Andre-Marie Ampere and the Current through a Battery

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.

Ampere’s experiment: A compass needle was deflected the same way when kept above the battery

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.

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