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Newton's law of gravitation was taken to be an example of action at a distance. The law described the force that one body would exert on another some distance away without any regard to what was in between and without any hint of a mechanism for transmitting the force.

the magnetic force on an electric current was not simply attractive or repulsive, it could cause rotation

Newton was aware that this was a feature of his 'System of the World' that many would find unattractive, but he also realized that he had no evidence on which to base a detailed explanation of gravitational forces. He contented himself with describing gravitational forces mathematically, and said in the Principia, that he would 'form no hypotheses' as to their cause.

Faraday, like others, was willing to accept this situation as far as a purely attractive force like gravity was concerned, or even for a force that could be attractive or repulsive like Coulomb's, but Faraday's own invention of the electric motor showed that the magnetic force on an electric current was not simply attractive or repulsive, it could cause rotation (see Figure 1.18). Faraday felt that for a wire to rotate around a magnet there had to be something, produced by the magnet but present at the location of the wire, that pushed the wire to one side rather than another. It was this agency, filling the space around the magnet, that Faraday eventually came to call a magnetic field.

Faraday's views about the nature of the magnetic field changed over time; for complex reasons, he spoke about his field as being different from an ether. Whatever his precise views, Faraday was convinced that fields held the key to understanding magnetic and electrical phenomena. He certainly felt that the curved pattern of lines revealed by sprinkling iron filings onto a sheet of paper placed over a magnet

Figure 1.21 Magnetic field lines, as revealed by sprinkling iron filings onto a sheet of stiff paper placed on top of a magnet. Click here for larger image (4.09kb)

showed the presence of a magnetic field. Like a collection of miniature compass needles, the filings showed the fieldís strength and direction in each region of space. However, he also realized that in order to provide convincing evidence of the reality of the field something more was needed, such as a demonstration that a disturbance at one point in the field would take a finite time to propagate through the field and have visible effects elsewhere. Faraday tried to observe such delays, but failed. Nevertheless, his belief in the physical reality of fields guided his experiments and lead him on to new discoveries.

When Maxwell started to work on electromagnetism he studied Faraday's experimental researches and, unlike most of his contemporaries, was impressed by the notion of a field. However, Maxwell had his own reasons for believing in an ether. In particular, he believed that an ether was necessary to account for the propagation of light, which was generally regarded as a kind of wavelike disturbance and was therefore thought to require a medium just as ocean waves require water. Maxwell therefore decided to combine Faraday's field ideas with the ether concept. He set out to treat electricity and magnetism in terms of fields that were themselves interpreted as manifestations of pressure, tension and motion within the ether.
Continue on to Electromagnetism and fields, part 4 of 4