When one reviews the progress made in the department of physics within the last ten years, he is struck by the change which has taken place in the fundamental ideas concerning the nature of electricity and matter. The change has been brought about in part by researches on the electric conductivity of gas, and in part by the discovery and study of the phenomena of radioactivity. It is, I believe, far from being finished, and we may well be sanguine of future developments. One point which appears to-day to be definitely settled is a view of atomic structure of electricity, which goes to conform and complete the idea that we have long held regarding the atomic structure of matter, which constitutes the basis of chemical theories.
At the same time that the existence of electric atoms, indivisible by our present means of research, appears to be established with certainty, the important properties of these atoms are also shown. The atoms of negative electricity, which we call electrons, are found to exist in a free state, independent of all material atoms, and not having any properties in common with them. In this state they possess certain dimensions in space, and are endowed with a certain inertia, which has suggested the idea of attributing to them a corresponding mass.
Experiments have shown that their dimensions are very small compared with those of material molecules, and that their mass is only a small fraction, not exceeding one one-thousandth of the mass of an atom of hydrogen. They show also that if these atoms can exist isolated, they may also exist in all ordinary matter, and may be in certain cases emitted by a substance such as a metal without its properties being changed in a manner appreciable by us.
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If, then, we consider the electrons as a form of matter, we are led to put the division of them beyond atoms and to admit the existence of a kind of extremely small particles, able to enter into the composition of atoms, but not necessarily by their departure involving atomic destruction. Looking at it in this light, we are led to consider every atom as a complicated structure, and this supposition is rendered probable by the complexity of the emission spectra which characterize the different atoms. We have thus a conception sufficiently exact of the atoms of negative electricity.
It is not the same for positive electricity, for a great dissimilarity appears to exist between the two electricities. Positive electricity appears always to be found in connection with material atoms, and we have no reason, thus far, to believe that they can be separated. Our knowledge relative to matter is also increased by an important fact. A new property of matter has been discovered which has received the name of radioactivity. Radioactivity is the property which the atoms of certain substances possess of shooting off particles, some of which have a mass comparable to that of the atoms themselves, while the others are the electrons. This property, which uranium and thorium possess in a slight degree, has led to the discovery of a new chemical element, radium, whose radioactivity is very great. Among the particles expelled by radium are some which are ejected with great velocity, and their expulsion is accompanied with a considerable evolution of heat. A radioactive body constitutes then a source of energy.
According to the theory which best accounts for the phenomena of radioactivity, a certain proportion of the atoms of a radioactive body is transformed in a given time, with the production of atoms of less atomic weight, and in some cases with the expulsion of electrons. This is a theory of the transmutation of elements, but differs from the dreams of the alchemists in that we declare ourselves, for the present at least, unable to induce or influence the transmutation. Certain facts go to show that radioactivity appertains in a slight degree to all kinds of matter. It may be, therefore, that matter is far from being as unchangeable or inert as it was formerly thought; and is, on the contrary, in continual transformation, although this transformation escapes our notice by its relative slowness.
In the beginning of the last century Coulomb and Ampère regarded each of the two kinds of electricity to be a fluid under the influence of central forces—repulsion existing between particles of the same fluid and attraction between particles of different fluids. Such forces would be proportional in the electric charge of the particles, and would vary in inverse ratio to the square of the distance between them. Starting with these hypotheses, and explaining suitably the observed facts relative to the different nature of conductors and dielectrics, they constructed a very perfect theory of electrostatic phenomena. An analogous theory for magnetism may be built up by assuming that the law of action between two magnetic poles is absolutely like the law of action between electrified particles. The electric current was regarded as the flowing of an electric fluid in a conductor. To establish a theory of electro-magnetics and electro-dynamic phenomena, it is necessary to bring in a third law of action-at-a-distance between the magnetic poles and the electric-current law of Laplace. All these theories in their entirety are founded on the laws of forces acting at a distance, in combination with the conception of electric fluids.
Faraday, although contemporaneous with this development, looked at the question from a different point of view. He did not believe in the possibility or lower of action-at-a-distance between electrified bodies, and thought that the forces which were exercised between them resulted from elastic tensions which established themselves in the intervening medium. These elastic forces comprise a tension in the direction of the lines of force and a pressure at right angles to them. In seeking to show the direct influence of the medium he was led to the discovery of the inductive power of dielectrics, and his belief in the essential part played by the intervening medium was thus strengthened. According to Faraday, the surfaces of charged conductors are to be regarded as surfaces of separation between regions where an electric field exists and fields of zero intensity. He was struck by the barrenness of the efforts that had been made to realize an absolute charge, and electric charges always appeared to him as the ends of tubes of force which traverse the dielectric.
