How does a
Last week, we explored magnets and how they work.
This week we will explain how a compass works.
When you put a magnet near metal, an invisible
force pulls the metal to the magnet. This invisible force is called
magnetism. In many ways, Earth acts like it has a big bar magnet buried
In order for the north end of the compass to point
toward the North Pole, you have to assume that the buried bar magnet has
its south end at the North Pole. If you think of the world this way,
then you can see that the normal “opposites attract” rule of magnets
would cause the north end of the compass needle to point toward the
south end of the buried bar magnet. So the compass points toward the
North Pole. The bar magnet does not run exactly along the Earth’s axis,
but is slightly off center. This skew is called declination.
A compass is really just a detector for very slight
magnetic fields. Since the needle of a compass is magnetic, the
invisible pull of Earth’s magnetic field causes the needle of a compass
to point north. Once the compass helps you find north, you can navigate
east, west, and south, too.
The magnetic field of the Earth is fairly weak on
the surface, but considering the Earth is almost 8,000 miles in
diameter, it is understandable. The magnetic field around the Earth has
to travel a long way to affect your compass. That is why a compass must
have a lightweight magnet and a frictionless bearing. Otherwise, there
just isn’t enough strength in the Earth’s magnetic field to turn the
The analogy that there is a big bar magnet buried
in the Earth helps to explain why the Earth has a magnetic field, but
not really what is happening. Scientists do not know for sure, but they
have a theory. The Earth’s core is thought to be made up of mostly
molten (liquid) iron. At the inner core, the pressure inside is so large
that the molten iron is “squeezed” into a solid. As the heat radiates
out from the solid inner core and the Earth spins, it causes the liquid
iron of the outer core to move in a rotational pattern. Scientists think
that this rotational force in the liquid iron causes the magnetic forces
around the Earth.