Fig. 15.—Alternating-current Double-transformation Power Plant for Generating Electric Waves (Fleming). a, alternator; H1H2, choking coil; K, signalling key; T, step-up transformer; S1S2 spark-gap; C1C2 condensers; T1T2, oscillation transformers; A, aerial; E, earthplate.
We turn, in the next place, to the consideration of those devices for putting more power into the aerial than can be achieved when the aerial itself is simply employed as the reservoir of energy. Professor Braun, of Strassburg, in 1899, described a method for doing this by inducing oscillations in the aerial by means of an oscillation transformer, these oscillations being set up by the discharges from a Leyden jar or battery of Leyden jars, which formed the reservoir of energy. The induction coil is employed to produce a rapidly intermittent series of electrical oscillations in the primary coil of an oscillation transformer by the discharge through it of a Leyden jar. Mr. Marconi immensely improved this arrangement, as described by him in a lecture given before the Society of Arts on May 17, 1901, by syntonising the two circuits and making the circuit, consisting of the capacity of the aerial and the inductance of the secondary circuit of the oscillation transformer, have the same time-period as the circuit consisting of the Leyden jars, or energy-storing condenser, and the primary circuit of the oscillation transformer, and by so doing immensely added to the power and range of the apparatus.
Starting from these inventions of Braun and Marconi, the author devised a double transmission system in which the oscillations are twice transformed before being generated in the aerial, each time with a multiplication of electromotive force and a multiplication of the number of groups of oscillations per second. This arrangement can best be understood from the diagram (see Fig. 15).
In this case a transformer, T, or transformers receive alternating low-frequency current from an alternator, a, being regulated by passing through two variable choking coils, H1 and H2, so as to control it. This alternating current is transformed up from a potential of two thousand to twenty, forty or a hundred thousand, and is employed to charge a large condenser, C1, which discharges across a primary spark-gap, S1, through the primary coil of an oscillation transformer, T1. The secondary circuit of the oscillation transformer is connected to a second pair of spark balls, S2, which in turn are connected by a secondary condenser, C2, and the primary circuit of a third transformer, T2 and the secondary circuit of this last transformer are inserted between a Marconi aerial, A, and the earth E. When all these circuits are tuned to resonance by Mr. Marconi's methods, we have an enormously powerful arrangement for creating electric waves, or rather trains of electric waves, sent out from the aerial, and the oscillations are controlled and the signals made by short-circuiting one of the choking coils.
Another transmitting arrangement, which involves a slightly different principle, and employs no oscillation transformer, is one due also to Professor Braun. In this case, a condenser and inductance are connected in series to the spark balls of an induction coil, and oscillations are set up in this circuit. Accordingly, there are rapid fluctuations of potential at one terminal of the condenser. If to this we connect a long aerial, the length of which has been adjusted to be one quarter of the length of wave corresponding to the frequency, in other words, to make it a quarter-wave resonator, then powerful oscillations will be accumulated in this rod. The relation between the height (H) of the aerial and the frequency is given by the equation 3 × 1010=4nH, where n is the frequency of the oscillations and H the height of the aerial in centimetres. The frequency of the oscillations is determined by the capacity (C) and inductance (L) of the condenser circuit, and can be calculated from the formula
That is, the frequency is obtained by dividing into the number 5,000,000, the square root of the product of the capacity in microfarads, and inductance in centimetres, of the condenser circuit. It will be found, on applying these rules, that it is impossible to unite together any aerial of a length obtainable in practice with a condenser circuit of more than a very moderate capacity. It has been shown that for an aerial two hundred feet in height the corresponding resonating frequency is about one and a quarter million.[22] As we are limited in the amount to which we can reduce the inductance of a discharge circuit, probably to something like a thousand centimetres, a simple calculation shows that the largest capacity we can employ is about a sixtieth of a microfarad. This capacity, even if charged at 60,000 volts, would only contain thirty joules of energy, or about 22·5 foot-pounds, which is a small storage compared to that which can be achieved when we are employing the above-described methods, which involve the use of an oscillation transformer. In such a case, however, it is an advantage to employ a spark-gap in compressed air, because we can then raise the voltage to a much higher value than in air of ordinary pressure without lengthening the spark so much as to render it non-oscillatory.
When employing methods involving the use of an oscillation transformer, it is possible to use multiple aerials having large capacity, and hence to store up a very large amount of energy in the aerial, which is liberated at each discharge. The most effective arrangement is one in which the radiator draws off gradually a large supply of energy from a non-radiating circuit, and so sends out a true train of waves, and not mere impulses, into the ether, and as we shall see later on, it is only when the radiation takes place in the form of true wave trains that anything like syntony can be obtained.
There are a number of variants of the above methods of arranging the radiator and associated energy-storing in circuit. Descriptions of these arrangements will be found in patents by Mr. Marconi, Professor Slaby and Count von Arco, Sir Oliver Lodge, Dr. Muirhead, Professor Popoff, Professor Fessenden and others. In all cases, however, they are variations of the three simple forms of radiator already described.
Returning to the analogy with the air or steam siren suggested at the commencement of this article, the reader will see in the light of the explanations already given, that all parts of the air-wave producing apparatus have their analogues in the electrical radiator as used in Hertzian wave telegraphy. The object in the one case is to produce rapid oscillations of air particles in a tube, which result in the production of an air wave in external space; in the other case, the arrangement serves to produce oscillations of electrons or electrical particles in a wire, the movements of which create a disturbance in the ether called an electrical wave. Comparing together, item by item, it will be seen, therefore, that the induction coil or transformer used in connection with electric-wave apparatus is analogous to the air pump in the siren plant. In the electrical apparatus, this electron pump is employed to put an electrical charge into a condenser; in the air wave apparatus, the air pump is employed to charge an air vessel with high pressure air. From the electrical condenser the charge is released in the form of a series of electrical oscillations, and in the air wave producing appliance, the compressed air is released in the form of a series of intermittent puffs or blasts. In the electrical wave producing apparatus, these electrical oscillations in the condenser circuit are finally made to produce other oscillations in an air wire or open circuit, just as the puffs of air finally expend themselves in producing aerial oscillations in the siren tube. Finally, in the one case we have a series of air waves and in the other case, a series of electrical waves. These trains of electric waves or air waves, as the case may be, are intermitted into long and short groups, in accordance with the signals of the Morse alphabet, and, therefore, the Hertzian wave transmitter, in whatever form it may be employed, when operated by means of a Marconi aerial, is in fact an electrical siren apparatus, the function of which is to create periodic disturbances in the universal ether of the same character as those which the siren produces in atmospheric air.