Everywhere else, except at the nodes, there is action more or less energetic, but there is perpetual calm.
But how can we tell where the nodes are? When we recollect that they are points at which no wave-motion at all takes place it is easy to see that we shall at those points get no spark in our detector. So what Hertz did was to set his oscillator going so that it threw waves upon a reflecting surface and then move his detector to and fro in the neighbourhood until he found the nodes. Between the nodes, as will be seen by an inspection of the curves once more, there are other points at which the wave-action will be twice as great as with the single wave, and so at those points the response of the detector would be especially energetic.
This mutual action between an incident wave and a reflected wave is termed "interference," and by it the wave-lengths of all the ethereal waves have been measured. The plan used in the case of light waves, although the same in principle, is somewhat different because of the extreme shortness of the waves.
So the experiments of Hertz not only showed that long electric waves existed, but that they were in all essentials similar to light, and their wave-lengths were ascertained. On that basis has been built up modern wireless telegraphy.
It may be interesting to mention at this point a very curious, and in a sense pathetic, incident. Professor Hughes, whose name is associated with certain well-known instruments for ordinary telegraphy, nine years before Hertz' discovery noticed that a microphone was affected by the action of an induction coil some distance away. He himself attached some importance to the matter, but he allowed himself to be dissuaded from following up the discovery by other scientists, more eminent than himself at the time, who thought that it was not a promising field for investigation. But for the influence of these friends he would possibly be the hero of this story in place of Hertz.
Professor Silvanus Thomson has said that he too noticed the sparks produced at a distance when a Leyden jar was discharged, but he makes no claim to precedence over Hertz, since, seeing the phenomenon, he did not perceive its real meaning, while Hertz, though a little later in time, realised the profound significance of it.
Hertz himself in his account of his experiments is generous enough to assert that, had he not discovered the waves when he did, he is quite certain that Sir Oliver Lodge would have done so.
Before proceeding to describe the principal apparatus used in the wireless station I should like to devote a little space to the explanation of a term which will come up again and again, and which represents that which is responsible, in the main, for the marvellous advances which the art of sending wireless messages has achieved in the last few years. I refer to "resonance."
It will be a great help if the reader will try for himself a simple, inexpensive little experiment. Stretch a string horizontally across a room and on to it tie two other strings so that they hang down vertically a little distance apart. To the ends of the two strings tie some small objects—a cotton reel on each will answer admirably. They will thus form two pendulums, and, to commence with, they should be just the same length. Having rigged all this up, give one pendulum a good swing. It will impart motion of a to-and-fro variety to the supporting string, which in its turn will pass that motion on to the other pendulum. In a very short time, then, the second pendulum will be vibrating like the first. Indeed the whole motion of the first will shortly become transferred to the second, so that the second will be swinging and the first still. Then the second will re-transfer its energy back to the first, and so they will go on until the original energy given to the first pendulum is exhausted. The point to be observed is the quickness with which one pendulum responds to the impulses given it by the other, and the ease with which the energy of the one passes to the other.
Now reduce the length of one pendulum. On setting the first in motion a certain irregular spasmodic action is to be observed in the second, but it is very different from the "whole-hearted" response in the previous instance. In the former case the second one responded naturally and readily to the first. Now its response is reluctant in the extreme. It moves somewhat because it is forced to, but it is apparently unwilling. Energy has to be impressed upon it. There is no readiness, because there is no sympathy between them.