Letter L, Forming, Therefore, a Very Narrow Box With Silver Sides.

The two silver plates are connected to the two long rods. As already explained, the metallic filings or finely powdered metal are not in their ordinary condition an electric conductor. Accordingly, if we connect to one of the silver plates one terminal of a battery joined in series with an electric bell, the other end of the bell being connected to the second silver plate, this battery cannot send a current through the bell, because the circuit is interrupted by the non-conductive metallic powder in the little box. Supposing, then, that we cause a spark to pass between the balls of the radiator, and start an electric wave. When this electric wave reaches the long rods connected to the receiving arrangement, it sets up in these rods a sudden electromotive force, and this electromotive force, as already explained, if of sufficient magnitude, causes the loose mass of metallic filings to pass from a non-conductive to a conductive condition. At that moment, therefore, the battery is able to send an electric current through the bell, and to cause it to ring. We can, however, stop the ringing by giving the little box containing the metallic filings a tap, which separates them from one another and interrupts the electric conductivity. The function of the two rods connected with the receiver is not quite the same as the function of the two rods connected to the radiator. In order to create a vigorous electric wave, we must have a radiator which possesses what is called considerable electric capacity, and also considerable inductance, and we can only do this in general by using long rods. On the other hand, at the receiving end the efficacy of the rods is due to the fact that they, so to speak, add together the electric strain taking place over a considerable distance; in other words, the electromotive force which is set up in the receiving circuit is dependent on the length of the rods. The longer, therefore, these rods, the greater is the distance at which we can obtain the effect which is shown to you with a given spark-length.

One point it is important to notice, and that is, that the rods of the receiver must be parallel to the rods of the radiator if we are to obtain any effect at a distance. If we turn the rods of the radiator round so that they are at right angles to those of the receiver, you see that no sparks produced at the radiator balls cause the bell in connection with the receiver to ring. The reason for this is because the electric strain, which is propagated out into the space, exists in a direction parallel to the radiator rods all along a line drawn perpendicular to the rods through the spark-gap. The receiver rods will not have electromotive force produced in them by this travelling line of electric strain unless they are parallel to its direction.

It is to be hoped that the above explanations have afforded indications of what is meant by an electric wave. On the other hand, there may be many who find it exceedingly difficult to derive clear ideas when the subject is presented to them clothed in such general terms as we have been obliged to use.

It may assist matters, therefore, if, before concluding this chapter, a word or two is said on the subject of recent investigation into the inner mechanism of an electric current and an electric strain. It is impossible to do this, however, without making mention, in the briefest possible way, of modern researches into the constitution of matter. If you can imagine yourselves furnished with a little crystal of ordinary table salt, chemically called chloride of sodium, and the means of cutting it up under an immensely powerful microscope, you might go on dividing it up into smaller and smaller pieces. If this process could be continued sufficiently far, we should ultimately obtain a very small fragment of salt, which, if still further divided, would yield two portions of matter not alike and not salt. This smallest possible portion of salt is called a molecule of sodium chloride. Chemical facts teach us that this molecule is made up of two still smaller portions of matter, which are called respectively atoms of chlorine and sodium.

We have good reason to believe that all solids, liquids, and gases are composed of molecules, and these are built up of atoms, few or many.

In the case of some substances, such as salt, the molecule is very simple and composed of two atoms. In other substances, such as albumen or white of egg, the molecules are very complicated and composed of hundreds of atoms. The word atom means something which “cannot be cut,” and until comparatively recent time the opinion was held that atoms of matter were the smallest indivisible portions of matter which could exist.

More than twenty-five years ago, Sir William Crookes showed, by numerous beautiful experiments, that in a vacuum tube, such as you have seen used to-day, a torrent of small particles is projected from the negative terminal when an electric current is passed through the tube. This stream of particles is called the cathode stream, or the cathode radiator. Within recent times, Sir Joseph Thomson has furnished a proof that this cathode stream consists of particles very much smaller than chemical atoms, each particle being charged with negative electricity. These particles are now called corpuscles, or electrons.

It has been shown that these electrons are constituents of chemical atoms, and when we remove an electron from an atom we leave the remainder positively electrified. An atom can, therefore, by various means be divided into two portions of unequal size. First, a very small part which is charged with negative electricity, and, secondly, a remaining larger portion charged with positive electricity. These two parts taken together are called ions, i.e. wanderers. The negative ions, or electrons, or corpuscles, taken together constitute what we call negative electricity, and up to the present no one has been able to show that the corpuscle can be unelectrified. Hence the view has been expressed that what we call electricity is a kind of matter, atomic in structure, and that these negative ions or corpuscles collectively are, in fact, the atoms of the electric fluid. These corpuscles can move freely in the interior of some solids, moving between the molecules of the solid just as little dogs can run about in and amongst a crowd of people in a street. In these cases the substance is called a conductor of electricity. In other substances the movement of the corpuscles is more restricted, and these constitute the various kinds of so-called non-conductors.

