Magnet and Magnetism-( Mystery)

 

Magnet and Magnetism

A small magnet can pick up metal objects such as paper clips and nails. A magnet seems to have some mysterious pulling power. Even if you lay a piece of paper over the nails, the magnet pulls them. This pulling power is called magnetism.

People first noticed magnetism thousands of years ago. They discovered that pieces of a certain kind of black rock had a strange force, which came to be called magnetism. No one knows just how the name was chosen. There is a story that the force was named for a shepherd called Magnes, who discovered that the iron nails in his sandals were pulled strongly when he stood on a large black stone. Another story is that the name came from a place called Magnesia, in Asia Minor, where this kind of stone was first found. There are fanciful stories, too, from ancient times about iron statues held in mid-air by magnets, and about magnetic mountains that could pull the nails out of wooden ships that sailed too near.

After a while, people discovered some useful facts about black rock or magnet. They found that when a long, thin piece was hung up by thread it would swing around and stop in a north-and-south position. The same end of the piece always pointed toward the north. This meant that such a magnet could be used as a compass.

Sailors began to use such compasses to guide their ships when the moon or stars could not be seen. The magnetic rock became known as lodestone, meaning "leading stone." To make a compass, a sailor took a sliver of lodestone and laid it on a piece of wood floating in a tub of water. This made it possible for the lodestone to turn freely into the north-south position.

There are other important facts about magnetism that you can easily find out for yourself. Lay a bar magnet on a pile of tacks and then pick it up. The tacks will cling to the bar mainly near its two ends. These two places, where the force is strong, are called the poles of the magnet. When the bar is used as a compass, one end swings to the north. The pole at this end of the bar is called the north pole.

One big question remains: Just what are the little unit magnets found in iron and steel and a few other materials? More than a century ago, the French scientist A. M. Ampère had the idea that these little magnets might be caused by tiny electric currents flowing inside the molecules of the material. Scientists already knew that magnetic fields could be produced by electric currents, and Ampère's idea proved to be right.

 It is now known that the currents that cause the unit magnets come from the movement of electrons inside the atoms of certain materials. Some of the magnetism comes from the circling of electrons around the center of the atom. But the main part is due to the spinning of an electron around its axis; each electron has a spin much like that of a toy top.

In most atoms, all these electron motions just cancel out, and so the material is non-magnetic. In a few kinds of atoms, including iron, the magnetic effects of the circling and spinning do not quite cancel out. What is left over makes these materials magnetic.

The story of the unit magnets is not quite finished. Unit magnets are not atoms or electrons, but special groups of atoms. In a material such as iron, the atoms gather in clusters. When they do this, all the little atom magnets in the group line up in the same direction. Such a cluster is called a magnetic domain. Domains are usually a few thousandths of an inch across and may contain millions of billions of atoms. Each domain acts like a little magnet. It is the unit magnet that Ampère first thought about. In an ordinary, unmagnetized piece of iron or steel, the tiny domains face in all directions and their effects cancel out. But if the material is put in a strong magnetic field, two things happen. The domains swing around, little by little, in the direction of the field. As they come into line, they may also grow bigger by taking over some atoms from the other domains, which then shrink in size. When a great many domains have been well lined up in one direction, the whole piece of iron or steel becomes a magnet.

Scientists know that disturbing a steel magnet or heating it can make it lose its magnetism. The reason is that such treatment lets some of the domains get out of their lined-up positions. If a piece of iron is heated until it glows dull red, then it cannot be magnetized at all.