On the 13th november, 1845, Faraday wrote: "It was only on the very strongest conviction that Light, Mag & Electricity must be connected that could have led me to resume the subject & persevere through much labour before I found the key. Now all is simplicity itself."
Maxwell's theory showed electricity and magnetism to be so tightly, and symmetrically interrelated as to constitute different facets of a single entity. It's quite a trick to divide the two, isn't it? I've so far worked with the aether being a latent magnetic field, which produces an electric field. After reading more about the notions concerning the aether held by Volta, Maxwell, Franklin, Thomson and Rankine, amongst others, I feel more comfortable imagining the aether as being an electric fluid, which then produces magnetic effects. These magnetic effects amount to the forces of attraction and repulsion. Also, when I observe the properties of hydrogen and helium near absolute zero (such as being a frictionless, super-conducting liquid metal), then working with the aether as an electric fluid seems to make more sense.
A magnetic fluid is almost indescribable. An electric fluid is a bit more manageable. It feels like magnetism describes something of what the aether does, and less about what the actually aether is. It appears then that tensions, or strains, or stress, within the electric fluid generate what we observe as a magnetic field.
In 1861 James Clerk-Maxwell (1831-79) published a mechanical model of the electromagnetic field. Magnetic fields correspond to rotating vortices with particles acting as idle wheels between them. Maxwell summised that these idle wheels "play the role of electricity". In a wire, the particles are free to flow and form an electric current. In space, they serve as counter-rotating idle wheels between vortices to make successive ones turn in the same direction. Maxwell understood that the particles which made-up the idle wheels, did not physically exist as particles. He had simply used them to describe the mechanics of an electromagnetic field. Once he had his calculations of EMR, Maxwell dropped the mechanical explanation, and stuck to the maths. Did Maxwell understand that it was the electric fluid that played the role of everything, both vortices and idle wheels, in the electromagnetic field? Certainly, the aether being an electrical fluid was neither a new or unknown idea.
Already by the 1730s, some natural philosophers began to concieve of the electric fluid as the most essential substance, that which bound all the Universe together in the sense of an aether. William Watson (1715-87) in the late 1740s introduced the notion that the electric fluid was an elastic fluid like air, and he strongly implied that it could have different degrees of compression within a conductor. Concepts of a pressure within the electric fluid became widespread in the 1770s.
In a dissertation written in 1766, Franz Mesmer had propounded the widely accepted theory that a "subtle fluid ...pervades the Universe, and associates all things together in mutual intercourse and harmony." In 1779 Allesandro Volta (1745-1827) through experimental studies, introduced his concept of the experimental studies, introduced his concept of the "degree of electric tension". He concieved the electric fluid to have a "greater tension when it comes to be concentrated in a smaller bulk."
A fluid helps explain the force of attraction. Surface tension is an attractive property of the surface of a liquid. Surface tension is caused by the attraction between the liquid's molecules by various intermolecular forces. In the bulk of the liquid, each molecule is pulled in all directions by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid, and are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air, or another liquid).
William John Macquorn Rankine (1820-72) had schematized a mechanical theory of the relation between radiant and bodily heat. He supposed that the bare atomic centres, possessing very small mass and exerting attractive forces on one another, constituted the elastic solid aether, whose vibrations propagated waves of light and heat. Normal matter, with its atomic centres dressed in atmospheres of self-repulsive fluid, absorbed radiant energy by somehow converting the wave vibrations of its atomic centres into thermal rotations in the atmospheres.
William Thomson, 1st Baron Kelvin (or Lord Kelvin) (1824-1907) supported the theory that heat was a form of motion. Electricity he tended to concieve as a fluid, whose accumulations produced tensions in aether and air and currents of which produced the rotational strains of magnetism. He regarded heat, Davy's "repulsive motion", as electricity in motion. Indeed, Thomson was anticipating the arrival of "...a complete theory of matter, in which all its properties will be seen to be merely attributes of motion." Thomson once wrote:
"The explanation of all phenomena of electro-magnetic induction, is to be looked for simply in the inertia and pressure of the matter of which the motions constitute heat. Whether this matter is or not electricity, whether it is a continous fluid impermeating the spaces between molecular nuclei, or is itself molecularly grouped; or whether all matter is continuous, and molecular heterogeneousness consists in finite vortical or other relative motions of contiguous parts of a body; it is impossible to decide, and perhaps in vain to speculate, in the present state of science."
