What does a compass look like? Be clear.

The invention of compass with physical direction indication consists of trilogy: Sina, magnetic needle and compass. All belong to China's invention.

Compass is one of the four great inventions in ancient China, and its invention has played an inestimable role in the development of human science and technology and civilization. In ancient China, the compass was first used for rituals, etiquette, military affairs and divination, and to determine the orientation when looking at geomantic omen. 1 1 At the end of the century or at the beginning of the 2nd century, China ships began to use compasses for navigation. Zhou Ping can talk in the Northern Song Dynasty: "Sailors know geography, watch the stars at night, watch the sun by day, and watch the compass in the dark."

Compass is an all-weather navigation tool applied to navigation, which makes up for the shortcomings of astronomical navigation and geographical navigation and opens a new era in the history of navigation. At the same time, navigation activities further promoted the development of compass.

China is regarded as the country that invented the compass in the world. According to ancient mine records, it first appeared in the area of Cishan (now Cishan, Handan City, Hebei Province) in the Warring States Period. The invention of the compass is the result of the long-term practice of the working people of Han nationality in China. Due to productive labor, people came into contact with magnetite and began to understand its magnetism. People first discovered the property that magnets attract iron, and later discovered the directivity of magnets. After many experiments and studies, a practical compass was finally invented. The earliest compass was made of natural magnets, which shows that the working people of Han nationality in China discovered the attraction of natural magnets and their iron very early. According to ancient records, as early as the Spring and Autumn Period and the Warring States Period, the productive forces developed greatly, especially agricultural production, which promoted the development of mining and smelting industries. In the long-term production practice, people know magnets from iron ore. The earliest compass was Sina.

The book Guanzi Dishu, written in the 7th century BC, recorded that "there are magnets on the top and copper and gold on the bottom". It means that if there is a magnet on the mountain, there is an iron ore hidden in the mountain. In the geographical masterpiece Shan Hai Jing, it is also recorded that there are many magnets in the mountains. It is recorded in the Notes on Water Classics that the front of Epang Palace in Qin State is made of magnets to prevent people from entering the palace to assassinate. If the bad guys wear dark armor and take hidden weapons into the palace, they will be caught in the door and found. This shows that people have long discovered the iron attraction of magnets and used it.

In the long-term production struggle, the working people of the Han nationality in China have further made use of the finger polarity of magnets to make a machine to indicate the direction, which is a compass. This is recorded in Youdu Pian and Guiguzi. It is recorded in Guiguzi that when Zheng people went to the deep forest to collect jade, they brought "Sina" in order not to get lost. This "Sina" is the mechanical compass. In Lun Heng, Wang Chong described "Sina" in the Eastern Han Dynasty. It is like a water spoon, which is ground with a natural magnet. The bottom of the spoon is spherical, the spoon is oval, and the handle of the spoon gradually shrinks into a cylinder. In order to determine the direction, there is also a "place", which is a copper or painted wooden board with a smooth circular groove in the middle, which may be outside the inner ring. The rack is marked with azimuth scale, and 24 orientations are represented by "stem" and "branch" (namely, A, B, C, D, Zi, Ugly, Yin and Mao, etc.). ) and gossip. When it will be thrown in the center of the field, its handle will usually stop in the direction of the guide. The above shows that Sina is the prototype of modern compass (magnetic compass). "Dao" was first put forward by Zhang Heng in "Tokyo Fu", which developed gradually through Wei, Jin, Southern and Northern Dynasties, Sui and Tang Dynasties, and lasted for more than 1000 years in the Song Dynasty. Shen Kuo, an outstanding scientist in the Song Dynasty, made a detailed discussion on the development of the compass at that time in Meng Qian Bi Tan. At that time, driven by the development of production and scientific experiments, especially the rise and expansion of navigation and foreign trade, the compass gradually developed. Shen Kuo summarized four installation methods of compass created by working people in practice. The first is the water float method, in which the magnetic needle floats on the water for guidance. Although relatively stable, it is easy to be unstable. The second method is nail rotation, and the magnetic needle is placed on the nail, which is flexible to rotate and easy to slide down; The third method is that the bowl lip rotates, and the magnetic needle is placed on the edge of the bowl mouth, which is flexible to rotate but easy to slide down; The fourth method is the spiral method, which can realize flexible and stable rotation by hanging the magnetic needle with silk thread. He also recorded the method of artificial magnetization, that is, "grinding a tit with a magnet can guide you." This artificial magnet is a big step forward. In addition, there have been "guiding fish", "dry needle" and "water needle". Dry needle and water needle laid the basic structural principle of modern compass (compass needle). In the process of studying the compass, Shen Kuo also found the existence of magnetic declination. In other words, the direction indicated by the compass is "always slightly east, not completely south". This is China's great contribution to geomagnetism.

Magnetic meridian plane

A vertical plane passing through the center of the earth, with a stationary magnetic needle indicating the north-south direction.

The great circle of the earth cut by the geomagnetic horizontal component line at a certain point on the earth's surface. Under the condition that the magnetic needle is only influenced by the geomagnetic field (without self-bias), its direction is magnetic meridian direction (that is, under the action of the geomagnetic field, the direction indicated by the axis of the magnetic needle when it is at a certain point). Due to the asymmetry of the geomagnetic pole and the irregularity of the magnetic field, magnetic meridian generally does not pass through the geomagnetic north and south poles. Therefore, the magnetic north direction pointed by the needle is not necessarily the direction of the geomagnetic north pole.

The direction of magnetic meridian can be determined by a compass.