crash were over in a fraction of a second. If the experimenter was asked to explain this mimic lightning, probably he said it was due to the flowing together of two charges of electricity previously communicated to two metallic sheets fixed respectively on the inner and outer surfaces of the jar, and no doubt this explanation was sufficient for its immediate purpose. But it was very far from telling the whole story. For what the observer saw was not, as he may have supposed, the result of a single rush of electricity, but was the outcome of a series of rushes backwards and forwards between the two discharging spheres, which followed one another at a rate that may have been as small as ten thousand, or as great as ten million, or even a hundred million, in a single second of time. The correctness of this description of the discharge of a Leyden jar has been established by examining the reflections of similar but less rapidly oscillating sparks in revolving mirrors, when there is seen in the reflection of each spark, not a continuous band of light, but definite fluctuations such as would correspond to a succession of separate discharges.

3

We all know that waves are generated in still water when It is disturbed by the shock of a falling stone; that sound-waves can be started in the air by the vibrations of a tuning-fork; we believe that luminous waves, or light waves, are started in the ether by the shock of flint hitting upon hard steel and, similarly, electricians, guided by Faraday, Clerk Maxwell and Fitzgerald, for some time have believed that the electric displacements which take place during the violent oscillations that constitute the discharges of a Leyden jar generate electromagnetic

3 Sir Oliver Lodge has shown that by using a large battery of jars the oscillations may be reduced to 5,000 or even to 500 per second, when it becomes comparatively easy to detect them with a mirror.

4

disturbances or waves, and thus radiate energy into the surrounding ether. For a long time, however, it was impossible to prove the existence of these electric waves, because, as I have already said, they are quite invisible to the human eye.

Dr. Joseph Henry seems to have come very near to the discovery of electric waves about sixty years ago, when, after describing how it had been found possible to magnetize steel needles by means of a single spark from the conductor of an electric machine, though the needles were thirty yards away and separated from the spark by two floors and two ceilings, he went on to say: "It may be inferred that the diffusion of motion in this experiment is almost comparable with that from a spark from a flint and steel in the case of light." But he missed the opportunity, and it was left to Hertz to make the actual discovery forty years later.

The Leyden jar in its ordinary form was not very suitable for the experiments designed by Hertz, and to obtain his oscillating discharges he employed a somewhat different arrangement of apparatus. You could construct a model of this apparatus from very simple materials. All you would have to do would be to obtain a few yards of stout copper wire well coated on its surface with paraffin wax or some other insulating material, to wind this wire round a large wooden reel, keeping its ends free, and then to dip the whole in melted paraffin to complete the insulation of the copper wire. Next you would have to obtain a much greater length of much finer copper wire, also well insulated, to wind this round the reel outside the inner coil of thick wire, and attach its two ends to an ar

A part of this energy at least never returns, and this, no doubt, is one reason why the discharge dies out after a life which only lasts for a fraction of a second.

rangement like that drawn below, in which A A' represent two metallic plates about sixteen inches square and B B two stout metallic wires each carrying a well-polished sphere C C. Finally, you would have to join the two ends of the stout inner wire of your coil to a small galvanic battery provided with an arrangement by which the current from the battery could be sent through the wire or be cut off from it smartly at short intervals." Then you would have a rough model of an "induction coil" with a Hertz oscillator.

B

I fear that home-made apparatus in the simple form described above would not enable you to reproduce the results obtained by Hertz. But if the home-made coil were replaced by a similar coil constructed by Mr. Apps, let us say, and if this were joined up to an oscillator such as that which I have described, you would be in a position to attempt to repeat these wonderful experiments. The process would

be quite simple. You would have to connect a galvanic battery to the coil. to connect the terminals of the thin, secondary wire of the coil to the oscillator, to start the automatic arrangement for making and breaking the current in the thick, primary wire of the coil, and to adjust the position of the spheres cc. This would bring about a rapid discharge of sparks between the spheres, producing an effect not unlike the discharge of a Leyden jar. but more continuous, and for that and other reasons more convenient to work with. The discharge of this instrument, like that of the Leyden jar. would not consist of a single spark, nor even of a succession of sparks all This should act automatically.

passing in the same direction, but of series of violent oscillations in which electricity would jump to and fro across the spark gap, as already described, at a rate corresponding to millions of jumps per second, the exact rate depending on the details of the construction of the coil and oscillator employed. Also this discharge, like the Leyden jar discharge, would, according to modern theory, generate waves in the ether which would travel with the velocity of light, and would be recognized by our eyes if only these were sensitive to large waves like electric waves.

