写作思路：根据题目要求，以介绍指南针作为主题，先介绍指南针是什么，接着写指南针的原理，最后围绕指南针展开描写。
Compass is a simple instrument identification range, also known as compass.
指南针是一种简单的仪器识别范围，又称指南针。
According to historical records, the compass first appeared in the Cishan area during the Warring States period.
据《史记》记载，最早出现在战国时期磁山地区的罗盘。
It used to be one of the four great inventions in ancient China. It was the result of the working people's understanding of magnetic objects in ancient China.
以前是中国古代四大发明之一，是中国古代劳动人民在长期实践中对磁性物体认识的结果。
The main parts are mounted on the shaft and can rotate the needle freely.
主要部件安装在轴上，可以自由转动针。
Under the action of the magnetic field, the needle can keep the tangential magnetic radial direction on the ground, and the magnetic needle points to the geographic north pole.
在磁场的作用下，针能在地面上保持切向磁径向方向，磁针指向地理北极。
This kind of performance is usually used in navigation, geodesy, travel and military fields.
这种性能通常用于导航、大地测量、旅行和军事等领域。
Therefore, the direction can be determined.
因此可以确定方向。