View source for André-Marie Ampère

{{Infobox scientist
| name = André-Marie Ampère
| birth_date = {{birth date|1775|1|20|df=y}}
| birth_place = Lyon, France
| death_date = {{death date and age|1836|6|10|1775|1|20|df=y}}
| death_place = Marseille, France
| nationality = French
| fields = Physics, Mathematics, Chemistry
| workplaces = École Polytechnique; Collège de France
| known_for = Ampère's force law, electrodynamics, the solenoid, molecular currents
}}

'''André-Marie Ampère''' (20 January 1775 &ndash; 10 June 1836) was a French physicist and mathematician who founded the science he named '''electrodynamics''', and who wrote down a force law between current elements that is still argued over on this wiki two centuries later.

Ampère was largely self-taught, reading his way through his father's library as a boy. His father was guillotined in the Terror of 1793, an event that marked him for life. He became professor at the École Polytechnique in 1809 and at the Collège de France in 1824.

His decisive work occupied a few extraordinary weeks. On 4 September 1820 Ampère heard of Ørsted's discovery that a current deflects a magnetic needle; by 18 September he had shown that two parallel currents attract one another and two antiparallel currents repel. He invented the helical winding he named the '''solenoid''' and showed it behaves as a magnet, and proposed that all magnetism is due to circulating molecular currents &mdash; a hypothesis vindicated a century later by electron orbital and spin magnetism. He devised the astatic needle and the first galvanometer.

The results were consolidated in his ''Mémoire sur la théorie mathématique des phénomènes électrodynamiques uniquement déduite de l'expérience'' (1826), whose subtitle &mdash; "uniquely deduced from experiment" &mdash; states his method. Maxwell called him the Newton of electricity. The SI unit of current is named for him.

==Two force laws==

Ampère's law between two current elements is a central-force law: the force lies along the line joining the elements, is equal and opposite, and depends on the angles the elements make with that line as well as with each other. It therefore obeys Newton's third law in the strong form, and it predicts '''longitudinal forces''' &mdash; tension or compression along the direction of the current itself.

The formulation used in textbooks today descends instead from Grassmann and the Biot&ndash;Savart&ndash;Lorentz force. For any pair of ''closed'' circuits the two give identical results, which is why the substitution is normally considered harmless. For isolated current ''elements'' they differ, and the Grassmann force is not generally along the line of centres. Whether the difference can be exhibited experimentally &mdash; and whether observed effects in exploding wires, railguns and liquid-metal conductors require Ampère longitudinal tension &mdash; is a real and unresolved experimental dispute, not merely a matter of formalism.

==On this wiki==

This wiki holds one of the largest collections anywhere of the Ampère-versus-Grassmann literature; see [[:Category:Electrodynamics]] and [[:Category:Electromagnetism]].

* [[Peter Graneau]] is the central figure. ''[[Ampere-Neumann Electrodynamics of Metals]]'' (1985) argues that the Lorentz formulation is inadequate and that electrodynamic reaction forces always act on their sources rather than on space. With [[Neal Graneau]] he wrote "[[Ampere Electrodynamics]]" (1993), "[[Only Ampere Forces Explain Railgun Recoil and Wire Explosions]]" (''[[Galilean Electrodynamics]]'', 1994) and ''[[Newtonian Electrodynamics]]'' (1996), and reported the effect experimentally in "[[Ampere Repulsion and Graneau's Exploding Wires]]" and "[[Ampere Repulsion Drives the Graneau-Hering Submarine and Hering's Pump]]".
* [[Andre K T Assis]] provided the scholarly foundation, including the complete English translation and analysis of the 1826 memoir, and connects Ampère's law to [[Wilhelm Eduard Weber|Weber]]'s electrodynamics and to [[Mach's Principle|Mach's principle]].
* [[Paul Wesley]] evaluated the contested Pappas&ndash;Moyssides bridge experiments in "[[Ampere's Original Force Law Compared with the Moyssides-Pappas Results]]" (1987), and [[Jan Olof Jonson]] argues the other way in "[[A Comparison between Ampere's Law, Coulomb's Law and the Lorentz Force]]" (2010) &mdash; the disagreement inside this wiki is worth noting.
* [[James Keele]] gives a derivation and a test in "[[Theoretical Derivation of Ampere's Law]]" (2002) and "[[Experiment with Ampere's Law and the Current Element]]" (2006).
* [[Domina Eberle Spencer]], with [[Parry H Moon]], [[Uma Y Shama]], [[Philip J Mann]] and colleagues, treats the same question from the postulate side in "[[The Force Between Current Elements]]" (''[[Physics Essays]]'', 1994).
* [[Thomas E Phipps]], "[[Ampere Tension and Newton's Laws]]" (''[[Apeiron]]'', 1993), places the issue in the context of Newtonian third-law consistency; [[Jorge A Guala-Valverde]] and [[Ricardo A Achilles]], "[[Ampere: The Avis Phoenix of Electrodynamics]]" (''Apeiron'', 2008), argue from homopolar torque production.
* [[Ionel Dinu]] gives an aether reading in "[[Bernoulli Equation for the Aether and Ampere's Effect]]" (2007), and [[Sidney Bertram]]'s "[[Faraday's and Ampere's Laws]]" (1992) derives both from a common base.

==See also==

* [[Michael Faraday]]
* [[Wilhelm Eduard Weber]]
* [[James Clerk Maxwell]]
* [[Peter Graneau]]
* [[Andre K T Assis]]
* [[Lorentz Force]]

[[Category:Scientist|Ampere Andre-Marie]]
[[Category:Electrodynamics]]
[[Category:Electromagnetism]]