A magnetic field is an invisible field that exerts a magnetic force on substances that are sensitive to magnetism. A classic example of one is the field created by an iron magnet; to see how the energy in such a field works, a small magnet can be placed under a piece of paper and iron filings sprinkled on it. As the filings respond to the magnetic field, they will slowly orient themselves along an axis. Larger examples include the Earth's magnetic field, and those that are created by other celestial bodies such as stars and planets.
Many people think of magnetism as a property of metal, particularly iron, since common household magnets are made from iron. Electrical currents are actually the force behind magnetic fields, which form as electrical charges move around. On a large scale like an electromagnet, the field is created by passing current through wires. In the case of a household magnet, it is created by the movements of electrons in their orbits. Depending on the material and environmental factors, the strength of the field can vary.
Magnetic fields have many properties that scientists and others have used over the centuries. In navigation, ships can orient themselves with the assistance of the Earth's magnetic field, which is, incidentally, located several degrees off the geographic poles. In scientific research, these fields can be used to gather information about a location or object; geologists, for example, use tools called magnetometers to measure ambient magnetism to learn more about the underlying rock and mineral materials.
Medicine uses it in things like diagnostic machines such as Magnetic Resonance Imaging (MRI) equipment. In this case, the field is carefully generated and controlled by the operator of the machine for the purpose of gathering information about the human body. In alternative medicine, some healers use magnets to achieve various desired effects. Scientists can also use other tools to create or alter magnetic fields for the purpose of learning more about the Earth.
The strength of magnetic fields varies widely. An MRI machine, for example, can pull the keys out of an unwitting doctor's pocket, while a household magnet can be knocked from the refrigerator door with little effort. There are also many different types of magnetic fields; an iron magnet, for example, displays ferromagnetism, while those created with the use of an external current are known as electromagnetic fields.