在2000多年前的春秋战国时代,我国劳动人民在采矿冶炼过程中,发现了以四氧化三铁为主要成分的磁石．磁石具有吸铁性,并且天然磁石在地球表面总是指向南北极．北宋的沈括在《梦溪笔谈》中记载:“方家以磁石磨针锋,则能指南,然常微偏东,不全南也．”“则能指南”说明地球周围有磁场,其磁场方向是大致南北方向的;“然常微偏东,不全南”说明磁石在地磁场中静止时的指向并不是正南正北方向,而是略微偏一些． In the Spring and Autumn and Warring States Period more than 2,000 years ago, the working people of our country found a magnet with iron oxide as the main component during the mining and smelting process. The magnet has iron absorption, and the natural magnet always points to the north and south poles on the surface of the earth. Shen Kuo of the Northern Song Dynasty wrote in “Meng Xi Bi Tan”: “The Fang family can use the magnet to sharpen the cusp, then they can use the guideline. However, they are often slightly eastward and incomplete in the south.” “The energy guide” shows that there is a magnetic field around the earth and its magnetic field direction It is roughly north-south direction; “it is often slightly east, not all south” shows that when the magnet is stationary in the geomagnetic field, the direction is not exactly south, north, but slightly.