Magnets are often called "magnets", which attract many iron filings tightly, just like a kind mother attracts her children, so people call them "magnets". Magnets attract iron because the magnetic poles at both ends of each magnet are different, one is called positive pole and the other is called negative pole. The earth where human beings live is also a huge natural magnet. The north pole and south pole of the earth also have different magnetic poles. The north pole of the earth is negative, and the south pole of the earth is positive. According to the principle that like magnetic poles repel and opposite magnetic poles attract, take a magnetic needle that can rotate freely. No matter where you stand on the earth, its positive pole always points to the north, and its negative pole always guides you.
During the Warring States period, people used the characteristics of magnets to make a tool to indicate the north and south-Sina. However, what Sina looked like in the Warring States period is impossible to verify. The textbook illustration "The History of China" and "Sina" are drawn according to the exhibit "Sina Model of Han Dynasty" of China History Museum. This model was made by later generations according to historical records and the physical objects of Han Dynasty sites unearthed underground. The site is made of bronze, with an inner circle and an outer circle, and the central round surface is polished very smoothly to ensure the accuracy of the direction indicated by the spoon body. Eight diagrams, heavenly stems, earthly branches and twenty-eight lodges are arranged in turn on the periphery of the central circle, with a total of twenty-four directions. The small spoon in the center of the site was ground with a whole natural magnet. The anode of the magnet was ground into a long handle of Sina, and the bottom of the spoon head was hemispherical and very smooth. When using, first put the site flat, then put Sina in the middle of the site, and move the spoon handle by hand to make it rotate. When Sina stopped, the spoon handle pointed in the direction of the south. This spoon-shaped Sina was used until the eighth century.
By the Song Dynasty, the working people had mastered the technology of making artificial magnets and made guide fish. Compass fish is cut into fish-shaped thin steel plates, two inches long and five points wide. The belly of the fish is partially sunken, so that the fish can float on the water like a boat. Then put the fish and the natural magnet together. Due to the attraction of magnets, steel plates are also magnetic through magnetic induction. The compass fish made by this artificial magnetic transfer method is much more convenient than Sina. As long as there is a bowl of water, you can tell the direction by putting the compass fish on the water. After long-term improvement, people rub the steel needle on the natural magnet, and the steel needle has magnetism. This kind of artificially magnetized steel needle can be said to be a regular compass. Shen Kuo mentioned in Meng Qian's Bi Tan that he had done four experiments on the use of the compass, namely, the float method, the hanging method, the nail method and the bowl and lip method. "Floating" is to put the compass in a bowl filled with water and let it float on the water to indicate the direction. The "hanging thread method" is to coat some wax in the middle of the magnetic needle, stick a silk thread on it, hang the silk thread on the wooden frame, and put a disk marked with orientation under the needle. When the pointer is stationary, the steel needle will indicate north and south. "Nailing method" is to put the steel needle on the surface of the nail and turn it gently. Because nails are smooth, magnetic needles can also play a guiding role like Sina. The "bowl lip method" is to put the magnetic needle on the smooth edge of the bowl and turn the magnetic needle, which will play the same guiding role as the nailing method. After careful observation and experiment, Shen Kuo also found that the direction indicated by the magnetic needle is not due south and north, but slightly northwest and southeast. This discovery is scientifically called magnetic declination.
China is not only the first country in the world to invent the compass, but also the first country to use it for navigation. 1 1 At the end of the century, the compass began to be used for navigation. Mi Zhu in the Northern Song Dynasty said in "Zhou Ping Ketan" that at that time, people on board knew the geographical direction, looked at the stars at night, looked at the sun during the day, and looked at the compass when it rained on cloudy days. Xu Jing, the envoy of the Northern Song Dynasty to North Korea, also said in the Map of Xuanhe's Mission to North Korea: When the fleet sets sail, it "looks at the stars at night, and if it is dark, it uses a floating needle". It can be seen that people engaged in navigation at that time had generally mastered the scientific knowledge of the guide. With the wide application of compass in navigation, the compass itself has also been improved. In the southern song dynasty, the magnetic needle and the azimuth device were assembled into a whole, which was the compass. In the Yuan Dynasty, there were artificial turtle guides and vertical needles to guide fish. The compass illustration in the textbook is a schematic diagram of the compass used in modern times.
The invention and application of compass not only make people overcome the difficulty of finding the direction when sailing, but also promote the development of world navigation and cultural exchanges. During the Southern Song Dynasty, some Arab businessmen and Persian businessmen often traded on fishing boats in China. They learned to make a compass and spread this method to Europe. It was not until the end of 12 century/the beginning of 13 century that some Arab and European countries began to use compasses to sail, which was more than 100 years later than our country.