Introduction
This is a short summary outlining errors I have seen countless times in astronomy books.
These errors relate to the way astronomers misunderstand of the laws of electricity and magnetism.

Magnetism
Magnetism is a property of an electric current. A current flowing through a coil will form an electromagnet. A piece of suitable ferromagnetic material can be made into a permanent magnet if it is subjected to a magnetic field under the right conditions. The usual process for modern permanent magnets is to heat the material above its Curie point (a temperature at which permanent magnetism cannot exist) and to allow the material to cool while being subjected to an intense magnetic field. The resulting magnet has the properties of all magnets. These are the magnet's ability to attract ferro-magnetic materials to itself and to induce an electric current into a nearby conductor when moved towards or away from the conductor. The magnetic field of a stationary magnet has no energy in itself. Energy is only involved in the movement of a magnet in the vicinity of a conductor or the movement of a conductor near a magnet.

Magnets always have two poles that are normally referred to as north and south poles. Technically these should be referred to as north seeking and south seeking poles. North seeking poles repel each other and south seeking poles do likewise. North and south seeking poles attract each other.

The magnetic field of a magnet always takes the shortest magnetic path. The magnetic path in ferromagnetic materials is very much shorter than in air. It is this property which enables magnets to attract and pick up ferromagnetic items.

There are no such things as magnetic lines of force. These are imaginary levels of magnetic field strength comparable to isobars on a weather map or contours on a physical map. Any explanation which refers to magnetic lines of force is meaningless.

Electricity and Magnetism.
The magnetic field caused by an electric current is always at 90 degrees with respect to the flow of the current.
The current induced in a conductor by the movement of a magnet near the conductor will create a magnetic field in opposition to the movement of the magnet. (This statement is a paraphrased version of Lenz's Law.) The movement of a magnet against the magnetic field created by the induced current involves work as defined in mechanics. The work done to move the magnet against the induced magnetic field requires an input of energy.

Astronomers' Errors
The most common error made is to assume that magnetism arises spontaneously. Many properties are ascribed to magnetism in and around the stars and planets as if the magnetism is an original actuating force. All magnetism is created by the flow of an electric current. In the field of astronomy most electric currents are created by the movement of hot ionised gas. All of the hot ionised gas in space is emitted from stars.Therefore all astronomical magnetic fields are created by flows of ionised gas emitted from stars. The gas may be diffuse as in the solar wind. Most currents are caused by ionised gas rotating around an astronomical body like a star or a planet. The flow of an ionised gas creates a magnetic field which can exert a force in a manner comparable to a lever. A lever in itself cannot exert a force unless energy is used to move the lever. In this instance the energy is in the form of the moving ionised gas.

The next most common error is to attribute the Earth's magnetism to movement of a conductor within the Earth's magnetic field. As Lenz's Law states, the magnetic field created by the induced current will be in opposition to the magnetic field that produced it. The induced current magnetic field will tend to cancel out the original magnetic field that induced the current in the first place. As the original magnetic field is effectively weakened, so will the induced current be reduced in proportion. The result will be no significant magnetic field produced by this means.

Earth's Magnetism
The Earth's outer atmosphere interacts with the solar wind and causes some of the solar wind to rotate around the Earth with it. This creates the so-called "Ring Current". Observations have shown that the solar wind is positively ionised. The ring current is of the opposite polarity to currents normally observed on Earth in electric conductors. The ring current produces a magnetic field of the same polarity as that observed: the magnetic pole near the geographic north pole is south seeking. It attracts the north seeking ends of compasses. There is no other mechanism that complies with the laws of physics which can explain the existence of a magnetic field around the Earth or any other astronomical body.

The Magnetism of Other Planets
The Voyager and other space probes have detected magnetic fields around most of the other planets in the solar system. Significant magnetic fields have only been found around planets which have significant atmospheres and spin relatively quickly. Planets without much atmosphere or do not spin much seem not to have significant magnetic fields. These observations support the hypothesis that all planetary magnetic fields are caused by rotating atmospheres interacting with the solar wind.

Wilf James
25/05/2008
Note:
This piece has veen written as a result of reading the astronomers' incorrect view of the Earth's magnetism repeated in Terry Pratchett's The Science of Discworld.

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