THE ELECTRON THEORY The modern theory is called the electron theory and was first formulated by H. A. Lorentz, in 1892. It has since been devel oped by H. A. Lorentz himself and many other physicists. H. A. Lorentz assumed that there is no such thing as magnetism, but that all magnetic fields are due to currents. He supposed that material bodies contain enormous numbers of minute atoms of electricity, to which their electrical properties are due. These electrons are supposed to excite a field in the space around them. Lorentz regarded space as filled with a medium called the aether which he supposed was at rest, even inside a moving body. The electrons are supposed to be extremely small, so that, even in a dense substance like gold, they occupy only a minute fraction of the space. The electrical properties of the aether, or space, in side material bodies are supposed to be identical with those in empty space. Each electron is supposed to excite its field exactly as if the other electrons were not present, so that the resultant field at any point is simply the resultant, or vector sum, of the fields due to all the electrons present. If a point is selected at random in a material body, there will almost always be nothing at the point except the electromagnetic field excited by the elec trons. If the point happens to be inside an electron, there will be at it a certain density of electricity in addition to the electro magnetic field. The electrical properties of material bodies are due to the electrons in them. When these properties are meas ured, the values obtained are average values over volumes contain ing enormous numbers of electrons. It is impossible to examine the microscopic structure of the distribution of the charges and field in a piece of matter.
Lorentz first obtained the electromagnetic equations for space, or aether, containing nothing but electricity and the field which it excites, and then showed how these equations could be trans formed into equations expressing the relations between the aver age values which can be observed experimentally. He supposed that conductors contain some free electrons which drift along in an electric field, so producing a conduction current. The electrons in insulators, he supposed, are not free, but are acted on by re storing forces proportional to the displacement of the electrons from their equilibrium positions. Lorentz supposed that the field excited by an electron is propagated out from it with the velocity, c, of light in empty space. Thus the field due to a charge at a point P, at a time t, is not the field excited by the charge at the time t, but that excited by the charge at the time t—r/c, where r is the distance of the charge from P at the time t-r/c. The dis tance r is different for different parts of an electron, and this has to be taken into account.
Lorentz applied his theory to the problem of optical and elec trical phenomena on a moving system, e.g., the earth, which is moving through space. He showed finally that no observable effects were to be expected, which was in agreement with all ex perimental results. The fact that no effects due to the motion of the earth through space can be detected is the basis of Einstein's theory of relativity, which may be regarded as a development of H. A. Lorentz's electromagnetic theory. (See RELATIVITY.) According to the electron theory, the dielectric current in an insulator is the sum of the dielectric current which would be obtained in a vacuum and that due to the displacement of the electrons. When an insulator is moving in a magnetic field, there is a force on the moving electrons which produces a displace ment, but there is no effect on the aether or space, since it is not supposed to move. The induced electromotive force in an insula tor, moving in a magnetic field, is therefore less than that to be expected on Hertz's theory, according to which the aether was sup posed to move with the moving body. It can be shown, by the electron theory, that the induced electromotive force in an in sulator, moving in a magnetic field, is equal to that in a conduc tor multiplied by the factor (K-1)/K, where K is the specific inductive capacity of the insulator. According to the Maxwell Hertz theory, the induced electromotive force should be equal to that in a conductor. It was shown experimentally, by H. A. Wilson in 1905, that the induced electromotive force in an in sulator, moving in a magnetic field, is equal to (K-1)/K of that in a conductor, in agreement with the electron theory.
H. A. Lorentz regarded the negative electrons as small spheres, of negative electricity, which he supposed contracted, in the di rection of motion, into spheroids when they moved. He calcu lated the electromagnetic momentum of such an electron, and showed that the corresponding mass, m varied with the velocity, according to the formula where is the mass when at rest, v the velocity of the electron, and c that of light in a vacuum. It was shown later that the mass actually does vary with the velocity in this way, but this is not now regarded as supporting H. A. Lorentz's conception of an elec tron, because it is believed that the mass of any kind of particle must vary with its velocity in the same way.