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Incoming links: Siméon Denis Poisson
Volume II, chapter VI, §442, contrasts Siméon Denis Poisson's induced molecular magnetization with Wilhelm Eduard Weber's pre-existing molecular magnets. A publisher transcription of that section is available at www.cambridge.org/core/books/abs/treatise-on-electricity-and-magnetism/webers-theory-of-magnetic-induction/672E4931AABCC94ED48AE9B952B85B48.
Several historical models represented the same broad magnetic observations in different ways:
- Magnetic fluids and poles. Opposite magnetic states were represented by two imponderable fluids, with attraction between unlike poles and repulsion between like poles. A magnetized body was described through separation of those fluids, often within elementary constituents. This gave useful mathematical pole descriptions, but the fluids were hypothetical entities, not liquids isolated in an experiment.
- André-Marie Ampère's molecular currents. Tiny circulating electric currents were proposed as the source of the elemental magnetic action. A current loop has a magnetic dipole moment, so an array of such loops can reproduce a magnet's external field. This connects magnetism to the observed magnetic action of an electric current.
- Wilhelm Eduard Weber's pre-existing molecular magnets. The elemental magnets were assumed already present in unmagnetized material, with their directions cancelling in aggregate. An applied field turned them towards common alignment, giving a net magnetization and a limiting magnetic saturation. This differs from supposing that the elementary magnetic separation is first created by the applied field.
James Clerk Maxwell's later treatment explicitly contrasts the induction model associated with Siméon Denis Poisson with Weber's permanently magnetic molecules and discusses the predicted saturation limit.[ref]
A Treatise on Electricity and Magnetism by James Clerk Maxwell. VolumeII, chapterVI, §442: Weber's theory of magnetic induction
Its 1873 date is retained; the chapter is used to explain the earlier hypotheses, not attributed to an 1865 publication.Experiments constrain these models. Cutting a magnet produces smaller magnets with both poles, rather than simply separating free north and south charges. A current loop and an ordinary magnet also have closely related external dipole fields. Such facts explain the utility of molecular dipoles and currents, but do not uniquely identify the microscopic constituents. Modern magnetic domains, electron spin and quantum magnetic ordering refine the account; classical loops and freely turning isolated molecular magnets are incomplete microscopic pictures of a real ferromagnet. An effective pole calculation must likewise be distinguished from claiming an observed isolated magnetic monopole.