Gravity and magnetism are still mysterious forces in many respects. Yet scientists have gathered considerable information about them. The term "gravity" -more accurately, "terrestrial gravity"- is applied to the gravitational force exerted by Earth.
Gravity is the force that pulls all materials toward the center of Earth. This force becomes weaker as one moves away from the center. When people weigh themselves, they are really measuring the force of gravity.
A person weighing 143 pounds (65 kilograms) is being pulled toward the center of Earth with a force of 143 pounds. Weight decreases farther from the center. So the same person will weigh slightly less on the top of a mountain than in a deep valley.
In 1600 the English physician Sir William Gilbert suggested that Earth behaves like a huge magnet, with north and south poles. This was confirmed by the German mathematician Carl Friedrich Gauss. In the early 1800s, Gauss demonstrated that the planet generates its own main magnetic field internally. The minor fluctuations that do occur are due to solar activity. When using a compass, the needle falls along the lines of force that run from one magnetic pole to the other. The magnetic poles do not correspond exactly to the geographic poles.