The "electric eye" or "resonator" by which Hertz succeeded in showing the existence of electric waves was very simple. It consisted of a piece of wire bent into a ring and provided at its ends with two polished metallic spheres, whose distance from each other could be adjusted very accurately by a fine screw provided for the purpose. The dimensions of this apparatus were varied, of course, according to those of the generator whose waves it was to detect. When this simple instrument was held horizontally in a proper position relatively to that of the "oscillator," and not too far off, the waves radiated by the former, as they reached the detector, set up secondary oscillations in the latter, and these soon made their existence manifest by small sparks which jumped across the gap between its knobs.

But though the sparks thus generated in the resonator strongly suggest the existence of waves in the neighborhood of the Hertz oscillator, they do not by themselves exactly prove the existence of these waves; and, in fact, something very like them had been observed by, among others, a well-known English physicist several years before Hertz made his experiments, without its dawning upon the

mind of our fellow-countryman that he had before him evidence of electric waves crossing space. But this idea did occur to Hertz, and he established the true nature of the phenomena by a series of convincing experiments, in the course of which he studied the properties of these waves, and made it clear that they were waves in the ether and differed from light waves only in size.

Light waves, as we know, can be reflected, refracted, and polarized, and Hertz established the existence of invisible electric waves passing across the space which separated his oscillator from his resonator, or receiver, by proving they could be reflected, refracted, and polarized like light itself. By placing his oscillator at the focus of a parabolic mirror, he produced waves that travelled across the laboratory to a second mirror, which reflected them, like a ray of light, to a focus, where he was able to recognize them by the sparks they produced when a detector was brought to that spot. In another experiment Hertz found that when he placed a great prism of pitch in the path followed by the waves, they passed as freely through this as light does through a prism of glass, and moreover that they were bent out of their course by the pitch prism much as a ray of light is bent or refracted by glass or rock crystal. In short, he showed by these and other tests that waves passed from the oscillator to the receiver, and that these waves answered to the tests for light. They were propagated in straight lines across space, they could be reflected, refracted, and polarized. These observations were soon fully confirmed by Sir Oliver Lodge and other electricians. In the course of his experiments, Hertz made the notable discovery that, unlike the more familiar, visible waves of light. electric waves pass freely through doors, wooden

floors, and even through stone walls and masses of pitch of great thickness, though all these things, as we very well know, are practically impenetrable to light. These novel facts do not, however, weaken the conclusions drawn by Hertz, for even glass, transparent as it seems to us, is opaque to the short ultraviolet light rays, though these pass freely through lenses and prisms made of quartz. Hence there is nothing inherently improbable in this interesting and peculiar quality of the large electric waves.

We now have before us some of the fundamental phenomena available for the purpose of "wireless telegraphy," and it may be pointed out that, in effect, Hertz not only discovered the existence of electric waves, but also despatched and received the first "ether wave" message when he generated electric waves in his laboratory at Karlsruhe and detected them sixteen or seventeen yards away from their point of origin, though closed doors and in some cases stone walls separated the apparatus which generated the waves from the receiver. In the Hertz experiment, when the coil was in action, one plate of the oscillator became positively and the other negatively electrified. When the effect became sufficiently intense, the electricity overcame the resistance of the air between the spheres cc and a spark crossed the spark gap. At each discharge, electricity from the positive plate, A let us say. rushed across the spark gap to A', and, overrunning itself, made A' positive, and left A, which at first was positive, negative, then instantly surged back again from A to A, then once more jumped from A to A', and so on over and over again, each oscillation occupying, perhaps, the one-hundred-millionth part of a second of time, according to the dimensions of the apparatus employed. At each oscillation a wave was generated, and travelled away

from the spark gap with the velocity of light. The intensity of this wave was not equal in every direction, but was greatest at right angles to the rods bearing the spheres, and at right angles to the plane of the metallic plates, and by holding his receiver in the position in which it came most completely under the influence of these waves Hertz was able to detect them in the manner already described. Those who are musical will readily admit the reasonableness of this interpretation of the action of a Hertz oscillator on a distant receiver, when they remember that the sound waves emitted by a tuningfork, after passing through the air, will set a second tuning-fork singing provided that both tuning-forks are tuned to the same note.

Here, then, we have in embryo the art of wireless telegraphy. It consists in producing electric waves similar to light waves, and in detecting them at a distance by means of a tuned or "syntonized" receiver.

I need hardly say that it is one thing to detect an electric wave fifteen or twenty yards away from its point of origin, and quite another thing to detect it after it has travelled scores or perhaps hundreds of miles over land or sea; and I must add that even when this is done there remain at least two difficult problems. First, to make the wave print the message it carries in black and white for our eyes to see; and, secondly, to secure that the message shall go into the hands intended to receive it and into no others. Now the second of these problems has proved very difficult, and thus it comes about that though great things have been done since Hertz taught us how to create and detect electric waves, the art of wireless telegraphy still seems by no means to have taken its final form.

