnetic fields are also produced by electric currents flowing through wires, the simplest example being the circular field produced by a current flowing in a straight wire (Corkscrew Rule).

III

Further Study

Magnetism can also be regarded as an aspect of electromagnetism. Magnetic forces are produced by the motion of charged particles such as electrons, indicating the close relationship between electricity and magnetism. The unifying frame for these two forces is called electromagnetic theory (see Electromagnetic Radiation). The most familiar evidence of magnetism is the attractive or repulsive force observed to act between magnetic materials such as iron. More subtle effects of magnetism, however, are found in all matter. In recent times, these effects have provided important clues to the atomic structure of matter. Magnetic fields not only influence magnetic materials, but also charged particles in motion. Generally, when a charged particle moves through a magnetic field, it feels a force that is at right angles both to its own velocity and to the direction of the field. Since the force is always perpendicular to the velocity, a particle in a magnetic field moves in a curved path. Magnetic fields are used to control the paths of charged particles in devices such as particle accelerators and mass spectrometers.

IV

History of Study

The phenomenon of magnetism has been known since ancient times. The mineral lodestone (see Magnetite), an oxide of iron that has the property of attracting iron objects, was known to the ancient Greeks, Romans, and Chinese. When a piece of iron is stroked with lodestone, the iron itself acquires the same ability to attract other pieces of iron. The magnets thus produced are polarized, that is, each has two sides or ends called north-seeking and south-seeking poles. Like poles repel one another, and unlike poles attract.

The compass was first used for navigation in the West some time after AD 1200. In the 13th century, important investigations of magnets were made by the French scholar, Petrus Peregrinus. His discoveries stood for nearly 300 years, until the English physicist and doctor William Gilbert published his book Of Magnets, Magnetic Bodies, and the Great Magnet of the Earth in 1600. Gilbert applied scientific methods to the study of electricity and magnetism. He pointed out that the Earth itself behaves like a giant magnet, and through a series of experiments, he investigated and disproved several incorrect notions about magnetism that were accepted at the time. Subsequently, in 1750, the English geologist John Michell invented a balance that he used in the study of magnetic forces. He showed that the attraction or repulsion between two magnetic poles decreases as the square of the distance between them increases. The French physicist Charles Augustin de Coulomb, who had measured the forces between electric charges, later verified Michell's observation with high precision.