Waves and ripples in water, air, and æther

by Sir J. A. Fleming

The electrical actions which take place in connection with an electric discharge between two conductors, one of which is charged positively and the other negatively, are exactly analogous to the above-described experiment with two air-vessels, one of which has air in it under compression, and the other has had the air removed from it. You will notice, however, that the oscillations of the air in the pipe in the air-vessel experiment depend essentially upon the fact that air is a substance which has inertia, or mass, and you will naturally ask what is it which has inertia, or its equivalent, in the electrical experiment? The answer to this question is as follows: Every electric circuit has a quality which is called inductance, in virtue of which an electric current cannot be started in it instantly, even under any electromotive force, and conversely when the current is started it cannot be immediately brought to rest. From the similarity of this quality of the circuit to the inertia of ordinary material substances, it has been sometimes called the electric inertia of the circuit. The word “inertia” really means inactivity, or laziness, but the term as used in mechanics implies something more than mere inactivity. It involves the notion of a persistence in motion when once the body is set moving.

When a material substance is in motion it possesses energy, and has the power of overcoming up to a certain point resistance to its motion. This energy-holding power, or capacity for storing up energy of motion, which is characteristic of all material substances, is a consequence of their inertia. The fact is otherwise expressed by stating that the mass of a material substance is one element in the production of energy of motion.

An electric current in one sense resembles a moving substance, for it is an exhibition of energy in association with matter. The current-energy is measured by the product of two factors: one is half the square of the current-strength, and the other is the inductance of the circuit. The analogy between the two cases may be more exactly brought out by pointing out that the energy of motion of a moving body is measured by the product of its mass and half the square of its velocity. Hence it follows that the power of overcoming resistance, or, in other words, of doing useful work or mischief, which is possessed by a heavy body in motion is proportional, not simply to its speed, but to the square of its speed. If a bullet, moving with a certain speed, can just pass through one plank 1 inch thick, then, when moving with twice the speed, it will pass through four such planks, and if moving with three times the speed, through nine planks of equal thickness. The energy of an electric current is similarly measured by the product of the inductance of the circuit and half the square of the current-strength. In the same or equal circuits two currents, the strengths of which are in the ratio of 1 to 2, have energies in the ratio of 1 to 4. The greater, therefore, the inductance of an electric circuit, the greater is the tendency of an electric current set flowing in it to run on after the electromotive force is withdrawn. The inductance of a circuit is increased by coiling it into a coil of many turns, and decreased by stretching it out in a straight line.

The important idea to grasp in connection with this part of the subject is that, just as there are two forms of mechanical energy, viz. energy of mechanical strain and energy of motion, so also there are two forms of electrical energy, viz. energy of electro-static strain and electric-current energy.

If, for instance, we bend a bow or extend a spring, this action involves the expenditure of mechanical energy, or work, and the energy so spent is stored up as energy of strain, or, as it is called, distorsional energy in the distorted bow or spring. When, however, the bow communicates its energy to the arrow or the spring to a ball, and so sets these in motion, we have in the flying arrow or ball a store of energy of motion. If a slip of steel spring is fixed at one end, and then set in vibration, we have a continual transformation of energy from the motional to the distorsional form. At one moment the spring is moving violently, and at the next it is bent to its utmost extent; and these states succeed each other. The store of energy in the vibrating spring is, however, gradually frittered away, partly because the continual bending of the steel heats it, and this heat dissipates some of the energy; but also because the spring, if vibrating quickly enough, imparts its energy to the surrounding air, and creates air waves, which travel away, and rapidly rob the vibrating spring of its stock of energy.

In a precisely similar manner all electrical oscillation effects depend upon the fact that electric energy can exhibit itself in two forms. In one form it is electro-static energy, or energy of electric strain. In this form we have it when we charge a Leyden jar. The glass is then, as explained, in a state of electrical strain, and its condition is analogous to that of a stretched spring. The same holds good when we have two conductors insulated from each other in air. We have then an electrical strain in the air. It is important, however, to notice that, since a perfect vacuum can support electric strain, it follows that, in the cases where air or glass constitute this non-conductor, or dielectric, of a condenser, the whole of the energy cannot be stored in the material substance, the glass or the air. The real storehouse of the energy is the æther, as modified by the presence of the ordinary matter in the same place.

When we discharge the Leyden jar or condenser, the electro-static energy in the dielectric disappears, and we obtain in its place an electric current in the connecting conductor; and this, as described, is an exhibition of energy in another form. If the resistance of the connecting conductor is small, then we have electrical oscillations established which consist in an alternate transformation of the energy from an electro-static form to the electric-current form.

At each oscillation some energy is frittered away into heat in the conductor, and if the conductor and condenser have a special form, energy may be rapidly removed from the system by the electric waves which are formed in the surrounding æther or dielectric. These waves consist in the propagation through the medium of lines of electric strain, just as an air wave consists in the propagation through the air of regions of air-compression, or a water wave consists in the propagation of an elevation on the surface.

Returning again to the discussion of the production of electrical oscillations, it is necessary to consider a little more in detail the manner in which we can create an electrical oscillation in what we have called an open electric circuit. Let me begin with an experiment, and it will then be easier for you to understand the particular points to be explained.