I have now discouraged the idea that there is just one monolithic practice, observing. We must now apply the same tactics to the other side of that old duet of theory and observation. Theory is no more one kind of thing than observation is. A rich but elementary example will illustrate this fact.

The Faraday effect

Michael Faraday (1791–1867), an apprentice bookbinder, got a job when he was 21 as assistant to Humphry Davy. He then advanced our knowledge and transformed our machinery. His two most lasting insights go hand in hand: the invention of the electric motor (and, conversely, the electric dynamo); and the realization that changes in current produce changes in magnetic intensity (conversely, rotation through a magnetic field generates current). There is also what is called the Faraday effect, or the magneto-optical effect. Faraday found that magnetism can affect light. This is of enormous historical importance. It suggested that there might be a single theory unifying light and electromagnetism. James Clerk Maxwell put it together by 1861, and systematically presented it in 1873. Faraday's effect had been experimentally demonstrated in 1845.

Faraday, a deeply religious man, was convinced that all the forces of nature must be interconnected. Newton made a space for unified science that lasted until 1800. In that year, as we saw in Chapter 10, William Herschel produced the problem of radiant heat. In the same year Guiseppe Volta made the first voltaic cell.