I should like, if space permitted, to continue my story by explaining the ingenious and often brilliant devices by

which Sir William Preece, Sir Oliver Lodge, Mr. Branly, Mr. Marconi, and others have brought us step by step towards the goal which all have aimed at. The main features of the problem are simple enough. In order that signals might be conveyed by waves in the ether over distances of hundreds of miles, it was necessary to increase enormously the energy of the oscillator, and those who have stood by the great Marconi installation in the fields at Poldhu, and can compare it in their minds with the little construction of wires and metallic plates employed by Hertz, will be able to form a fair, if rather vague, idea of what has been done in this direction. It was necessary, also, to supplement the more powerful generating installations by far more delicate receivers than those of Hertz; and, finally, it was necessary to devise means of recording the messages, and to discover some way of tuning or syntonizing the receivers in order that the messages might never reach any one except those who were entitled to receive them. Some of these requirements have been fulfilled in a considerable degree. Every one knows that installations capable of generating waves that can be detected at considerable distances from their sources exist on land and sea by the dozen and the hundred. The Hertz receiver has been replaced by others far more delicate, and these in their turn have been so applied that when brought under the influence of electric waves they set in action contrivances for printing messages more or less similar to those already in use in the old-fashioned system of sending messages by means of electricity. Yet, in spite of all this progress, there have been difficulties. Much has been accomplished, but the thing has never been a perfect success.

Let us suppose that we desired to give a signal by making one tuningfork start a musical note in another

placed some distance away from the first. In such a case we could only hope to succeed if the transmitter and the receiver were in tune with each other. Hopeless confusion would arise if this condition were not fulfilled, or if it were impossible to produce with the tuning-fork used as transmitter a sustained train of vibrations of a definite character. Now sound waves are aerial waves, and may not, strictly speaking, be compared with the ethereal waves of wireless telegraphy, but, nevertheless, this illustration will serve to indicate the kind of obstacle which has stood in the way of those who have been engaged upon the problem of wireless or, as it is sometimes called, ethereal telegraphy. For in the case of the oscillator and resonator used in wireless telegraphy it is necessary that the transmitter shall emit sustained vibrations of definite character, like the note emitted by an efficient tuning-fork; and this condition is so difficult, if not impossible, to secure by means of the spark discharge of Leyden jars or Hertzian oscillators that not a few of those who have followed the progress of experiment on the subject have thought for some time that in the end it might prove necessary to abandon the use of spark discharges altogether and seek a solution in some other direction. Today this forecast seems not far from being realized, for quite recently an alternative mode of treating the problem has come to the front.

When a current of electricity passes between two rods of gas carbon placed with their ends pretty close to one another but out of actual contact, we find at the gap between the ends of the rods a very intense source of light. This is the familiar electric "arc." Now the electric arc is a very Caliban among the known sources of light. It is difficult to control, and when ever so little out of hand is apt to produce, if it is a large arc, roars and screams of

the most nerve-shattering description. But in spite of the difficulty thus introduced Mr. Duddell, a distinguished electrician, has contrived so to order the proceedings of the "are" when it is fed by a continuous current, that not only does he make it roar like a sucking dove, but actually educes from it musical notes-indeed, I had almost said, uses it as a musical instrument; and what is even more interesting and important to us, this modern genius of the lamp has shown us that if the two carbon rods of the arc are connected to what is known as a "shunt circuit" possessing self-induction and including a condenser, there are produced in the shunt continuous trains of electric oscillations which are in harmony, so to speak, with the vibrations of the arc. Unfortunately, the oscillations produced in this way do not exceed thirty or forty thousand a second, and slow oscillations like these, as you will understand, would be quite useless for the purposes of wireless telegraphy. Fortunately, however, the suggestion .conveyed by Mr. Duddell's experiments has not been fruitless. His discoveries have set others thinking and working. with the result that Mr. Poulsen, an eminent Danish physicist, has gone a step further and shown us that by running an arc similar to that of Mr. Duddell under the influence of a powerful magnet, at the same time lengthening it, that is increasing the gap between the carbon points, and surrounding it with an atmosphere of the light gas hydrogen, he can increase the frequency of its oscillations so immensely that as many as a million per second can be obtained.

We can form a helpful picture of the difference between Mr. Poulsen's method of producing "undamped" electric waves and its older rival, if, once more, we compare sound waves with electric waves and use an image suggested, I believe, by Mr. Poulsen