About 400 years ago, a physician named William Gilbert published "De Magnete" (in English, "On the Magnet"), a study of magnetism. It offered the first rational explanation of the magnetic properties of Earth. This document heralded the modern era of physics and astronomy, paving the way for experiments by scientists like Galileo, Kepler, Newton and countless others.
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The first magnetic compasses were invented by the Chinese around 221 to 206 B.C. during the Qin Dynasty for use by fortune tellers. In the eighth century, the Chinese developed compasses for navigation similar to the ones we still use today.
Like the Earth, each magnet has a north pole and a south pole, and opposite poles attract. Experiment with two magnets by putting them together. If they stick together, they are opposite poles. Flip one over and they will repel each other.
Curious Behavior of Atoms
- Everything is made up of atoms. Each atom has electrons circling a nucleus, and the electrons create a magnetic field with a north and south pole. Most things--like flesh, wood or plastic--have random placement of atoms, so they can't be magnetic. Iron, nickel and cobalt are different. Groups of atoms, called domains, line up together to create magnetic properties.
Anything that contains iron, nickel or cobalt can be temporarily magnetised. Steel contains iron, so you can magnetise things like paper clips, screws and nails. Rub them on one pole of a magnet, over and over, always in the same direction. For a while, they will stick to each other and act like weak magnets. This happens because contact with magnets causes the atoms inside the items to line up all their poles in the same direction.
Heated iron can't be magnetised. Thermal energy from heating makes the atoms in the iron jiggle. Jiggling atoms can't line up properly, so heated iron loses its magnetism. The temperature where this occurs is called the Curie point after Pierre Curie, the physicist who discovered it.
Magnets Around Your House
A strong magnet can erase computer hard drives, credit cards, video and audio tapes, and stop a windup watch. Magnets will also change the colour properties of TV and computer screens.
Magnets are used in lots of household items. Some examples of common household things that contain magnets are stereo speakers, headphones, telephones, microwaves, doorbells, refrigerator door seals, dishwashers, computers and TVs.
Dairy farmers and vets feed or insert magnets in cows to prevent hardware disease. When cows graze, they often ingest things like bits of wire and nails left in the field by careless workers. These odds and ends become lodged in the tissues, causing inflammation and irritation. The cow loses her appetite and can't produce milk or gain weight. A magnet in a cow's stomach will pull all the iron bits together keep the cow from getting sick.
Water is diamagnetic, which means it is repelled by both magnetic poles. A very strong magnet can repel things that are made up mostly of water, like grapes and living creatures. Researchers at NASA's jet propulsion lab in California have levitated frogs and mice. Scientist Yuanming Liu speculates that she could probably use the same principles to build a magnetic field strong enough to levitate a human, but it would cost way too much.
Maglev trains operate using magnetic repulsion. Magnets are attached to the track and to the underside of the train, making the train "float" above the track. Since there is no friction, these trains travel very fast. The highest recorded speed is 343 miles per hour by a Japanese train.
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