Magnetic Curves

Before 1820–1850.

Static electricity. Electric current can be produced by chemical processes. Magnets attract and repel, point North, the exact direction in which they point varies in complex ways. Iron makes good magnets, and louses up compasses.

Oersted

Discovered that wires with current flowing are attracted/repelled by magnets.

Ampère

Discovered that wires with current flowing attract each other, and ... .

Barlow

Studies how to use compasses in the presence of iron.

Compasses don't just point North. The direction in which they point is a complex function of where they are, the time of day, ... . In order for them to be useful for navigation, one therefore needs maps of what they do. The maps are maps of Earth with little arrows all over them, and the arrows form lines. 182

Barlow needs to know how a piece of metal near a compass affects the compass, and he needs to know in 3 dimensions. There is an obvious way to represent the data: he moves a compass around an object and makes a map with arrows showing the effect of the object at each location. That might not have been obvious were it not for the maps of Earth's magnetism.

Faraday

Does a lot of work with picturing the influence of magnets using iron filings. 187

He discovers that iron concentrates the lines, and that diamagnetic materials disperse or repel them. That gives him a kind of picture of the concentrating properties of iron. He can picture attraction by or repulsion by magnets by thinking of the lines as wanting to be together. He also thinks of the lines around a magnetic pole as being like an octopus with tentacles pulling like magnets and iron toward them.

Induction

If you move a wire in a magnetic field, that produces a current. Faraday discovers that the strength of the current is proportional to the rate at which the wire cuts the lines.

Diamagnetism

Nonmagnetizable things (things other than iron) are repelled by magnets.

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Not only induction, but the other phenomena can be made quantitative by using the lines too.

Faraday's pictorial discoveries, plus the non-Newtonian character of the phenomena give people a reason to take the lines seriously.

Maxwell's theory of electromagnetism, which is first post-Newtonian theory, is a theory of fields. He obtained it, in large part, by looking for a way to mathematize Faraday's drawings and metaphors about octopuses and so on. According to Maxwell's theory there should be "electromagnetic radiation" that moves at exactly the speed of light, and so light is electromagnetic radiation. You can also make "light" using electromagnetic processes, that is, radio waves. Special relativity comes out of problems with Maxwell's theory. General relativity and quantum mechanics both take the fundamental structures of the world to be not things (like Newton) but fields.

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Thus, Faraday's fields, and his thinking of pieces of iron as nothing more than field concentrators has taken ove r our picture of the world completely. However, it was not arrived at as a part of any theory. It arose out of a convenient way to represent certain observations (Barlow) and out of certain experimental results (lines formed using iron filings). The pictures are antecedent to any theory. One important kind of experiment is experiments that explore novel phenomena.

Experimental practice led to the "theoretical" idea of fields, and theories were devised about fields only after that happened. "Theoretical" entities can be the result of experiment, not of theory or of attempts to explain the experiments.

-- ShaughanLavine - 11 Apr 2006