CHAPTER XIX.

THE ELECTRO-MAGNETIC THEORY OF LIGHT AND

THE ETHER.

THE various phenomena of the action between magnets-the induction phenomena between neighbouring circuits, a current of induction rising in one circuit whenever an electric current is started in a neighbouring circuit, and thus manifesting energy-lead us to believe that the energy has been transferred from the exciting circuit across space by means of some medium filling that space. The energy has disappeared from the exciting circuit, and has reappeared in the induction circuit. It must have existed during the time of its disappearance and reappearance in the intervening space. We are therefore forced to believe in some medium which serves to convey this energy.

The old fluid theories implied that when a body was electrified it had something upon it which was called electricity. According to the modern views, we regard the ether around the body as charged with energy which is the result of the work we have done in charging the body. This energy in the ether is the energy of motion. There is a state of strain in the ether which we term a polarized condition. Around a positively charged body this polarization has a certain direction and a certain amount. With a negatively

charged body this polarization is in an opposite direction. It is suggested that these polarizations may be like right-handed and left-handed rotations or twists. When we electrify a conductor we store up energy around the conductor in the ether. The work we do is spent in changing the state of the medium. When a body is discharged, the medium returns to its original state, and the energy is dissipated as heat in the electric spark or as heat in the conductor. The electric current is therefore the manifestation of energy in the ether along the wire through which the current appears to flow. According to Poynting's theory, we have seen that the electric energy produced by a battery or a dynamo does not flow along the wire-for instance, the overhead wire of an electric railroad-but it produces this strained condition in the ether, and the ether relieves itself along the wire. What we call the flow of the electrical current is therefore not in the same direction as the flow of electrical energy.

When a Leyden jar is discharged the knobs of the jar become alternately positive and negative. The medium around the jar is therefore polarized alternately in opposite directions. This polarization starts from the knob and spreads through space, at each point of which there are to-and-fro motions, and waves of opposite polarizations are sent through the medium, carrying the energy which had been stored up in the Leyden jar. There is a periodic or to-and-fro movement in the ether, and if we could make a Leyden jar of molecular dimensions charge it and discharge it, we could produce a periodic movement in the ether which is analogous to that which occurs in the propagation of light. Maxwell's electro-magnetic theory of light supposes that the periodic motions which constitute light are of the same nature

as those which arise when the positive and negative conditions of the ether are rapidly alternated in the case of the discharge of a Leyden jar. Light, heat, and electricity are therefore manifestations of electromagnetic waves which come to us from the sun.

In our study of electric waves we have used their light manifestations in order to trace their phenomena. Thus, when a Leyden jar is discharged through a great circle of wire properly placed in a room, we can send electro-magnetic waves through brick walls and detect them in neighbouring rooms by the sparks that are excited in a similar circle of wire connected to another similar Leyden jar. By photographing the latter spark, we can say, in popular language, that we have photographed by means of waves that have passed through a brick wall. We shall see later that it is possible to photograph by means of electric waves which have passed through opaque metallic screens, which cut off entirely the light rays so considered.

Our eyes can see an electric spark very much farther. than we can detect the electro-magnetic waves by any other instrument than the eye. The eye really detects them in the form of light. It is true also that we can not detect the heat waves sent out by the spark so far as we can perceive the electric waves. The heat waves are nearer in length to the electric waves which we can detect than the light waves. The most delicate thermometer will not show any indication of heat at a distance of ten feet from a powerful spark if we prevent the electromagnetic waves from surging in its mass, and if we depend upon the direct radiation of heat through the ether.

We are thus certainly as advantageously placed at present in regard to measuring electro-magnetic waves generated by a spark as we are in regard to measuring

heat waves which accompany them. We can, however, measure the long waves of heat which come to us from the sun, yet we can not detect the long electro-magnetic waves. Prof. J. J. Thomson has shown that if an electrical charge on a sphere is disturbed in any sudden way, it can oscillate to and fro in the time taken by light to travel a certain number of times the diameter of the sphere, depending on the wave length of the electric wave. Prof. O. J. Lodge, in quoting this calculation, remarks: "An electrostatic charge on the whole earth would surge to and fro seventeen times a second. On the sun an electric swing lasts six and a half seconds. Such a swing as this would emit waves 19 × 105 kilometres or 1,200,000 miles long, which, travelling with the velocity of light, could easily disturb magnetic needles and produce auroral effects, just as smaller waves produce sparks in gilt wall paper (Rhigi's resonators), or as the still smaller waves of Hertz produce sparks in his little resonators, or, once more, as the waves emitted by electrostatically charged vibrating atoms excite corresponding vibrations in our retina."*

With our present methods of studying electric waves we are compelled to use electric sparks which do not succeed each other continuously. The electric waves from one spark are thousands of miles on their way before another spark occurs. The dying out of the waves, or what we have called their damping, is due to the rapid radiation of the energy of the spark into space. Hertz has calculated this loss of energy in the case of a small spark from two spheres of fifteen centimetres radius (about six inches) so arranged that they sent out waves four hundred and eighty centimetres long (one

* Lightning Conductors and Lightning Guards, p. 260.

hundred and ninety-two inches). He estimates that such an oscillator giving waves of this length must be supplied with energy at the rate of twenty-two horse power per second if the intensity is to be kept constant. "At a distance of twelve metres (thirty-nine and a half feet) from the spheres or oscillator the intensity of the radiation is equal to the intensity of the solar radiation at the surface of the earth.” *

From this calculation we see what an enormous amount of energy is radiated in a stroke of lightning. This consideration of the rapid damping of electric waves sent out by electric sparks is interesting from the point of view of the endeavour to obtain light by rapid electric oscillations.

The hypothesis of an ether filling all space, a medium by means of which light, heat, and electricity are transmitted to the earth from the sun, seems untenable to many minds, largely because we are obliged to attribute to this medium extraordinary qualities of rigidity and elasticity, and to regard it of such extreme tenuity that it escapes detection by direct measurement; for it does not impede the motion of bodies.

If one critically examines the advance that has taken place in the applications of electricity, one can see that even the practical man's chief consideration is in regard to the medium around his electro-magnets and his currents. In designing dynamos and motors for the transmission of power, one of the main principles to be borne in mind is that movable parts of iron or copper will arrange themselves in a magnetic field so as to diminish as far as possible the magnetic resistance of such a field. By magnetic resistance we mean the dif

* Preston, Theory of Light, p. 444.