The Story of the Lodestone
Posted by Greg MacDonald on
Long ago, the ancient Chinese navigated across trackless deserts because they knew that a type of stone, now called lodestone (from the Old English word, lãd= course) or magnetite (from Magnesia, in Thessaly, Greece, where deposits of magnetite were found) would align itself in an approximately north/south direction when suspended from a thread.
Some ancient Greeks believed that there were magnetic islands, made of lode-stone, that could attract the nails and other iron objects in a ship. Vessels that disappeared at sea were said to have been pulled helplessly to these islands. (There was even a story, not nowadays believed to be true, that Archimedes used an incredibly strong lodestone to pull the nails from enemy ships, sinking them!)
The thirteenth century soldier and scholar Petrus Peregrinus (Peter the Pilgrim) studied magnets, and found that when a piece of lodestone was broken, each piece became a complete magnet. In his study Epistola de Magnete, Peregrinus also reported that the north-seeking ends of a lodestone would repel one another, and that the north seeking end of one lodestone would attract the south-seeking end of another.
Demonstrate this phenomenon with two bar magnets.
Some of the first European navigators floated a magnetized needle or a lode-stone fragment in a bowl of water. The needle showed them the direction of north, making it possible for them to find their way across oceans when there was no land in sight.
In the seventeenth century, William Gilbert shaped a lodestone into a sphere, and showed that a small piece of lodestone placed nearby behaved like the navigator’s compass magnet. He called the ends of the lodestone poles.
Magnets have a north (seeking) pole and a south (seeking) pole, named after the pole of the Earth that they seek. What is confusing here is that the north pole of a magnet and the north pole of the Earth are ACTUALLY opposite poles! The north pole of a magnet is actually a south pole, as it is attracted to the Earth’s north pole!
We now know that all magnets have an invisible magnetic field around them, which we can actually see under the right circumstances.
Demonstrate magnetic field lines using iron filings sealed in a flat-surfaced, transparent container.
We still have a great deal to learn about magnets. We do know that magnets and electricity are related (a fact that you and I use every day). We have also learned that members of the animal kingdom knew about magnetism long before the ancient Chinese and Greeks began their studies. There are bacteria that can sense magnetic fields, and we have also discovered that birds, fruit flies, and many other animals have a magnetic sense.
But these are stories for another day …