The Earth and its magnetism

The magnetic field of the Earth

As long ago as 600 BCE, the Greeks knew that a certain form of naturally occurring iron ore, known as lodestone, has the property of attracting small pieces of iron. Lodestone contains magnetite, an oxide of iron (Fe₃ O₄) which is a magnetic material.

Historically, lodestone was important in providing one of the first clues towards establishing the connection between magnetic materials and the Earth. For example, in the Middle Ages, crude navigational compasses were made by attaching pieces of lodestone to wooden splints floating in bowls of water. These splints always lined up in a north–south direction on the Earth's surface and such early observations led to naming the ends of magnets poles.

We now know much more about the shape and nature of the magnetic field due to the Earth. In its simplest form we might describe it as being rather like the magnetic field of a bar magnet with ends near the Earth's geographical north and south poles. However there is evidence that there are strong local variations in strength and direction of the field, and even that the field undergoes complete reversals in direction over long periods of time.

The locations of the geographical and magnetic poles are relatively close. However, Arctic explorers seeking to reach the geographic North Pole would be hundreds of miles adrift if all they used was their compass. Their compass would direct them to the magnetic north.