4.2 Early Models of Electricity

Ordered Abode of Reason or a Factory?

Is electricity a fluid of some sort contained within a wire? Or is it the result of fields pervading the otherwise free space around charges and currents? And if it is fields, what is going on in the medium to give rise to the activity and behavior we call “fields?” Now might be a good time to examine what electricity and magnetism are at a fundamental level and the speculations, explanations, and models investigators have proposed.

Commenting on the “weakness” of the English mind, the French physicist, Pierre Maurice Marie Duhem (1861–1916), had this to say about the English predilection for models:

In the treatises on physics published in England, there is always one element which greatly astonished the French student; that element, which nearly invariably accompanies the exposition of a theory, is the model….

The French or German physicist conceives, in the space separating two conductors, abstract lines of force having no thickness or real existence; the English physicist materializes these lines and thickens them to the dimensions of a tube which he will fill with vulcanized rubber.... Such is the famous model of electrostatic action imagined by Faraday and admired as a work of genius by Maxwell and the whole English school.

The employment of similar mechanical models, recalling by certain more or less rough analogies the particular features of the theory being expounded, is a regular feature of the English treatises on physics. Here is a book [Oliver Lodge’s Modern Views of Electricity] intended to expound the modern theories of electricity and to expound a new theory. In it there are nothing but strings which move around pulleys, which roll around drums, which go through pearl beads, which carry weights; and tubes which pump water while others swell and contract; toothed wheels which are geared to one another and engage hooks. We thought we were entering the tranquil and neatly ordered abode of reason, but we find ourselves in a factory [[i]].

Figure 4.8 offers a depiction of Maxwell’s mechanical model for self-induction, and the behavior of field lines.

Figure 4.8 A “provisional” mechanical model of a current interacting with a surrounding dielectric medium, left, a model for a field line which when rotated exhibits tension along its axis and pressure along its sides [[ii]], center, and, William Thomson, Baron Kelvin (1824–1907) [[iii]], right.

However much Duhem might disdain the English turning his temple of reason into a factory, Faraday, Maxwell, and their successors established the fundamental principles of electromagnetism using mechanical models like these. As Heaviside noted, Kelvin (see Figure 4.8) “was most intensely mechanical, and could not accept any ether unless he could make a model of it” [[iv]]. If mechanical models helped in the discovery of electromagnetism, what then are the promising models available to us as we seek to understand how electromagnetism works?

To answer this question, we must back up to review how we came to understand electricity and magnetism and what evidence led investigators to the fluid model of electricity. Magnetic and electric phenomena were known in ancient times and ascribed to “souls,” or vague “atmospheres,” “effluvia,” or “spheres of influence.” The magnetic compass was the first practical application of the science. Early investigators generated static electricity through friction and studied its behavior. Originally interpreted as two different kinds, Benjamin Franklin (1706–1790) and others concluded there was only one kind of electrical fluid normally present in materials. Electric effects were ascribed to an excess or deficit of this fluid. Franklin’s model–updated to account for atomic theory and electron flow–is still fundamental to understanding electricity and electronics, today.

Next time: 4.2.1 Ancient Views

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References

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[i] Duhem, Pierre, The Aim and Structure of Physical Theory, New York: Antheneum, 1962, pp. 69-71.

[ii] Lodge, Oliver, Modern Views of Electricity, 3^rd. ed., London: Macmillan & Co., 1907, p. 183, and p. 170, respectively.

[iii] William Thomson, Baron Kelvin. Photograph. Credit: Wellcome Collection. CC BY

[iv] Heaviside, Oliver, EMT v3. p. 479.