Tribute to Hans Christian Oersted, 200 Years after His Experiment – Part II

In Part I of this tribute, we looked at some of the background of Oersted’s famous experiment. We saw that Oersted was philosophically inclined to believe in the existence of a link between electricity and magnetism, but had not actually formulated any hypothesis or experiment to specifically investigate it. We touched upon the fact that Oersted observed a slight deflection of the magnetic needle in the lecture experiment, but he kept it aside and did not immediately communicate it to anyone.

Oersted wanted to be sure that the movement of the magnetic needle was not due to any other causes – for example, a stray electrostatic charge that could have caused any needle, even non-magnetic, to deflect. He had been using a battery that was only moderately strong, so he set about building a much larger and more powerful battery. This took several weeks, and required the help of craftsmen who had the relevant skills.

Remember that Oersted believed in the unity of all the forces of nature, not just electricity and magnetism. In his mind’s eye, it was only when a current made a wire red hot that it emitted a magnetic force along with the heat and light. So in his experiment he was using a very thin platinum wire with a high resistance to get a glow on passing a current.

Oersted imagined the magnetic effect of the current to be radiating outwards from the wire, just like heat and light. This had a profound influence on where Oersted would have kept the magnetic needle to check for deflections, as illustrated in the figure below.

If the magnetic field had radiated outwards from the wire as Oersted imagined it, he would have expected the needle to show the maximum deflection when the needle was kept beside the wire, forming a horizontal plane.

This was an accidental factor that greatly diminished his chances of seeing a proper deflection. As we know today, the magnetic field lines form a circular loop around the wire. A needle kept beside the wire would not have deflected much, since the magnetic field lines at that point would have been in a vertical plane, perpendicular to the needle’s plane of free rotation.

Oersted’s best chance of seeing a large deflection would have been to keep the needle right above the wire, but in his conception of a radially emitted magnetic force, that was the position in which the needle would deflect the least!

Even after he got his new powerful battery, we see that Oersted was labouring under two hypotheses that stacked the odds against him:

  1. The magnetic effect was produced only when the wire emitted heat and light as well.
  2. The magnetic effect was emitted by the wire radially outward.

The first hypothesis made Oersted use extremely thin wires in his circuit to get the wire glowing. This meant that the current he got from the battery was far weaker than it could have been, if he hadn’t been thrown off track by his need to make the wire red hot.

Oersted did not record when and how exactly he realised that the wire need not be red hot. We can only surmise that he most likely discovered it by accident. He must have moved the compass around and must have seen the needle remain deflected by the same angle even near one of the connecting leads far away from the red hot wire.

Once he realised that he did not need a red hot wire, Oersted started using a thicker wire that could carry a stronger current. This led him to soon recognise that the direction of the magnetic force was not as he had imagined it to be, but actually formed a circular loop around the current-carrying wire.

By this point, Oersted had understood the gravity of his discovery. He naturally didn’t want to be beaten to it by anyone else, so he rushed to write a report of his experiments and sent it to several scientists across Europe on 21 July 1820.

What happened afterwards is much better known – the invention of the electromagnetic motor and generator, electromagnets and the telegraph – all of which can be traced back to this seemingly insignificant and academic experiment.

As we complete 200 years of this experiment, I feel that this is a rich story to delve into with students. Nahum Kipnis, an eminent science historian and educator, recommends getting students to actually repeat Oersted’s experiment as he did it, first with red hot wires and let them refute Oersted’s hypotheses and discover the circular magnetic field lines themselves. It would fit right into a historical-investigative approach to teaching electromagnetism.

Reference

  1. Nahum Kipnis. Chance in Science: The Discovery of Electromagnetism by H.C. Oersted. https://www.academia.edu/10253171/Chance_in_Science_The_Discovery_of_Electromagnetism_by_H_C_Oersted

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