The corpuscle, being a small charge of negative electricity, creates in all surrounding space a state called electric force. It is impossible to expound this action more in detail without the use of mathematical reasoning of a difficult character. Suffice it to say that this electric force must be a particular condition of strain or motion in the æther. If the corpuscle is in rapid motion, it creates in addition another kind of strain or motion called magnetic force. The electric force and the magnetic force are related to each other in free space in such a manner that if we know the difference between the values of the electric force at two very near points in space, we are able to tell the rate at which the magnetic force is changing with time in a direction at right angles to the line joining these near points in space. We cannot specify in greater detail the exact nature of these states or conditions which constitute magnetic force and electric force, until we know much more than we do at present about the real nature of the æther. The two fundamental qualities of the æther are, however, its capacity to sustain these states we call the magnetic force and the electric force.

The electrons of which we have spoken not only give rise to electric and magnetic force when in movement, but they are themselves set in motion by these forces. Thus electric force at any point moves electrons placed at that spot, and an electron in motion is affected and has its direction of motion changed when magnetic force acts on it.

Leaving further remarks on the relations of atoms, electricity, and æther until the end of the last lecture, we may conclude the present one by explaining the manner in which the observed facts connected with a Hertz oscillator are interpreted in terms of this electron hypothesis of electricity.

Take the simple case of two long insulated metal rods separated by a spark-gap. The process of charging one rod positively and the other negatively consists in forcing more corpuscles, or negative ions or electrons, into one conductor and removing some from the other. Any source of electromotive force, such as a dynamo or induction coil, is, on this hypothesis, a sort of electron-pump, which pumps electrons from one conductor and puts them into another. One conductor, therefore, gains in electron-pressure, and the other loses.

The excess of electrons in one conductor endeavour to escape, and a strain is produced on the electrons or atoms in the surrounding dielectric or air, which may be looked upon as the effort of the electrons, more or less tethered to the atoms, to escape. The air in the spark-gap is subjected to the most intense strain, and when this reaches a certain intensity some of the electrons are torn away from their atoms, and the air in the gap then becomes a conductor. The excess of electrons in one conductor rush through the channel thus prepared, and this constitutes an electric current. The first rush carries over too many electrons to equilibrate the electron-pressure, and hence the first torrent of migrating electrons in one direction is followed by a back-rush in the opposite one, this again in turn by another in the original direction, and so the equality in the number of electrons in each conductor is only established after a gradually diminishing series of to-and-fro rushes of electrons across the air-gap. This action constitutes a train of electrical oscillations. At the same time that these operations are going on in and between the conductors, the electrons attached to the atoms of the air or other dielectric all around are being violently oscillated. These oscillations may not proceed to such an extent as to detach electrons from their atoms, but they are sufficient to create rapidly reversed electric and magnetic forces. It appears that the very rapid movement to and fro of an electron causes a wave in the æther, just as the rapid movement of the hand through water causes a wave in water, or the vibration of the prong of a tuning-fork creates a wave in the air.

The electron has some grip on the æther, such that the sudden starting or stopping of the electron makes a disturbance which we may popularly describe as a splash in the æther. Hence, if a large number of electrons are suddenly started into motion in the same direction, the effect on the æther is something like casting a multitude of stones on the surface of still water, or the simultaneous action of a number of small explosions in the air. Anything, therefore, which, so to speak, lets the electrons go gradually, or softens the first rush, is inimical to the production of a vigorous electric wave. On the other hand, anything which causes the first rush of electrons from one conductor to another across the air-gap to be very sudden is advantageous, and results in a powerful wave. Experience shows that the nature of the metal surfaces, whether polished or rough, has a great influence on the wave-making power of the radiator. If the spark-balls or surfaces are rough and not polished, it seems to tone down the violence of the first electron rush, and the wave-making power of the oscillator is not so great as if the balls are polished.

At this point, however, it will be best to withhold further discussion on points of theory until we have considered the facts to be brought before you in the next lecture, showing that the electric radiation manufactured by means of electric oscillations is only one variety of a vast range of æther waves, some forms of which are recognizable by us as light and radiant heat.