Part 33

Such a near bond of connection between man and the rest of the animate creation is regarded by many as derogatory to our dignity. But we have already had to exchange the pleasing conceptions indulged in by poets and theologians as to the high position in the scale of being held by our early progenitors for humble and more lowly beginnings, the joint labours of the geologist and archæologist having left us in no doubt of the ignorance and barbarism of Palæolithic man.

It is well, too, to remember that the high place we have reached in the scale of being has been gained step by step, by a conscientious study of natural phenomena, and by fearlessly teaching the doctrines to which they point. It is by faithfully weighing evidence without regard to preconceived notions, by earnestly and patiently searching for what is true, not what we wish to be true, that we have attained to that dignity, which we may in vain hope to claim through the rank of an ideal parentage.

JAMES CLERK MAXWELL

A Treatise on Electricity and Magnetism

James Clerk Maxwell, the first professor of experimental physics at Cambridge, was born at Edinburgh on November 13, 1831, and before he was fifteen was already famous as a writer of scientific papers. In 1854 he graduated at Cambridge as second wrangler. Two years later he became professor of natural philosophy at Marischal College, Aberdeen. Vacating his chair in 1860 for one at King's College, London, Maxwell contributed largely to scientific literature. His great lifework, however, is his famous "Treatise on Electricity and Magnetism," which was published in 1873, and is, in the words of a critic, "one of the most splendid monuments ever raised by the genius of a single individual." It was in this work that he constructed his famous theory if electricity in which "action at a distance" should be replaced by "action through a medium," and first enunciated the principles of an electro-magnetic theory of light which has formed the basis of nearly all modern physical science. He died on November 5, 1879.

I.--The Nature of Electricity

Let a piece of glass and a piece of resin be rubbed together. They will be found to attract each other. If a second piece of glass be rubbed with a second piece of resin, it will be found that the two pieces of glass repel each other and that the two pieces of resin are also repelled from one another, while each piece of glass attracts each piece of resin. These phenomena of attraction and repulsion are called electrical phenomena, and the bodies which exhibit them are said to be "electrified," or to be "charged with electricity."

Bodies may be electrified in many other ways, as well as by friction. When bodies not previously electrified are observed to be acted on by an electrified body, it is because they have become "electrified by induction." If a metal vessel be electrified by induction, and a second metallic body be suspended by silk threads near it, and a metal wire be brought to touch simultaneously the electrified body and the second body, this latter body will be found to be electrified. Electricity has been transferred from one body to the other by means of the wire.

There are many other manifestations of electricity, all of which have been more or less studied, and they lead to the formation of theories of its nature, theories which fit in, to a greater or less extent, with the observed facts. The electrification of a body is a physical quantity capable of measurement, and two or more electrifications can be combined experimentally with a result of the same kind as when two quantities are added algebraically. We, therefore, are entitled to use language fitted to deal with electrification as a quantity as well as a quality, and to speak of any electrified body as "charged with a certain quantity of positive or negative electricity."

While admitting electricity to the rank of a physical quantity, we must not too hastily assume that it is, or is not, a substance, or that it is, or is not, a form of energy, or that it belongs to any known category of physical quantities. All that we have proved is that it cannot be created or annihilated, so that if the total quantity of electricity within a closed surface is increased or diminished, the increase or diminution must have passed in or out through the closed surface.

This is true of matter, but it is not true of heat, for heat may be increased or diminished within a closed surface, without passing in or out through the surface, by the transformation of some form of energy into heat, or of heat into some other form of energy. It is not true even of energy in general if we admit the immediate action of bodies at a distance.

There is, however, another reason which warrants us in asserting that electricity, as a physical quantity, synonymous with the total electrification of a body, is not, like heat, a form of energy. An electrified system has a certain amount of energy, and this energy can be calculated. The physical qualities, "electricity" and "potential," when multiplied together, produce the quantity, "energy." It is impossible, therefore, that electricity and energy should be quantities of the same category, for electricity is only one of the factors of energy, the other factor being "potential."

