Central figures in this active period in the history of electromagnetism were Michael Faraday and James Clerk Maxwell. Faraday was responsible for the concepts of fields and lines of force, which have come to be the most important ideas in the mathematics and in the imagery of electromagnetism. It was also Faraday who arrived at the view that the atomic basis of matter is essentially electrical in nature, and that a definite amount of electricity is associated with one mole of a substance. To Maxwell we owe the prediction of electromagnetic radiation and the first general mathematical formulation of electromagnetic theory. Faraday (and, simultaneously, his contemporary Joseph Henry in America) discovered one law of induction, which, using modern terminology, we can express in this way: A changing magnetic field creates an electric field. Maxwell later formulated the other law of induction: A changing electric field creates a magnetic field. The great significance of these laws of induction lies in the fact that they express connections between fields alone, with no reference to charge or to poles. Accordingly, they govern the interaction of fields even in space far removed from matter.
Although the laws of induction stand properly as separate significant statements about electric and magnetic fields, both have interesting close connections with the earlier discoveries of Oersted and Ampère. One remarkable fact is that, from a strictly logical point of view, the experiments of Faraday and of Henry showing the induction of an electric field by a changing magnetic field need not have been performed at all. With the benefit of our present comprehensive knowledge of electromagnetism, we recognize that the result of the experiment was to have been expected. This was of course not known at the time. Faraday’s result was rightly regarded by his contemporaries as a significant new discovery. Without it the path to the unification of electromagnetism would have been longer and harder.
Here is how to see that the results of the Faraday experiment was “to be expected.” Instead of moving a magnet near a stationary coil of wire, as Faraday and Henry did, and observing that a current is induced—which we now interpret by saying that a changing magnetic field creates an electric field—we could leave the magnet stationary and move the coil (exactly the same relative motion). Then there is no changing magnetic field. Instead there is motion of the electrons in the wire through a magnetic field. This means, in keeping with Ampère’s earlier result, that these electrons experience a force, and since they are free to move within the wire, they do move, creating a current. So whether we say that the current is generated by an induced electric field or by a magnetic force, the result is the same. This is an example of relativity at work. All that matters is the relative motion of the magnet and the coil. Whether either—or neither—is “at rest” doesn’t matter.