Waves and ripples in water, air, and æther

by Sir J. A. Fleming

Fig. 80.

It should be noted, however, that whilst there are more or less definite limits to the wave-lengths of the eye-affecting radiation, and probably also to the actinic, or photographic radiation (radiation of some wave-lengths being both visible and actinic), rays of every wave-length are in some degree thermal, or heat-producing. The term dark-heat radiation is, however, generally restricted to radiation of that wave-length which is non-visible and non-actinic. This mode of presenting the facts will call your attention again to the narrow limits of sensibility of the human eye as compared with those of the ear.

The above-mentioned range of wave-lengths does not, however, exhaust our powers of æther-wave production. If we skip over six octaves lying below the limits of the longest dark-heat wave with which we are acquainted, we should arrive at a wave whose wave-length is about 4000μ, or 4 millimetres. At this point we encounter the shortest æther waves which have yet been made by means of electrical oscillations in the fashion first discovered by Hertz.

It is not possible to define exactly the wave-length limits of radiation as yet made by means of electrical oscillations. Lampa has experimented with æther waves made by the Hertz method, the wave-length of which was not more than 4 millimetres. Professors Lodge, Rhigi, Bose, Trouton, the author, and many others, have carried out quasi-optical experiments with electrically made æther waves, the wave-length of which ranged from a few millimetres to several inches. Hertz’s own work was chiefly done with æther waves from 1 or 2 feet to 30 or 40 feet in wave-length. More recently, æther waves of 800 to 1000 feet in wave-length have been employed in wireless telegraphy. Perhaps we shall not be wrong in saying that we are acquainted with sixteen or seventeen octaves of æther-wave radiation which is made electrically, and is usually called the Hertz radiation.

Between the radiation of greatest wave-length which proceeds from hot or incandescent bodies such as the sun, the electric arc, or a hot ball, and that of the shortest wave-length which has been created by means of electrical oscillations set up in some form of Hertz oscillator, there is a range of six octaves of æther waves which, so far as we know, have not yet been manufactured or detected. Herein lies an opportunity for much future scientific work. We have to discover how to create and recognize these interconnecting wave-lengths. From the fact that all Hertz waves travel with the same speed as light, and from our ability to imitate, as you have seen, the well-known optical phenomena with Hertz radiation of short wave-length, the great induction has been made that all æther waves have the same essential nature, and that invisible actinic rays, light rays, dark-heat rays, and Hertz rays are all of them æther waves of various wave-lengths and amplitudes. Thus we see, as Maxwell long ago predicted, that light in all probability is an electro-magnetic phenomenon, and therefore all optical effects must be capable of receiving an electro-magnetic explanation. The inclusion thus made of the whole science of Optics within the domain of Electricity and Magnetism is one of the grandest achievements of Physical Science. It stands second only to Newton’s great discovery of universal gravitation, which reduced all Physical Astronomy to pure Dynamics, and showed that the force concerned in the falling of a stone is identical with that which holds the planets in their orbits, and controls the motions of galaxies of suns.

At the end of the last chapter it was explained that these Hertz radiations are created in the æther by the suddenly starting, stopping, or reversing the motion of crowds of electrons, which are, as it were, instantly released from a state of pressure or tension, and set moving inside a straight insulated conductor, which forms an open electric circuit. The radiations we call light and dark heat are probably, therefore, started in a similar manner by vibrations of the electrons which form parts of, or which build up, atoms. There are many physical phenomena which seem to show that the electrons which we can detach from atoms in a high vacuum tube are capable of vibrating freely in definite periods when in connection with their atom. If the atoms are able to move freely, and if each is practically independent, as is the case in a gas, and if they are then caused to radiate by any means, the radiation emitted by the vibration of these electrons consists of certain definite wave-lengths. Hence, when we form the spectrum of an incandescent gas, we find it to consist of several detached bright lines, each corresponding to one particular wave-length, and we do not obtain a uniformly graduated band of coloured light. If an atom is struck by colliding with another, and then left to itself, it appears as if the electrons which compose it and form part of it are set in vibration, and each executes its oscillation in some definite period of time. An atom has, therefore, been compared to a “collection of small tuning-forks,” which, if rudely struck, would result in the emission of a set of air-wave trains, each one corresponding in wave-length to one particular tuning-fork which emitted it. Hence, if we could administer a blow to such a congeries of tuning-forks, and then analyze the compound sound, we should obtain a sound spectrum consisting of separated tones—in other words, a bright line spectrum of the complex sound. Supposing, however, that we have a mass of atoms much more closely in contact, as in the case of a solid body, the continual collisions between the atoms and the closer contact between them cause the vibrations of the electrons to be “forced,” and not “free.” Hence the electrons are compelled to execute all varieties of irregular motion, and these predominate over their regular free natural vibrations. Accordingly, the waves emitted are of a large variety of wave-length, and when the radiation is analyzed by a prism, we obtain a continuous spectrum, or band of many-coloured light, as the result of the separation of the rays of different wave-lengths present in it.

It is this fact which renders our present method of creating artificial light so excessively uneconomical.

All our practical methods for making light consist in heating a solid body in one way or another. In the case of the electric light we heat electrically a carbon rod or filament, or else, as in the Nernst lamp, a rod composed of magnesia and the rare earths. In the case of the lime-light we heat a cylinder of lime. In an ordinary gas or candle flame we heat small particles of carbon, and the same is the case even in the sun itself.

But this process manufactures not only the single octave of radiation which can affect our eyes as light, but a dozen other octaves of radiation to which they are insensible. Hence it follows that of the whole radiation from a gas flame, only about 3 per cent. is eye-affecting light, the remainder is dark heat. In the case of an incandescent electric lamp, this luminous efficiency may amount to 5 per cent., and in the electric arc to 10 or 15 per cent. There is, however, always a great dilution of the useful light by useless dark heat.