Electricity is treated as a substance in most theories of the subject, but as there are two kinds of electrification, which, being combined, annul each other, a distinction has to be drawn between free electricity and combined electricity, for we cannot conceive of two substances annulling each other. In the two-fluid theory, all bodies, in their unelectrified state, are supposed to be charged with equal quantities of positive and negative electricity. These quantities are supposed to be so great than no process of electrification has ever yet deprived a body of all the electricity of either kind. The two electricities are called "fluids" because they are capable of being transferred from one body to another, and are, within conducting bodies, extremely mobile.

In the one-fluid theory everything is the same as in the theory of two fluids, except that, instead of supposing the two substances equal and opposite in all respects, one of them, generally the negative one, has been endowed with the properties and name of ordinary matter, while the other retains the name of the electric fluid. The particles of the fluid are supposed to repel each other according to the law of the inverse square of the distance, and to attract those of matter according to the same law. Those of matter are supposed to repel each other and attract those of electricity. This theory requires us, however, to suppose the mass of the electric fluid so small that no attainable positive or negative electrification has yet perceptibly increased or diminished the mass or the weight of a body, and it has not yet been able to assign sufficient reasons why the positive rather than the negative electrification should be supposed due to an excess quantity of electricity.

For my own part, I look for additional light on the nature of electricity from a study of what takes place in the space intervening between the electrified bodies. Some of the phenomena are explained equally by all the theories, while others merely indicate the peculiar difficulties of each theory. We may conceive the relation into which the electrified bodies are thrown, either as the result of the state of the intervening medium, or as the result of a direct action between the electrified bodies at a distance. If we adopt the latter conception, we may determine the law of the action, but we can go no further in speculating on its cause.

If, on the other hand, we adopt the conception of action through a medium, we are led to inquire into the nature of that action in each part of the medium. If we calculate on this hypothesis the total energy residing in the medium, we shall find it equal to the energy due to the electrification of the conductors on the hypothesis of direct action at a distance. Hence, the two hypotheses are mathematically equivalent.

On the hypothesis that the mechanical action observed between electrified bodies is exerted through and by means of the medium, as the action of one body on another by means of the tension of a rope or the pressure of a rod, we find that the medium must be in a state of mechanical stress. The nature of the stress is, as Faraday pointed out, a tension along the lines of force combined with an equal pressure in all directions at right angles to these lines. This distribution of stress is the only one consistent with the observed mechanical action on the electrified bodies, and also with the observed equilibrium of the fluid dielectric which surrounds them. I have, therefore, assumed the actual existence of this state of stress.

Every case of electrification or discharge may be considered as a motion in a closed circuit, such that at every section of the circuit the same quantity of electricity crosses in the same time; and this is the case, not only in the voltaic current, where it has always been recognised, but in those cases in which electricity has been generally supposed to be accumulated in certain places. We are thus led to a very remarkable consequence of the theory which we are examining, namely, that the motions of electricity are like those of an incompressible fluid, so that the total quantity within an imaginary fixed closed surface remains always the same.

The peculiar features of the theory as developed in this book are as follows.

That the energy of electrification resides in the dielectric medium, whether that medium be solid or gaseous, dense or rare, or even deprived of ordinary gross matter, provided that it be still capable of transmitting electrical action.

That the energy in any part of the medium is stored up in the form of a constraint called polarisation, dependent on the resultant electromotive force (the difference of potentials between two conductors) at the place.

That electromotive force acting on a dielectric produces what we call electric displacement.

That in fluid dielectrics the electric polarisation is accompanied by a tension in the direction of the lines of force combined with an equal pressure in all directions at right angles to the lines of force.

That the surfaces of any elementary portion into which we may conceive the volume of the dielectric divided must be conceived to be electrified, so that the surface density at any point of the surface is equal in magnitude to the displacement through that point of the surface reckoned inwards.

That, whatever electricity may be, the phenomena which we have called electric displacement is a movement of electricity in the same sense as the transference of a definite quantity of electricity through a wire.

II.--Theories of Magnetism

Certain bodies--as, for instance, the iron ore called loadstone, the earth itself, and pieces of steel which have been subjected to certain treatment--are found to possess the following properties, and are called magnets.

If a magnet be suspended so as to turn freely about a vertical axis, it will in general tend to set itself in a certain azimuth, and, if disturbed from this position, it will oscillate about it.