By the end of the 1830s, Faraday had decided to pursue two problems. The first was to examine why magnetism was specific to only two metals iron and nickel with a view to discovering if other materials had magnetic properties. The second was to develop and sustain experimentally a non-atomic theory of matter.
He pursued the second topic in his lecture of 19th January 1844 entitled A speculation touching electric conduction and the nature of matter. Here he proposed the existence of point atoms, where lines of force (of magnetism, for instance) would meet, and what were formerly perceived as material atoms would be the result of a combination of forces acting at those points.
However, there was little experimental evidence for this theory. In particular, since such atoms should be structurally similar, it was peculiar that only iron and nickel evinced magnetic properties. So the two problems Faraday identified at the end of the 1830s become linked and he approached the whole thing experimentally.
In September 1845, he passed polarised light through a piece of high refractive index glass placed on the poles of an electro-magnet. When he turned the electro-magnet on, he found that the state of polarisation of the light had changed. This is now called the magneto-optical or Faraday effect. Discovering it was the first stage in his efforts to make magnetism a universal property of matter.
In November he found that a piece glass hung between the poles of an electro-magnet would align itself with the lines of force of the magnet. He called this phenomenon diamagnetism. Very quickly he found that all solid materials had some form of magnetic property. In 1847, extending his work to gases, he found that oxygen was more strongly magnetic than iron.
In his early experiments, Faraday used both a piece of glass that he had made during the glass work he did in the late 1820s and some powerful lamps he had been testing for Trinity House (see lighthouses
). This materials basis for Faraday's discoveries illustrates, as does so much of his work, the closeness of the practice of his science theory with his practical work.
Faraday had found experimental evidence that magnetism was a universal property of matter and for his views on the nature of matter and space. He outlined his new understanding in his lecture Thoughts on ray-vibrations delivered on 3rd April 1846. This lecture was later seen by many scientists as laying the foundations of the field theory of electro-magnetism.
Although Faraday had used the word 'field' in 1845, he used it then in a purely descriptive sense to describe the space surrounding a magnet. During the first half of the 1850s, he developed arguments for the reality of the field, which by 1851 he defined in terms of lines of force.
The mathematisation of field theory by William Thomson (later Lord Kelvin, 1824-1907) and James Clerk Maxwell (1831-1879), in consultation with Faraday, led to the theory becoming and remaining one of the fundamental concepts of modern physics. Furthermore, field theory helped solve problems connected with the construction of long-distance underwater telegraph cables in the 1850s and 60s, and later formed the basis of wireless technology.