If the hand-key is raised or pressed, it is possible to make long or short torrents of secondary sparks.
Suppose, then, that we connect to the secondary spark-balls two long insulated rods, and place the spark-balls about ¹⁄₄ inch apart. On pressing the hand-key, we obtain a peculiarly bright crackling spark between the balls, which is an oscillatory spark, and at the same time, as already described, electrical oscillations are set up in the rods and electric waves given off. We may represent to ourselves these electrical oscillations in the rods as similar to the mechanical vibrations which would be set up in a long elastic wooden rod, clamped at the middle and set in vibration at the ends. Or we may consider them similar to the fundamental vibrations of an open organ-pipe, the middle of the pipe corresponding with the middle of the rod. In comparing the mechanical vibrations of the rod or the acoustic vibration of the air in the organ-pipe with the electrical oscillations in the long rods, we must bear in mind that the displacement of the rod or the air in the organ-pipe at any point corresponds with electrical pressure, or potential, as it is called, at any point in the long oscillator. Hence, bearing in mind the remarks in the fourth lecture, it will be evident to you that just as the length of the air wave emitted by the open organ-pipe is double the length of the pipe, so the length of the electric wave thrown off from the pair of long rods is double their total length.
Instead of using a pair of rods for the electrical oscillator, it was found by Mr. Marconi to be an improvement to employ only one insulated rod, held vertically, and to connect it to one spark-ball of the coil, and to connect the opposite spark-ball to a metal plate buried in the earth. Then, when the spark-balls are placed a little apart and the hand-key pressed, we have a torrent of oscillatory sparks between the “earthed ball” and the insulated rod ball. This sets up in the rod electrical oscillations, which run up and down the rod. It is easy to show that there is a strong electric current passing into and out of the rod by connecting it to the spark-ball by means of a piece of fine wire. When the sparks are taken, we find this wire will become hot, it may be red hot, or sometimes it may be melted.
By applying the principles already explained, it is not difficult to demonstrate that in the case of an oscillator consisting of a single rod connected to one spark-ball the electric waves thrown off are in wave-length four times the length of the rod.
The electrical actions taking place, therefore, are as follows: At each interruption of the primary current of the spark-coil there is an electromotive force created in the secondary circuit, which gradually charges up the insulated rod until it attains a state in which it is said to be at a potential or electrical pressure of several thousand volts. The spark then happens between the balls, and the rod begins to discharge.
This process consists, so to speak, in draining the electric charge out of the rod, and it takes the form of an electric current in the rod, which has a zero value at the top insulated end, and has its maximum value at the spark-ball end.
Also, when the oscillations take place, we have variations of electric pressure, or potential, which are at a maximum at the upper or insulated end, and have a zero value at the spark-ball end. From the rod we have a hemispherical electric wave radiated. In the language of wireless telegraphists, such a simple insulated rod is called an insulated aerial, or an insulated antenna.
A simple insulated aerial has, however, a very small electrical capacity, and it can store up so little electric energy that the whole of it is radiated in the first oscillation. Hence, strictly speaking, we have no train of electric waves radiated, but merely a solitary wave or electric impulse. The effect on the æther thus produced corresponds to the effect on the air caused by the crack of a whip or an explosion, and not to a musical note or tone as produced by an organ-pipe.
We can, however, make an arrangement which is superior in electric wave-making power, as follows:—
