Historical context

Lodestone, rich in the mineral magnetite (Fe$_3$O$_4$), was known for its qualities of attraction thousands of years ago. Historical accounts vary, but they indicate that ancient Egyptian, Greek and Central American civilisations were familiar with it. The Chinese first used a compass as a fortune-telling device and subsequently as a directional indicator somewhere between 400 B.C. and 100 B.C., but surprisingly it was not until later in the first millennium A.D. that a needle compass was used for navigation.

In the thirteenth century Petri Pergrinus (Pierre de Maricourt) outlined the direction to which the needle would point at various positions around a lodestone, and from this ascertained that magnets had two regions, north and south.

The Elizabethan scientist William Gilbert demonstrated that the Earth was a giant magnet (Gilbert and Mottelay, 1600, 1991) and that this was responsible for the directional alignment of a compass needle, additionally observing that the attractive effects of amber were, contrary to general belief at that juncture, not magnetic: we now know this is a form of electrical attraction. Gilbert prepared and presented Queen Elizabeth I of England with a magnetite model to demonstrate the magnetic behaviour of the Earth (figure 1.1) called a terrella, or ``little earth''. When the terrella was aligned with the poles of the Earth it would spin on its axis.

Gilbert is also responsible for providing the north-south polar analogy between magnets and the Earth's poles, and disposing of most of the magical legends surrounding magnetism, though he did develop the somewhat esoteric notion that the Earth had an anima, or ``soul'' which was the source of the magnetic field. The anima was effective up to the orbis virtutis: the ``orb of virtue''.

Figure 1.1: William Gilbert's magnetic model of the Earth

Gilbert can be credited with establishing magnetism as a scientific field. His work fascinated Galileo Galilei who, influenced by Gilbert's work (BBC, 2004), hypothesised that the Earth orbited the Sun rather than the popular perception of the time which was that the Sun (and everything else) revolved around the Earth.

In the mid-eighteenth century John Michell proposed that the attractive force between two magnets can be calculated using the inverse square law, i.e. that if the two entities are half as far apart, the force between them will be four times greater. Charles Augustin de Coulomb verified this experimentally and indicated that if one were to split a magnet then two new poles would be created (figure 1.2, left).

A professor at the University of Copenhagen, Hans Christian Oersted, observed during a demonstration that the needle of a compass was deflected whenever he turned on an electric current; this was the first recorded instance of the relationship between magnetism and electricity. André Ampère, a French physicist, confirmed this and just one week after the initial observation by Oersted had developed an equation to calculate the magnetic force between electric currents.

Figure 1.2: Coulomb's theory (left) was that if one were to break a magnet into two parts then two new poles would form at the broken ends. Magnetic field lines or ``lines of force'' (right) as demonstrated by Michael Faraday.

Towards the end of the 1830s Michael Faraday propounded the concept of lines of force, nowadays known as magnetic field lines, as a way of visualising the magnetic field of an object (figure 1.2, right); these can be seen when dusting iron filings around a traditional bar magnet. Faraday was also responsible for creating the electric generator and motor.

During the 1850s and 1860s James Clerk Maxwell developed mathematical equations derived from mechanical models which described the electricity and magnetism, the relationship between them, and Faraday's lines of force. These equations were published in 1873 and defined classical electromagnetism.