On Action at a Distance

I must ask you to go over very old ground, and to turn your attention to a question which has been raised again and again ever since man began to think.

— On Action at a Distance

The question is that of the transmission of force. We see two bodies at a distance from each other exert a mutual influence on each other's motion. Does this mutual action depend on the existence of some third thing, some medium of communication, occupying the space between the bodies, or do the bodies act on each other immediately, without the intervention of anything else?

— On Action at a Distance

The mode in which Faraday was accustomed to look at phenomena of this kind differs from that adapted by many modern inquirers, and my special aim will be to enable you to place yourselves at Faraday's point of view, and to point out the scientific value of that conception of lines of force which, in his hands, became the key to the science of electricity. ...

— On Action at a Distance

Why... should we not admit that the familiar mode of communicating motion by pushing and pulling... is the type and exemplification of all action between bodies, even in case in which we can observe nothing between...

— On Action at a Distance

But the advocates... maintain that even when the action is apparently the pressure of contiguous portions of matter... that a space always intervenes between... that so far from action at a distance being impossible, it is the only kind of action which ever occurs, and that the favorite old vis a tergo... exists only in the imagination of schoolmen.

— On Action at a Distance

[A]s for those who introduce ætherial, or other media... without any direct evidence... or any clear understanding of how the media work... the less these men talk about their philosophical scruples about admitting action at a distance the better.

— On Action at a Distance

The doctrine of direct action at a distance... was first asserted by , in his preface to the Principia... According to Cotes, it is by experience that we learn that all bodies gravitate. We do not learn in any other way that they are extended, movable, or solid. Gravitation, therefore, has as much right to be considered an essential property of matter...

— On Action at a Distance

[I]t was most essential that Newton's method should be extended to every branch of science to which it was applicable—that we should investigate the forces with which bodies act on each other... before attempting to explain how that force is transmitted. No men could be better fitted to apply themselves to the first part of the problem, than those who considered the second part quite unnecessary.

— On Action at a Distance

But Ampère, by a combination of mathematical skill with experimental ingenuity, first proved that two electric currents act on one another, and then analysed this action as a result of a system of push-and-pull forces between the elementary parts of these currents.The formula of Ampère, however, is of extreme complexity, as compared with Newton's law of gravitation, and many attempts have been made to resolve it into something of greater apparent simplicity.

— On Action at a Distance

Thus Faraday, with his penetrating intellect, his devotion to science, and his opportunities for experiments, was debarred from following the course of thought which had led to the achievements of the French philosophers, and was obliged to explain the phenomena to himself by means of a symbolism which he could understand, instead of adapting what had hitherto been the only tongue of the learned.

— On Action at a Distance

This new symbolism consisted of those lines of force extending themselves in every direction from electrified and magnetic bodies, which Faraday in his mind's eye saw as distinctly as the solid bodies from which they eminated.

— On Action at a Distance

The mathematicians saw in this experiment nothing but a method of exhibiting at one view the direction in different places of the resultant two forces, one directed to each pole of the magnet; a somewhat complicated result of a simple law of force.

— On Action at a Distance

But Faraday, by a series of steps as remarkable for their geometrical definiteness as for their speculative ingenuity, imparted to his conception of these lines of force a clearness and precision far in advance of that with which the mathematicians could then invest their new formulæ.

— On Action at a Distance

[E]ach individual line has a continuous existence in space and time. When a piece of steel becomes a magnet, or when an electric current begins to flow, the lines of force do not start into existence each in its own place, but as the strength increases new lines are developed within the magnet or current, and gradually grow outwards, so that the whole system expands from within, like in our former experiment.

— On Action at a Distance

Thus every line of force preserves its identity during the whole course of its existence, though its shape and size may be altered to any extent.

— On Action at a Distance

[E]very question relating to the forces acting on magnets or currents, or to the induction of currents in conducting circuits, may be solved by the consideration of Faraday's lines of force. In this place they can never be forgotten. By means of this new symbolism, Faraday defined with mathematical precision the whole theory of electro-magnetism, in language free from mathematical technicalities, and applicable to the most complicated and simplest cases.

— On Action at a Distance

He observed that the motion which the magnetic and electric force tends to produce is invariably such a to shorten the lines of force and allow them to spread out laterally from each other. He thus preserved in the medium a state of stress, consisting of a tension, like that of a rope, in the direction of the lines of force, combined with a pressure in all directions at right angles to them.

— On Action at a Distance

This is quite a new conception of action at a distance, reducing it to a phenomenon of the same kind as that action at a distance which is exerted by means of the tension of ropes and the pressure of rods.

— On Action at a Distance

When the muscles of our bodies are excited... the fibres tend to shorten themselves and at the same time expand laterally. A state of stress is produced in the muscle, and the limb moves. This explanation of muscular action is by no means complete...

— On Action at a Distance

For similar reasons we may regard Faraday's conception of a state of stress in the electro-magnetic field as a method of explaining action at a distance by means of the continuous transmission of force, even though we do not know how the state of stress is produced.

— On Action at a Distance

[O]ne of Faraday's most pregnant discoveries, that of the magnetic rotation of polarised light, enables us to proceed... Of two circularly polarised rays of light, precisely similar in configuration, but rotating in opposite directions, that ray is propagated with greater velocity which rotates in the same direction as the electricity of the magnetizing current.

— On Action at a Distance

It follows... as Sir W. Thomson has shewn by strict dynamical reasoning, that the medium when under the action of magnetic force must be in a state of rotation... [i.e.,] that small portions of the medium, which we may call molecular vortices, are rotating, each on its own axis, the direction of this axis being that of the magnetic force.

— On Action at a Distance

Here, then, we have an explanation of the tendency of the lines of magnetic force to spread out laterally and to shorten themselves. It arises from the of the molecular vortices.

— On Action at a Distance

We have thus found that there are several different kinds of work to be done by the electro-magnetic medium if it exists. We have also seem that magnetism has an intimate relation to light, and we know that there is a theory of light which supposes it to consist of the vibrations of a medium. How is this luminiferous medium related to our electro-magnetic medium?

— On Action at a Distance

[T]he only kind of electro-magnetic disturbance which can be propagated through a non-conducting medium is a disturbance transverse to the direction of propagation, agreeing... with what we know about that disturbance which we call light.

— On Action at a Distance

And these lines must not be regarded as mere mathematical abstractions. They are the directions in which the medium is exerting a tension like that of a rope, or rather, like that of our own muscles. The tension of the medium in the direction of the earth's magnetic force is in this country one grain weight on eight square feet. In some of Dr Joule's experiments, the medium has exerted a tension of 200 lbs. per square inch.

— On Action at a Distance

But the medium, in virtue of the very same elasticity by which it is able to transmit undulations of light, is also able to act as a spring. When properly wound up, it exerts a tension, different from the magnetic tension, by which it draws oppositely electrified bodies together, produces effects through the length of telegraph wires, and when of sufficient intensity, leads to the rupture and explosion called lightning.

— On Action at a Distance

These are some of the already discovered properties of that which has been called vacuum, or nothing at all. They enable us to resolve several kinds of action at a distance into actions between contiguous parts of a continuous substance. Whether this resolution is of the nature of explication or complication, I must leave to the metaphysicians.

— On Action at a Distance