Waves and ripples in water, air, and æther

by Sir J. A. Fleming

Now, at this point I must recall to your recollection that, in speaking about the production of air waves, I pointed out that one condition essential to the production of an air wave was that there must be a very sudden application or release of the air-pressure, such as is caused by an explosion or escape of compressed air. We cannot produce an air wave by moving any object such as a fan slowly to and fro through the air. In order to produce an air wave we must strike the air a very sudden blow, or, which comes to the same thing, we must apply and remove a very sudden pressure to the air; and under these circumstances we start into existence an air wave, which travels away from the vibrating or rapidly moving body, and continues its journey out into surrounding space.

I want to show you that, in the case of the Hertz oscillator, these very sudden reversals of electric strain in the air or space round about it, which take place at the moment when the oscillatory spark passes between the knobs, creates in a similar manner what is called an electric wave, which travels out into the space around. The point you must appreciate is, that just as an air wave conveys away to distant places a rapidly alternating compression made in the air by a vibrating body at a particular place, so an electric wave conveys away to distant places an alternating electric strain, which is originated at some point in the medium by the oscillatory discharge of some form of condenser. Before, however, we can demonstrate this fact, we must have some means for detecting the influence of what we call an electric wave. You will remember that, in the case of experiments with air waves, I used a sensitive flame in order to make evident to you the presence of waves in the air which you could not see, so here I must use an appropriate detector for electric waves, the operation of which will render evident to us the existence in the space round our electric oscillator of the electric waves we cannot see.

Time will not permit me to discuss all the different forms of electric-wave detector which have been invented. For our present purposes we must limit ourselves to the description of one plan, which depends on the remarkable fact that finely powdered dry metal or metallic filings are non-conductors of the electric current until they are subjected to an electromotive force exceeding a certain value, when the metallic filings at once pass into a conductive condition.

If you recall the remarks made just now in connection with the special electrical properties of air and other gases, you will notice that there is a remarkable similarity between the electrical behaviour of air at ordinary pressures to electromotive force, and that of a loose mass of metallic filings. Both the air and the metallic filings are non-conductors as long as the electromotive force acting on them does not exceed a certain value, but if it exceeds this critical value, they pass at once into a conductive condition. The fact that pieces of metal in loose contact with one another behave in a similar manner was discovered more than twenty years ago by the late Professor D. E. Hughes, who, as you may perhaps know, was the inventor of a printing telegraph, the microphone, and many other most important electrical instruments. Professor Hughes was a great genius, and in many respects in advance of his age. He it was who undoubtedly discovered that an electric spark has the power of affecting at a distance the electric conductivity of a metallic junction consisting of two metals in loose contact.

The peculiar behaviour of metallic filings under electromotive force, and under the influence of electric sparks at a distance, was subsequently rediscovered by Professor Branly; and the effect of an electric oscillatory spark in changing the conductivity of a light metal contact was also rediscovered by Sir Oliver Lodge, and the phenomena investigated by many other observers. I can show you the experiment on a large scale in the following manner:⁠—

Fig. 70.—A metal disc-coherer.

I have here a number of aluminium discs, the size of sixpences, stamped out of thin metal, and these are arranged in a sort of semi-cylindrical trough between two terminal screws, so that the discs are very lightly pressed together. Under these circumstances the pile of metal discs is not a conductor, and it will not pass the electric current from a battery which is joined up in series with an electric bell and the pile of discs ([see Fig. 70]). Supposing, however, that I make an oscillatory spark in proximity to this pile of metal discs, as I can do by taking the discharge from a large Leyden jar near it; the pile of discs at once becomes a conductor; the electric current from the battery can then pass through it, and the bell rings. Such an arrangement has been named by Sir Oliver Lodge a coherer, because, under the action of the oscillatory spark, the discs cohere or stick together. We can separate the discs by giving them a sharp rap, and then the operation can be again repeated.

A much more sensitive arrangement can be made by taking a small box of wood through the bottom of which pass two nickel wires which are parallel to one another, but not in contact. In this box is placed a small quantity of very finely powdered metallic nickel or nickel filings, and if the quantity of these filings is properly adjusted, it is possible to make an arrangement which possesses the property that there is no conductivity between the two nickel wires under ordinary circumstances, but that they become conductively connected to one another the moment an oscillatory electric spark is made in the neighbourhood. We shall speak of this contrivance as an electric wave indicator, and we shall employ it in subsequent experiments to enable us to detect the presence of an electric wave.

We must then return for a moment to the consideration of the production of electrical oscillations in circuits of various kinds. I trust it has been made plain to you that if two metallic surfaces, separated by a non-conductor such as air or glass, are acted upon by an electromotive force, the non-conductor becomes electrically strained. Another way of stating this is to say that a positive charge of electricity exists on one metal surface, and a negative charge on the other. The only objection which can be raised to expressing the facts in this manner is that it fastens attention rather upon the conductors than upon the insulator, which is the real storehouse of the energy. If these two metal surfaces are then connected together by a conductor of low resistance, the charges disappear by a series of oscillations, and the result is an electric current in the conducting circuit connecting the plates, which rushes backwards and forwards in the circuit, but gradually diminishes in strength until it completely dies away. You may picture to yourselves the electrical effect as analogous to the following experiment with two air-vessels: Supposing we have two strong steel bottles, into one of which we compress a quantity of air, and in the other we make a vacuum by pumping out nearly all the air. These vessels would correspond with two conductors, one charged with positive electricity and the other with negative. Imagine these vessels connected by a wide pipe in which is placed a tap or valve, which can be opened suddenly so as to permit the air to rush over from the full vessel to the empty one. If this is done, it is a matter of experience that the equality of pressure between the two vessels is not at once established, but in virtue of the inertia quality of the air, it only takes place after a series of oscillations of air in the pipe. In rushing over from the full vessel to the empty one the air, so to speak, overshoots the mark, and the state of the vessels as regards air-pressure is exactly interchanged. The air then rushes back again, and it is only after a series of to-and-fro movements of the air in the pipe that an exact equality of pressure in the two vessels is attained.