Every-day Science: Volume 6. The Conquest of Nature

by Henry Smith Williams; Edward Huntington Williams

Such being the case, we must consider this theory of Franklin's somewhat in detail. Perhaps we cannot do better than state the theory in the words of the celebrated physicist, Dr. Thomas Young, as given in his work on natural philosophy, published in 1807. By quoting from this old work we shall make sure that we are not reading any modern interpretations into the theory. "It is supposed," says Young, "that a peculiar ethereal fluid pervades the pores, if not the actual substance of the earth and of all other material bodies, passing through them with more or less facility, according to their different powers of conducting it; that particles of this fluid repel each other, and are attracted by particles of common matter; that particles of common matter also repel each other; and that these attractions and repulsions are equal among themselves, and vary inversely as to squares of the distances of the particles. The effects of this fluid are distinguished from those of all other substances by an attractive or repulsive quality, which it appears to communicate to different bodies, and which differs in general from other attractions and repulsions by its immediate diminution or cessation when the bodies, acting on each other, come into contact, or are touched by other bodies.... In general, a body is said to be electrified when it contains, either as a whole or in any of its parts, more or less of the electric fluid than is natural to it.... In this common neutral state of all bodies, the electrical fluid, which is everywhere present, is so distributed that the various forces hold each other exactly in equilibrium and the separate results are destroyed, unless we choose to consider gravitation itself as arising from a comparatively slight inequality between the electrical attractions and repulsions."

The salient and striking feature of this theory, it will be observed, is that the electrical fluid, under normal conditions, is supposed to be incorporated everywhere with the substance of every material in the world. It will be observed that nothing whatever is postulated as to the nature or properties of this fluid beyond the fact that its particles repel each other and are attracted by the particles of common matter; it being also postulated that the particles of common matter likewise repel each other under normal conditions.

At the time when Franklin propounded his theory, there was a rival theory before the world, which has continued more or less popular ever since, and which is known as the two-fluid theory of electricity. According to this theory, there are two uncreated and indestructible fluids which produce electrical effects. One fluid may be called positive, the other negative. The particles of the positive fluid are mutually repellent, as also are the particles of the negative fluid, but, on the other hand, positive particles attract and are attracted by negative particles. We need not further elaborate the details of this two-fluid theory, because the best modern opinion considers it less satisfactory than Franklin's one-fluid theory. Meantime, it will be observed that the two theories have much in common; in particular they agree in the essential feature of postulating an invisible something which is not matter, and which has strange properties of attraction and repulsion.

These properties of attraction and repulsion constituted in the early day the only known manifestations of electricity; and the same properties continue to hold an important place in modern studies of the subject. Electricity is so named simply because amber—the Latin electrum—was the substance which, in the experience of the ancients, showed most conspicuously the strange property of attracting small bodies after being rubbed. Modern methods of developing electricity are extremely diversified, and most of them are quite unsuggestive of the rubbing of amber; yet nearly all the varied manifestations of electricity are reducible, in the last analysis, to attractions and repulsions among the particles of matter.

As to the alleged immaterial fluids which, according to the theories just mentioned, make up the real substance of electricity, it was perfectly natural that they should be invented by the physicists of the elder day. All the conceptions of the human mind are developed through contact with the material world; and it is extremely difficult to get away, even in theory, from tangible realities. When the rubbed amber acquires the property of drawing the pith ball to it, we naturally assume that some change has taken place in the condition of the amber; and since the visible particles of amber appear to be unchanged—since its color, weight, and friability are unmodified—it seems as if some immaterial quality must have been added to, or taken from it. And it was natural for the eighteenth-century physicist to think of this immaterial something as a fluid, because he was accustomed to think of light, heat, and magnetism as being also immaterial fluids. He did not know, as we now do, that what we call heat is merely the manifestation of varying "modes" of motion among the particles of matter, and that what we call light is not a thing sui generis, but is merely our recognition of waves of certain length in the all-pervading ether. The wave theory of light had, indeed, been propounded here and there by a philosopher, but the theory which regarded light as a corpuscular emanation had the support of no less an authority than Sir Isaac Newton, and he was a bold theorist that dared challenge it. When Franklin propounded his theory of electricity, therefore, his assumption of the immaterial fluid was thoroughly in accord with the physical doctrines of the time.

MODERN VIEWS

But about the beginning of the nineteenth century the doctrine of imponderable fluids as applied to light and heat was actively challenged by Young and Fresnel and by Count Rumford and Humphry Davy and their followers, and in due course the new doctrines of light and heat were thoroughly established. In the light of the new knowledge, the theory of the electric fluid or fluids seemed, therefore, much less plausible. Whereas the earlier physicists had merely disputed as to whether we must assume the existence of two electrical fluids or of only one, it now began to be questioned whether we need assume the existence of any electrical fluid whatever. The physicists of about the middle of the nineteenth century developed the wonderful doctrine of conservation of energy, according to which one form of force may be transformed into another, but without the possibility of adding to, or subtracting from, the original sum total of energy in the universe. It became evident that electrical force must conform to this law. Finally, Clerk-Maxwell developed his wonderful electromagnetic theory, according to which waves of light are of electrical origin. The work of Maxwell was followed up by the German Hertz, whose experiments produced those electromagnetic waves which, differing in no respect except in their length from the waves of light, have become familiar to everyone through their use in wireless telegraphy. All these experiments showed a close relation between electrical phenomena, and the phenomena of light and of radiant heat, and a long step seemed to be taken toward the explanation of the nature of electricity.

The new studies associated electricity with the ether, rather than with the material substance of the electrified body. Many experiments seemed to show that electricity in motion traverses chiefly the surface of the conductor, and it came to be believed that the essential feature of the "current" consists of a condition of strain or stress in the ether surrounding a conductor, rather than of any change in the conductor itself. This idea, which is still considered valid, has the merit of doing away with the thought of action at a distance—the idea that was so repugnant to the mind of Faraday.

So far so good. But what determines the ether strain? There is surely something that is not matter and is not ether. What is this something? The efforts of many of the most distinguished experimenters have in recent years been directed toward the solution of that question; and these efforts, thanks to the new methods and new discoveries, have met with a considerable measure of success. I must not attempt here to follow out the channels of discovery, but must content myself with stating briefly the results. We shall have occasion to consider some further details as to the methods in a later chapter.

Briefly, then, it is now generally accepted, at least as a working hypothesis, that every atom of matter—be it oxygen, hydrogen, gold, iron, or what not—carries a charge of electricity, which is probably responsible for all the phenomena that the atom manifests. This charge of electricity may be positive or negative, or it may be neutral, by which is meant that the positive and negative charges may just balance. If the positive charge has definite carriers, these are unknown except in association with the atom itself; but the negative charge, on the other hand, is carried by minute particles to which the name electron (or corpuscle) has been given, each of which is about one thousand times smaller than a hydrogen atom, and each of which carries uniformly a unit charge of negative electricity.