“Not Resting Until It Points South”: Tracing the Story of the Invention of the Compass

The picture above is the Sinan model.

The picture below shows a model of a compass made by hanging a thread. Image courtesy of Visual China

Recently, the BeiDou-3 global satellite navigation system was completed and put into operation, making China the third country in the world to independently own a global satellite navigation system. When talking about satellite navigation systems, we have to mention my country's earliest navigation tool - the compass.

It took a long time for our ancestors to understand and master the magnetism of materials. During production labor, people came into contact with magnetite, discovered the properties of magnets attracting iron and the directionality of magnets, and finally invented the compass. During the Song Dynasty, our ancestors had already mastered a lot of knowledge about magnetism. Wen Tianxiang, a patriotic poet of the Southern Song Dynasty, was detained by Yuan soldiers for several days during his mission to Beiying and his return to Fuzhou. He wrote "Yangtze River": "I traveled to the North Sea with the wind for a few days, and returned from the Yangtze River. My heart is like a magnetic needle stone, and I will not rest until it points to the south." The poem was collected in "Zhi Nan Lu" to express the author's loyalty to the Southern Song Dynasty and his determination to return to the south at the risk of his life.

Sinan has not been widely used due to its own defects

The ancestor of the compass appeared around the Warring States Period. It was made of natural magnets and looked like a spoon. It could be placed on a smooth "ground" to keep balance and could rotate freely. When it was still, the handle of the spoon would point to the south. The ancients called it "Sinan". The use of Sinan was recorded in the book "Guiguzi" during the Warring States Period. The people of Zheng State brought Sinan with them when they went to collect jade to ensure that they would not get lost.

In the Spring and Autumn Period, people were able to grind nephrite and jadeite with a hardness of 5 to 7 degrees into utensils of various shapes, so they could also make Si Nan from natural magnets with a hardness of only 5.5 to 6.5 degrees. Wang Chong, a thinker in the Eastern Han Dynasty, made a clear record of the shape and use of Si Nan in Lun Heng. Si Nan is made of a whole piece of natural magnet and is grinded into a spoon shape. The handle of the spoon is used as the compass, and the center of gravity of the entire spoon falls exactly in the middle of the bottom of the spoon. The spoon is placed on a smooth plate, which is square outside and round inside. The four directions of the stems and branches are engraved around it, which together form 24 directions. This design was completed after the ancients carefully observed many magnetic phenomena in nature, accumulated a lot of knowledge and experience, and studied for a long time. The emergence of Si Nan is a practical application of people's understanding of the polarity of magnets.

However, Si Nan had many defects, such as the difficulty in finding natural magnets, the difficulty in processing, the poor measurement accuracy, the large size and the inconvenience in carrying. These were the main reasons why Si Nan failed to be widely used for quite a long time.

Artificial magnetization method turns iron sheet into compass

In ancient times, people often cut thin iron sheets into fish shapes. The belly of the "fish" was slightly concave, like a small boat. After being magnetized, it floated on the water and could indicate north and south. At that time, this was used as a game, called "compass fish", which was the prototype of the compass. In the Northern Song Dynasty, Zeng Gongliang recorded the method of making and using the compass fish in "Wu Jing Zong Yao": "Use thin iron sheets to cut, two inches long, five fen wide, with sharp heads and tails like fish shapes, place them in charcoal fire and burn them. When they are red, use an iron seal to seal the fish head out of the fire, with the tail facing the child position, dip it in a basin of water, and stop when the tail is submerged for a few minutes, and collect it in a sealed container. When using it, place a bowl of water flat in a windless place, and let the fish float on the water surface, with its head always facing noon."

This is an artificial magnetization method that uses the earth's magnetic field to magnetize the iron sheet. That is, the red-hot iron sheet is placed in the direction of the meridian. The molecules inside the red-hot iron sheet are in a relatively active state, so that the iron molecules are arranged along the direction of the earth's magnetic field to achieve the purpose of magnetization. Dipping the iron sheet into water can quickly fix this arrangement, and tilting the fish tail slightly downward can increase the degree of magnetization. The invention of the artificial magnetization method has played a huge role in the application and development of the compass, and it is also a major event in the history of the development of magnetism and geomagnetism.

The invention of the compass is the result of the Chinese working people's understanding of the magnetism of objects in long-term practice. By the Song Dynasty, this knowledge was already quite rich. Shen Kuo, a scientist in the Northern Song Dynasty, introduced four methods of magnetic needle installation in "Mengxi Bitan".

The water floating method is to put a few rushes on the magnetic needle and let it float on the water. The bowl lip rotation method is to place the magnetic needle on the edge of the bowl, and the magnetic needle can rotate. The fingernail rotation method is to place the magnetic needle on the fingernail. Since the nail surface is smooth, the magnetic needle can rotate freely. The thread hanging method is to apply some wax on the middle of the magnetic needle, stick a piece of silk, and hang it in a windless place to indicate the direction.

Derived from the two major systems of water acupuncture and dry acupuncture

Shen Kuo once compared the above four methods. He pointed out that the biggest disadvantage of the water float method is that the water surface is easy to shake, thus affecting the measurement results. The bowl lip rotation method and the fingernail rotation method have low friction and are very flexible to rotate, but they are easy to fall off. Shen Kuo prefers the thread hanging method, which he believes is a more ideal and feasible method. In fact, the four methods pointed out by Shen Kuo have summarized the two major systems of compass devices so far - water needle and dry needle.

Shen Kuo mentioned another method of artificial magnetization in Mengxi Bitan: "The experts used magnets to grind the needle tip, and it could point south." According to Shen Kuo, people at that time could make the needle magnetic by rubbing it with magnets. From the current point of view, this is a method of using the magnetic field of natural magnets to make the arrangement of magnetic domains inside the steel needle tend to a certain direction, so that the steel needle shows magnetism. This method is simpler than the geomagnetic method, and the magnetization effect is better than the geomagnetic method. The invention of the friction method is not only the earliest in the world, but also created important technical conditions for the emergence of magnetic pointers with practical value.

Chen Yuanliang of the Southern Song Dynasty introduced another method of making a compass fish and a compass turtle in his book "Shilin Guangji". This compass fish is different from the one recorded in "Wujing Zongyao". It is made of wood and shaped like a fish, the size of a finger. A natural magnet is placed in its belly, with the south pole of the magnet pointing to the fish head. After sealing it with wax, a needle is inserted from the fish mouth to make a compass fish. When it is floated on the water, the fish head will point south, which is also a type of water needle.

The compass turtle was a new device popular at the time. A natural magnet was placed inside the belly of a wooden turtle. A smooth small hole was dug under the belly of the wooden turtle and aligned and placed on the sharp bamboo nail at the top of the wooden board. In this way, the wooden turtle was placed on a fixed and freely rotatable fulcrum. Since the friction at the fulcrum was very small, the wooden turtle could rotate freely. However, it was not used for navigation, but for magic. This was the forerunner of the later dry compass.

(The author is a researcher at the China Science and Technology Museum)