The outer core is in many ways even less well understood, though everyone is in agreement that it is fluid and that it is the seat of magnetism. The theory was put forward by E. C. Bullard of Cambridge University in 1949 that this fluid part of the Earth's core revolves in a way that makes it, in effect, an electrical motor, creating the Earth's magnetic field. The assumption is that the convecting fluids in the Earth act somehow like the currents in wires. Exactly what happens isn't known, but it is felt pretty certain that it is connected with the core spinning and with its being liquid. Bodies that don't have a liquid core—the Moon and Mars, for instance—don't have magnetism.
外核在许多方面被了解得更少，虽然大家都认为它是液态的，是产生磁力的地方。1949年，剑桥大学的E·C·布拉德提出一种理论，认为地核的液态部分在以某种方式转动，实际上成了一台电动机，创建了地球的磁场。他认为，地球里面对流的液体，在某种意义上起着电线里的电流的作用。到底是怎么回事现在还不清楚，但大家觉得比较肯定的是，磁场的形成与地核的转动有关系，与地核是液态的有关系。没有液态核的物体——比如月球和火星——就没有磁力。

We know that Earth's magnetic field changes in power from time to time: during the age of the dinosaurs, it was up to three times as strong as now. We also know that it reverses itself every 500,000 years or so on average, though that average hides a huge degree of unpredictability. The last reversal was about 750,000 years ago. Sometimes it stays put for millions of years—37 million years appears to be the longest stretch—and at other times it has reversed after as little as 20,000 years. Altogether in the last 100 million years it has reversed itself about two hundred times, and we don't have any real idea why. It has been called "the greatest unanswered question in the geological sciences."
我们知道，地球磁场的强度在不停地变化：在恐龙时代，磁场的强度是现在的三倍。我们还知道，它平均每隔50万年左右自我逆转一次，虽然那个平均数包含着很大程度的不可预测性。上一次逆转发生在大约75万年以前。有时候，几百万年也没有变化——最长的时间似乎是3700万年，有时候，不到20万年就发生一次。在过去的1000万年里，地球磁场总共发生了大约200次逆转，原因搞不清楚。这一直被称为“地质科学里最大的未解问题”。