A. K. T. Assis is a Brazilian scholar who has written some intriguing books on physics and the history of physics. He has very generously made all these books available for free access on his website – https://www.ifi.unicamp.br/~assis/books.htm.
I have been flipping through the book Ampere’s Electrodynamics by Prof. Assis. It requires a much closer study, but I wanted to write about one thing that stood out for me.
All this while, I had thought of the development of electromagnetism as a linear timeline. I had thought that starting with Oersted’s experiment, other scientists had added bit by bit to the understanding of electromagnetic phenomena, until Maxwell synthesised all the knowledge into one single theory. This book showed me that the actual history was very different.
It seems that there were many differences of opinion between Ampere and the other prominent scientists of the time regarding how to interpret the phenomena. The following snapshot from the book’s Contents pages promises an enticing discussion of these scientific controversies.
The most remarkable aspect of Ampere’s theory was that he imagined all magnetic phenomena as arising from electric currents. In this, almost all his contemporaries disagreed with him vehemently.
For Ampere, there was no ‘magnetic field’ – it was simply a manifestation of an interaction between currents. He hypothesised that there were circular electric currents inside magnets and inside the Earth, which we know today to be true.
The following muse (edited for readability) from Ampere’s writings is a striking example of the role of imagination and creativity in science:
“Suppose we had known that a magnetic needle is influenced by an electric current into a position perpendicular to the wire before knowing that a magnetic needle points to the geographical north. Then, would not the simplest idea and the one that would occur immediately to anyone who wanted to explain it be that there is an electric current inside the Earth?
Andre Marie Ampere
Assis’ book also includes a complete English translation of Ampere’s classic work written in 1826, Theory of Electrodynamic Phenomena, Uniquely Deduced from Experience.
I hope to write more on this when I do manage to study this book in detail.
Ampere’s experiments on the attraction and repulsion of two current-carrying wires depending on the direction of the currents, are well known. I recently learned about another experiment of his – seemingly simple and perhaps trivial – but actually very significant. It was in the book ‘Ampere’s Electrodynamics’ by the brilliant Brazilian scholar A. K. T. Assis, that I read about this experiment.
It was September of 1820. A few months earlier, in April, Hans Christian Oersted had demonstrated the effect of an electric current in a wire on a magnetic compass kept near it. Ampere came to know of this experiment and repeated it.
Ampere’s unique addition to Oersted’s experiment was to check whether there was a similar effect on a compass needle kept near the battery. The battery widely used at the time was a trough battery – a large rectangular box having several compartments filled with acid in which pairs of metal plates were dipped.
The magnetic needle was deflected in the same way when kept above the battery, as it would have been if kept above the wire. Ampere showed that a current flowed through the battery. And that this current was in the same direction as the current in the wire, forming a closed loop.
That’s obvious to us today, but we must realise that this was less than three decades since Volta first invented the battery and how it worked was still a mystery to everyone. This was a time when the most familiar electrical phenomena were all electrostatic in nature. The only other source of electricity scientists knew about were a Leyden jar (a capacitor made from a glass jar) which gave brief bursts of current.
Against this backdrop, what Ampere discovered raises more questions than answers. If the current flowed in a closed loop in the circuit, that meant the current inside the battery flowed from the negative terminal to the positive terminal! This was the opposite of the direction you would expect the current to flow if it were driven by electrostatic forces.
So, Ampere indirectly proved that there were non-electrostatic forces in the battery that caused the current.
What caused a battery to produce a current had already become a raging debate at the time and wouldn’t be settled until over half a century later. That is a topic that deserves several posts, and I’ll return to later.