Do You Know What An Electromagnetic Compass Is?

An electromagnetic compass (as opposed to a gyroscopic Compass) is placed on a fulcrum with little friction by means of a small light magnet balance. A magnet is often called a pointer. One end of the pointer is marked "n" for North, or drawn in a different color to indicate that it points north. On the surface, this is all the compass.

The reason the Compass works is more interesting. As you can imagine, a huge bar magnet is buried inside the earth. To make the North Point of the compass point to the north pole, it is necessary to assume the southern end of a bar magnet buried in the north pole, as shown on the right. 

If you understand the world, you will find that the electromagnetic universal "opposite attraction" law will make the north end of the compass point to the south end of the embedded bar magnet. As a result, the compass will always point to the North Pole.

The axis of the bar does not exactly coincide with the axis of the earth's rotation. This deviation is called bias, and most high-quality maps show different regional biases.

The magnetic field on the earth's surface is quite weak. After all, planets are almost 13000 kilometers in diameter, so it takes a long distance for the magnetic field to affect the compass. 

That's why the compass needs very light magnets and has no friction support. Otherwise, the magnetic field on the earth's surface is not strong enough to rotate the pointer.

Although the analogy to "a giant bar magnet buried in the core" explains why the earth has a magnetic field, it is not. So what is the actual situation?

No one knows for sure, but there is a working principle that has been widely spread. As mentioned above, it is believed that the core of the earth is made up of molten iron (red). But the core pressure is so high that the ultra-high temperature iron crystal becomes a solid. 

The heat from the iron core generates convection and rotates the earth, so that molten iron enters the rotating mode. It is believed that these rotating forces in the molten iron weaken the magnetic force around the rotating axis.

The fact is that the earth's magnetic field is so weak that the compass is just a detector that can detect the weak magnetic field produced by anything. That's why we can use a compass to detect the small magnetic fields produced by live wires.