Introduction: The Aether

(1) The most obvious mechanical phenomenon in electrical and magnetical experiments is the mutual action by which bodies in certain states set each other in motion while still apart.

The first step to reduce these into scientific form is to ascertain the magnitude and direction of the force acting between the bodies.

This force depends on the relative position of the bodies and on their electric or magnetic condition.

This led to the assumption of something either at rest or in motion in each body, constituting its electric or magnetic state, and capable of acting at a distance according to mathematical laws.

In this way mathematical theories of statical electricity, of magnetism, of the mechanical action between conductors carrying currents, and of the induction of currents have been formed.

In these theories the force acting between the two bodies is treated with reference only to the condition of the bodies and their relative position, and without any express consideration of the surrounding medium.

These theories assume the existence of substances the particles of which have the property of acting on one another at a distance by attraction or repulsion.

The most complete development of a theory of this kind is that of M.W. Weber[1], who has made the same theory include electrostatic and electromagnetic phenomena.

He assumes that the force between two particles depends on their relative velocity and on their distance.

This theory, as developed by MM. W. Weber and C. Neumann[2], is ingenious and comprehensive in its application to the phenomena of statical electricity, electromagnetic attractions, induction of current and diamagnetic phenomena.

(2) There are mechanical difficulties in the assumption of particles acting at a distance with forces which depend on their velocities.

These difficulties have led me to think that they are produced by actions which go on in the surrounding medium as well as in the excited bodies.*

(3) I propose a dynamical theory of the Electromagnetic Field.

It has to do with the space in the neighbourhood of the electric or magnetic bodies.

It assumes that in that space there is matter in motion that produces electromagnetic phenomena.

(4) The electromagnetic field is that part of space which contains and surrounds bodies in electric or magnetic conditions.

It may be filled with any kind of matter, or we may endeavour to render it empty of all gross matter, as in the case of Geissler’s tubes and other so called vacua.

There is always enough of matter left to receive and transmit the undulations of light and heat.

The transmission of these radiations is not greatly altered when transparent bodies are substituted for the vacuum.

This is why the undulations are those of an ethereal substance, and not of the gross matter.

The presence of gross matter merely modifies the motion of the ether.

The phenomena of light and heat show that there is an ethereal medium filling space and permeating bodies.

It is capable of:

• being set in motion
• transmitting that motion from one part to another
• communicating that motion to gross matter so as to heat it and affect it in various ways.

(5) The energy communicated to the body in heating it must have formerly existed in the moving medium.

The undulations had left the source of heat some time before they reached the body.

During that time, the energy must have been half in the form of motion of the medium and half in the form of elastic resilience.

From these, W. Thomson[3] has:

• argued that the medium must have a density capable of comparison with that of gross matter.
• even assigned an inferior limit to that density

(6) We may therefore receive the existence of a pervading medium of small but real density.

This medium is capable of being set in motion, and of transmitting motion from one part to another with great, but not infinite, velocity.

The parts of this medium must be so connected that the motion of one part depends in some way on the motion of the rest.

At the same time, these connections must be capable of a certain kind of elastic yielding, since the communication of motion is not instantaneous, but occupies time.

The medium is therefore capable of receiving and storing up 2 kinds of energy:

• the “actual” energy depending on the motions of its parts
• the “potential” energy, consisting of the work which the medium will do in recovering from displacement in virtue of its elasticity.

The propagation of undulations consists in the continual transformation of one of these forms of energy into the other alternately.

At any instant, the amount of energy in the whole medium is equally divided, so that half is energy of motion, and half is elastic resilience.

(7) This medium is capable of other kinds of motion and displacement than those which produce the phenomena of light and heat.

Some of these may evidenced to our senses by the phenomena they produce.

(8) The luminiferous medium is acted on by magnetism.

Faraday[4] discovered that when a plane polarized ray traverses a transparent diamagnetic medium in the direction of the lines of magnetic force produced by magnets or currents in the neighbourhood, the plane of polarization is caused to rotate.

M. Verdet[5] has since discovered that if a paramagnetic body, such as solution of perchloride of iron in ether, be substituted for the diamagnetic body, the rotation is in the opposite direction.

W. Thomson[6] has pointed out that no distribution of forces acting between the parts of a medium whose only motion is that of the luminous vibrations, is sufficient to account for the phenomena but that we must admit the existence of a motion in the medium depending on the magnetization, in addition to the vibratory motion which constitutes light.

The rotation by magnetism of the plane of polarization has been observed only in media of considerable density.

But the properties of the magnetic field are not so much altered by the substitution of one medium for another, or for a vacuum, as to allow us to suppose that the dense medium does anything more than merely modify the motion of the ether.

Does the ethereal medium have a motion of going on wherever magnetic effects are observed?

We suppose that this motion is rotation, having the direction of the magnetic force as its axis.*