Waves and ripples in water, air, and æther

by Sir J. A. Fleming

The proportion of the light or eye-affecting radiation to the dark heat in the total radiation from any source of light increases with the temperature, but it is not always merely a question of temperature. Thus the electric arc is hotter than a candle flame, and the sun is hotter than the electric arc. Hence, whilst the luminous rays only form 3 parts out of 100, or 3 per cent. of the radiation of a candle, they constitute 10 to 15 per cent. of those of the electric arc, and more than 30 per cent. of those of the sun. On the other hand, the glow-worm and the fire-fly seem to have possession of a knowledge and an art which is as yet denied to man. It has been shown by Professor Langley and Mr. Very that nearly the whole of the radiation from the natural torch of the fire-fly is useful light, and none of it is useless dark heat. Hence these photogenic (light-producing) insects have the art, which we have not, of creating cold light, or unadulterated luminous radiation.

At the present moment in ordinary incandescent or glow-lamp electric lighting we require to expend an amount of power, called one horse-power, to produce illumination equal to that of 600 candles. Supposing, however, that all our power could be utilized in generating merely the rays useful for vision, or which can impress our eyes, we might be able to create by the expenditure of one horse-power more than twenty times as much illumination, that is, a light equal to 12,000 candles.

These figures show us what rewards await the inventor who can discover a means of generating æther waves having wave-lengths strictly limited to the range lying between the limits 0·4μ and 0·7μ without, at the same time, being obliged to create radiation comprising longer waves which are not useful for the purpose of rendering objects visible to us. For the purposes of artificial illumination we require only the æther waves in this one particular octave, and nothing else.

This increase in the efficiency of our sources of artificial illumination is only likely to be brought about when we abandon the process of heating a solid substance to make it give out light, and adopt some other means of setting the electrons in vibration.

It is almost impossible to discuss the subject of æther waves without some reference to the most modern utilization of them in the so-called wireless telegraphy. Without entering upon the vexed questions of priority, or on the historical development of the art, we shall simply confine our attention here to a consideration of the methods employed by Mr. Marconi, who has accomplished such wonderful feats in this department of invention.

We have already seen that when two insulated conductors are placed with their ends very near together, and are then electrified, one positively and the other negatively, and then allowed to be suddenly connected by an electric spark, they constitute an arrangement called an electrical oscillator. If the conductors take the form of two long rods placed in one line, and if their contiguous ends are provided with spark-balls separated by a small gap, we have seen that we have shown that, under the above-mentioned conditions, electric currents of very high frequency are set up in these rods. For creating these oscillations, an instrument called an induction coil or spark-coil is generally employed. You will understand the arrangements better if a brief description is given first of the spark-coil itself as used in wireless telegraphy.

Fig. 81.—A 10-inch induction coil for wireless telegraphy (Newton).

The appliance consists of a large bundle of fine iron wires, which are wound over with a long coil of insulated wire. This forms the primary coil. It is enclosed entirely in a tube of ebonite. One end of this coil is a contact-breaker, which automatically interrupts an electric current flowing from a battery through the primary coil ([see Fig. 81]). A hand-key is also placed in the circuit to stop and start the primary current as desired. Over the primary coil is a very long coil of much finer silk-covered copper wire, called the secondary coil. The length of this coil is very considerable, and may amount to many miles. The secondary coil is divided into sections all carefully insulated from each other. Another important part is the condenser. This consists of sheets of tinfoil laid between sheets of waxed paper, alternate tinfoil sheets being connected. The arrangement forms virtually a Leyden jar, and one set of tinfoils is connected to one side of the automatic break, and the other to the adjacent side. When, therefore, the primary circuit is interrupted by the break, the condenser is at that moment thrown into series with the primary coil. The rapid interruption of the primary current causes a secondary current in the fine-wire coil. The automatic contact-breaker makes from ten to fifty such interruptions per second. At every “break” of the primary a very high electromotive force is generated in the secondary circuit, which may amount to many hundreds of thousands of volts. This very high secondary electromotive force is able to cause an electric discharge in the form of a spark between brass balls connected to the secondary circuit terminals. Coils are generally rated by the length (in inches) of the spark they can produce between brass balls about ¹⁄₂ inch in diameter. The coil most commonly used in wireless telegraphy is thus technically termed a “10-inch induction coil,” from the length of the spark this particular type of coil can produce.

If the insulated brass balls, called the spark-balls, connected to the secondary terminals, are placed an inch or so apart, and the hand-key in the primary circuit is closed, a battery connected to the primary circuit will send a rapidly interrupted current through the primary coil, and a torrent of sparks will pass between the spark-balls. The primary current of the 10-inch coil is usually a current of 10 ampères, supplied at a pressure of 10 volts.