Back to Contents

Note: The Danish/Norwegian letter ě, when properly pronounced sounds a little like the "oh" in neighbor.

In the Summer of 1820 Hans Christian ěrsted, a professor of physics at the University of Copenhagen, made one of the most spectacular discoveries in the history of Science. In the middle of teaching a course in physics he performed the first experiment that demonstrated a relationship between electricity and magnetism. The apparatus of the experiment consisted of a magnetic compass near a wire and a voltaic pile (a primitive electric battery) to whose opposite poles ěrsted could connect the opposite ends of the wire to make an electric current flow in the wire. ěrsted pointed out to his students that the needle of the compass shifted its orientation when a current flowed in the wire and that the needle returned to its original orientation when he broke the circuit and thus made the current stop flowing. A certain folklore has it that ěrsted made this discovery purely by accident, but ěrsted's own account (presumably supported by his students) disputes that interpretation: as preparation for the experiment ěrsted had mentioned and commented on observations that travelers had made over the years that their magnetic compasses moved erratically whenever thunderstorms passed overhead.

Prior to conducting his classroom experiment, ěrsted had hypothesized that something in thunderstorms exerts a magnetic force upon travelers' compasses. He knew, as a result of Benjamin Franklin's famous kite experiment, that thunderstorms contain electricity and that lightning is a powerful electric current, so he guessed that of all the phenomena associated with thunderstorms, the electric incandescence of the lightning deflected the compass needles. He designed his experiment to abstract that one feature from the thunderstorm and to recreate it in miniature in his classroom so that he could test his hypothesis in accordance with the standard Scientific Method. Once he had confirmed his hypothesis he conducted further experiments to refine it, subsequently discovering that it was not the incandescence, but rather the electric current alone that exerted the magnetic force in his experiments.

Though he effectively halted his study of the relationship between electricity and magnetism at that point, he could have gone farther and discovered electromagnetic induction. Aside from a lack of imagination, nothing prevented from taking additional steps. Those steps involved little more than applying certain rules of Newtonian mechanics, which rules he knew as well as did any other physicist of the time. Had he done that, he might have reasoned from his discovery as follows:

1) An electric current (A) exerts a force upon a magnet (B),

2) By Newton's third law of motion, if A exerts a force upon B, then B exerts an equal and oppositely directed force upon A; therefore,

3) A magnet (B) exerts a force upon an electric current (A).

Prior to taking the next step ěrsted would have had to clarify the concept of an electric current in a way that avoids any confusion in what follows. Because an electric current consists of electric charges in motion, he could have created the concept of a pure electric current by imagining that he had deposited electric charges upon a silk thread and then moved the thread in a direction parallel to its length. He might have imagined, for example, pulling the thread off one spool and winding it onto another one. If the thread were to pass between the poles of a strong magnet, the magnet would, in accordance with Statement #3 above, deflect the thread sideways. ěrsted could then have imagined moving with the thread while a student sat by the magnet. He would then have reasoned:

4) The student (C) sees the thread deflected away from a straight line by the magnet;

5) By the Principle of Relativity, any phenomenon that Reality manifests to Observer C it must also manifest to Observer D; therefore,

6) ěrsted (D) sees the thread deflected away from a straight line where it passes through the magnet.

We think of Relativity as originating with Albert Einstein in 1905, but the basic principle appeared in the scientific literature as long ago as 1633. That's when Galileo Galilei published his quickly suppressed book, "Dialogue on the Two Chief World Systems". In that book Galileo described the principle of Relativity (though he didn't call it that) by noting that there is no experiment one can perform that will reveal one's uniform motion in a straight line relative to some putative absolute state of rest; in particular, he noted that a man occupying a windowless cabin aboard a ship would be unable to make any experiment that would reveal whether the ship was sailing on a calm sea or was tied to a dock in a port. Isaac Newton offered a similar description in his Principia. ěrsted certainly knew of it and in this case might have used it as a version of the law of noncontradiction. ěrsted would have brought that noncontradiction into play by claiming that uniform relative motion cannot so deform space and time that a deformation of a thread into a curve for one observer would be hidden from another observer, who would see the thread uncurved: the thread must appear curved to both ěrsted and his student.

But for ěrsted the electrically charged thread would not be moving and, thus, would not constitute an electric current. In his frame the magnet would be moving and the charged thread would be stationary, so he would reason:

7) Only an electric force can move a stationary electric charge;

8) The moving magnet is moving a stationary electric charge; therefore,

9) A moving magnet exerts an electric force.

That last statement gives us the principle of electromagnetic induction and the scientific foundation upon which engineers base electric generators. Designed to make wires wound upon rotors spin within arrays of stationary magnets (only relative motion between the wires and the magnets counts, after all), electric generators are the equivalent of voltaic piles, but with the advantage that they can generate electricity from anything that can spin the rotors (steam engines and falling water are two common prime movers). Thus, ěrsted could have deduced the principle of electromagnetic induction in 1820 and laid the foundation for our modern electrically-driven civilization eleven years earlier than Michael Faraday actually did.

We call the mathematical expression of Statement #9 Faraday's law and it is the third of the four fundamental equations of electromagnetic theory (the equations are called Maxwell's Equations). Physicists have so named the law because Faraday discovered it in August 1831, doing so in a way more consistent with the folklore surrounding ěrsted's discovery; that is, by accident.

Once physicists saw from ěrsted's experiment that an electric current causes a magnetic effect, they speculated that some kind of reciprocal relationship must exist between electricity and magnetism; that is, they speculated that, in some way, a magnet could be made to cause an electric effect. Faraday hypothesized that he could obtain such an effect by using the magnetic effect of an electric current in one wire to induce the desired electric effect in a second wire. In order to test that hypothesis Faraday wound two wires around an iron core, attached the ends of one wire to a galvanometer (a device that, by embodying ěrsted's experiment, detects the flow of electric current with a magnetized needle), and attached the ends of the other wire to a voltaic pile through a switch. The test immediately falsified Faraday's hypothesis: the galvanometer showed that no current flowed in the one wire when current flowed in the other wire. However, Faraday was sufficiently observant to notice that the needle of the galvanometer twitched whenever he opened or closed the switch. He inferred then that it was not a steady current, but rather a changing current, in a wire that induces a current to flow in a neighboring wire. Applying his own concept of forcefields (developed as an "aid to the imagination" when he saw that iron filings scattered on paper were rearranged by the presence of a magnet or of a current-carrying wire), he then deduced the rule that a changing magnetic field (of which the field of a moving magnet is an example) generates an electric field, which rule we now know in its mathematical form as Faraday's law.

We may well ask why it took physicists eleven years to progress from ěrsted's experiment to the discovery of electromagnetic induction and had to discover the latter by accident to boot. It was certainly not for lack of interest. No, I believe that an overemphasis on empiricism led to an Icarus complex, a dread of allowing the imagination to soar to high. It took time for physicists to gain confidence in the techniques implied in the use of Faraday's "aids to the imagination". They had to be shown the way by outsiders, such as James Clerk Maxwell and Albert Einstein. Today the use of carefully contrived fantasies is an important part of theoretical physics and that use has evolved modern physics into a subject that Hans Christian ěrsted would barely recognize.


Back to Contents