Copyright Goodheart-Willcox Co., Inc. 98 Electricity Copyright Goodheart-Willcox Co., Inc. History of Magnetism What is the mysterious force called magnetism? Scientists have proposed theories about the nature of magnetism and have learned how to use this force, but no one has ever seen it. You may have a horseshoe-shaped magnet at home and have discovered that it will pick up nails and other metal objects. If you have hiked, you may have used a compass and observed that it points in the general direction of north. The compass is a common application of magnetism. Magnets have been known of since antiquity. Early Greek scientists discovered that small pieces of iron ore attracted small pieces of iron. Because iron ore was found near Magnesia in Asia Minor (present day Turkey), it was named magnetite. The word magnetism is derived from magnetite. Lodestones Medieval Chinese sailors used small magnetite stones fastened to wooden sticks fl oating in a container of liquid. The stick, with stone attached, would turn in the direction of north, and enable the navigator to chart the ship’s direction. This device was the earliest compass the magnetite stones became known as “leading stones,” or lodestones. A material or metal that has the property of attracting metals such as iron and steel is known as a magnet. The attracted materials are called magnetic materials. All magnets have two poles: the pole that is attracted toward north is called the north pole the opposite pole is called the south pole. Between the north and south poles exists a magnetic fi eld consisting of many lines of magnetic force. This magnetic fi eld is frequently referred to as the magnetic fl ux, Figure 12-1. Permanent Magnets Permanent magnets are made in many shapes and sizes, Figure 12-1. They all have north and south poles and they all have magnetic fi elds. Materials used in the manufacture of such magnets have the ability to retain their magnetism over long periods of time. High-carbon steel and special alloys, such as alnico (a mixture of aluminum, nickel, and copper), exhibit this property. Low-carbon steel and iron do not retain magnetism, but they do conduct or concentrate a magnetic fi eld very well. They have a high permeability. Permeability is the characteristic of a material that conducts magnetic lines of force. How is a magnet made? According to the molecular theory of magnetism, in an unmagnetized steel bar molecules are arranged in random order, as illustrated in Figure 12-2. When this steel bar is rubbed with a magnet or placed in a strong magnetic fi eld, all of the molecules line up in the same direction. Each molecule assumes a north and a south Magnet S N Flux Goodheart-Willcox Publisher Figure 12-1. A bar magnet showing magnetic fl ux. Goodheart-Willcox Publisher Figure 12-2. In a non-magnetized piece of steel, the molecules are in random positions.