In The Feynman Lectures on Physics volume 2, Magnetostatics, section 13-4 Mr. Feynman tells us that there were "three critical experiments and a brilliant theoretical argument given by Ampère" that helped in determining the the magnetic field by a steady current. However he says he is going to skip those and come directly upon the results.

Can you please tell me what were those 3 experiments and that argument by Ampere. Thank you.


Ampère devised more than 3 null experiments in the 1820s, but perhaps Feynman is referring to these:

"Galvanic spirals and helices produce the same effects as magnets":
"Galvanic spirals and helices produce the same effects as magnets"

Experiment showing equivalence between helical current and bar magnet:
helical current = bar magnet

Experiment of two parallel rectilinear currents:
∥ rectilinear currents

(images source)

Here's the English translation of Ampère's work where he explains his experiments and derives his force law:

See also:

and the other excellent articles translated into English on Ampère et l'histoire de l'électricité.

Lastly, Feynman seems to think the Maxwell equation named after Ampère ("Ampère's" circuital law) was actually due to Ampère. It was not; it was due to Heaviside. Ampère's force law is something completely different.

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