Michael Faraday (1791-1867), unlike his contemporaries, was not convinced that electricity was a material fluid that flowed through wires like water through a pipe. He thought of electricity as a vibration or force that was somehow transmitted as the result of tensions created in the conductor. Faraday's conviction that an electric current gives rise to lines of magnetic force arose from his idea that electricity was a form of vibration and not a moving fluid. A current thus appeared to be the setting-up of a state of tension in the wire, OR the collapse of such a state. Faraday christened this special state inside the wire, the "electrotonic state". In his worldview, space was occupied by fields comprising "lines of force".
Faraday saw the "lines of force", which are revealed by sprinkling iron filings on a sheet of paper held over a magnet, not only as geometrical lines but also as physical lines stretched elastic bands with an extra sideways repulsion. For him, these physical stresses could be used to explain magnetic force. The idea of magnets inducing some kind of strain in their surroundings was of great importance.
Sometimes, as much as Faraday did not want to use the term 'aether', it appears that the aether is exactly what he is describing. It's almost as if he is aware that space is an electric fluid, but refuses to use the words 'aether', or 'electric fluid' to illustrate his ideas. Later, Maxwell argued that the fluid would flow from source to sink precisely the same lines as Faraday's "lines of force". In fact, the "lines of force" (or rather the space between the lines) could be considered exactly as thin tubes of steadily-flowing, continuous, incompressible fluid.
In 1855, when Faraday was sixty-four, he told his niece, Constance Reid, "How few understand the physical line of force! They will not see them, yet all the researches on the subject tend to confirm the views I put forth many years since. Thomson of Glasgow seems almost the only one who understands them. He is perhaps the nearest to understanding what I meant. I am content to wait, convinced as I am of the truth of my views."
Tellingly, it was Thomson who went on to create a hypothesis based on the aether. In April 1867, in a formal paper 'On vortex motion', Thomson began to publish mathematical foundations for "...the hypothesis, that space is continuously occupied by an incompressible frictionless liquid acted on by no force, and that material phenomena of every kind depend solely on motions created in this liquid".
Perhaps Faraday was influenced in his thinking by time served under Davy. Sir Humphry Davy (1778-1829) supplied the argument that heat is not matter, but "repulsive motion". He recognised that heat was behind the repulsive force "which prevents the actual contact of the corpuscles of bodies". Davy's wrote: "The phenomena of repulsion have been supposed, by the greater part of chemical philosophers to depend on a peculiar elastic fluid; to which the names of latent heat, and caloric, have been given." He was able to summise that the "motion or vibration of the corpuscles of bodies must be necessarily generated by friction and percussion. Therefore, we may reasonably conclude that this motion or vibration is heat, or the repulsive power."
At first it might appear that Davy was trying to dispel all notions of the aether. I don't think this was quite the case. His biographer, Dr J A Paris, attributes Davy as saying "The electric fluid is considered as light in a condensed state" and "that the great quantity of this fluid almost everywhere diffused over our earth..."
Maybe Davy was resolved to dispel ideas about heat being a material thing. In doing so, it removes all possibility that heat has any potential energy. Indeed, if heat is not matter, where then is matter gaining its energy from to generate heat? I think "repulsive motion" can quite easily slip into being one of the properties of an electric fluid. If anything, I think Davy was trying to confirm that heat was brought from outside the material world, through vibration, from the aether.
It could be said that Davy and Faraday share similar notions about the importance of vibration, and its impact on the material world. Faraday believed that these vibrations were responsible for electricity, and Davy thought them responsible for heat. For those that have held a cable which feeds an over-loaded electric shower, and had that awful whiff of burning rubber, will be all too familiar with the reality that electrical resistance creates heat.
I shall end this post with a question posed by Thomson, which was enough to raise both my eyebrows (Roger Moore stylie) and that is, "What is centrifugal force but a repulsive motion?" So, if the 'cold' electric fluid acts out centripetal forces as it enters the vortex, it then appears to be heated as it exits as a centrifugal force. Heat... vibration... repulsion..... hmmmm.
Great Physicists, By William H. Cropper
The Life of Sir Humphry Davy, By John Ayrton Paris