Chinese scientists discovered magnetic forces occurring naturally Chinese scientists discovered magnetic forces occurring naturally in a rock called lodestone. Magnetic compasses were first made from this stone around the 5th Century AD and used in the Chinese art of Feng Shui, also known as geomancy. Practitioners of Feng Shui believe that orienting furniture, buildings and tombs according to a magnetic compass will harmonize them with the earth's magnetic field (known as the "cosmic breath").
By the 11th Century, floating or water-compasses guided Chinese ships. One hundred years later, these magnetic compasses were used aboard European ships. As noted in Voyagers, Columbus used a magnetic compass on his first trans-Atlantic trip. Columbus discovered, through his compass sightings, that the North Star (Polaris) was not exactly due north. In 1492, Polaris was off by about 3.5o, which caused it to circle the sky like every other star.
1. Figure 16 shows how to make your own magnetic compass. This type was used by Columbus on all 4 voyages to the Americas. The materials needed include a film canister, a pushpin, two sewing needles, cardboard and a bar magnet.
Insert the pushpin through the lid of the canister so its point pokes through the top. Snap the lid on the canister to form the compass stand. Cut out a 2-inch circle and mark off the compass directions. As an art project, students can create their own designs for the dial's face.
Magnetize two sewing needles by placing them in contact with a bar magnet. Tape both needles to the underside of the compass dial. Position the needles near the center of the dial as shown in the figure 16. It is important that the dial remain balanced. Make a tiny hole in the center of the dial and rest the dial on the tip of the pushpin. The pushpin should be supporting the dial without binding it. Any friction will prevent the dial from turning. Reposition the needles as needed to make the dial balance.
Demonstrate how a compass detects a magnetic field by positioning the bar magnet near the compass dial. Reversing the bar magnet will cause the dial to spin away.
2.) Certain metals affect magnets. For this reason, iron is especially troublesome on board ships. Iron interferes with the compass needles, causing them to point incorrectly. This problem is called deviation. With so little iron on board Columbus' ships, there was little worry about deviation.
Place different materials (such as aluminum cans, iron nails, rocks and wood) near the magnetic compass constructed in Activity 1. Have the students record which materials cause the dial to turn.
Exploring Navigation, A Publication of the Exploratorium, pp. 30-35, Spring 1991.