Maxwell, captivated by the ideas of Faraday, endeavored to explain them in mathematical language. He demonstrated that there does not exist in a mathematical view any incompatibility between theories based upon laws of action-at-a-distance and Faraday’s theory of continuous action; and that by assigning a reasonable value to the tensions and pressures which Faraday conceived to exist in the dielectric, an electrostatic theory could be constructed identical to that which is derived from the law of action-at-a-distance. While Maxwell does not specify precisely the nature of electricity, he treats of it generally as a fluid whose displacement in a conductor gives rise to a resistance proportional to the velocity of the flow, while its displacement in a dielectric produces an elastic reaction. In a dielectric, displacement can only occur at the time when the field changes. One of the essential ideas of Maxwell was to consider the displacement of electricity in the dielectric as an electric current to which he gives the name of “currents of displacement.” Currents of displacement, according to Maxwell, behave like ordinary currents, in the sense that they produce magnetic fields. Every open circuit in a conductor, following the opinion of Maxwell, is completed by a current of displacement in the dielectric, so that there exist only closed circuits.
The system of the six differential equations, called Maxwell’s equations, brings out in mathematical form the relation which exists at each point of an electromagnetic field between the current of displacement and the magnetic field, as well as between the rate of change of the magnetic induction and the resulting electric field. These perfectly symmetrical relations show that all variations of an electric field cause a magnetic field, and vice versa. Starting from these equations, Maxwell proved that every perturbation of an electro-magnetic field should be propagated in a vacuum, with a velocity equal to that of light in a vacuum, and he draws the conclusion that the medium which transmits electro-magnetic actions in the vacuum is the same as that which transmits light, and that light is very likely an electro-magnetic phenomenon. This conception has served as the basis of the electromagnetic theory of light, now universally adopted as the result of the experiments of Hertz and numerous physicists upon the electro-magnetic waves. In the development of the ideas of Faraday and Maxwell, a preponderating influence was attributed to the role of the dielectric medium, so that little attention was paid for some time to the nature of electricity; and this question was relegated to a subordinate place, and received only an indirect interpretation. There was no longer the conception of charges of electricity localized in a determined region, nor of a fluid flowing through a conductor. The main conceptions were of energy localized in the dielectric medium and the differential equations which determined the field in the medium. Recent progress in research has brought us back to a more concrete conception of the nature of electricity.
The first impulse in this direction was the result of investigations of electrolysis and modern theories of this phenomenon. It was established with certainty that the passage of electricity in the electrolyte is always accompanied by the transportation of matter. Electrolytes are aqueous solutions of acids, bases, and mineral salts, or these bodies in a fused condition. It is now admitted that the molecules of a dissolved substance are totally or partially dissociated in two ions—one ion formed by the metal, or hydrogen, and charged positively; another formed by the acid radical, and charged negatively. When there is set up in the solution an electric field the ions move toward the electrodes of contrary sign, transporting across the liquid their charges, which they give up to the electrodes, and themselves become free in a neutral state. Ions are, then, the actual carriers of electricity in electrolytes, and the current is a current of convection. It follows from Faraday’s laws that all monovalent ions carry the same amount of charge, q, corresponding to 96,600 coulombs per gramme of ions, while an ion of valence, n, carries a charge nq. There cannot be conceived in electrolysis an isolated charge of electricity less than that carried by a monovalent atom, such, for example, as hydrogen in the ionic state. The atomic structure of electricity is therefore an immediate and necessary consequence of the atomic structure of matter.
It is by no means evident, a priori, that this conception can be generalized and that the other known cases of conduction are susceptible of an analogous interpretation; but this seems to be coming to pass. The study of the electrical conductivity of gases has borrowed from the theory of electrolysis the idea of charged ions, vehicles of the current; and the phenomena are satisfactorily accounted for by the hypothesis that the current in a gas is a current of convection. But the vehicles of the current are not here the same as in an electrolyte. It is believed that an ionized gas gives rise to two ions, of which one is that minute thing which we call an electron, the other being the remainder of the molecule deprived of the electron. By ingenious methods the number of ions present in a given volume of gas has been counted and the charge carried by each one determined. This charge is equal to that transported by an atom of hydrogen in electrolysis, and thus we find this presented to us the second time as the smallest quantity of electricity which can be isolated.
All the phenomena of conduction across a gas under the influence of different forms of radiation or in the disruptive discharge at varying tension appear to be susceptible to explanation by the theory of the ionization of gases.
Attempts have been made to explain the conduction of metals in a similar way, and it is probable that this also may be considered as a current of convection whose vehicles are the electrons set free in the metal. Thus we arrive at the conclusion that electric currents through all forms of matter are currents of convection, or, in other words, the displacements of electric charges. Besides this it has been proved that any such displacement gives rise to a magnetic field.
The conception of the existence of atoms of electricity which is thus brought before us in the phenomena of conduction plays an essential part in modern theories of electricity like that of Lorentz. This theory retains the fundamental idea of Faraday and Maxwell, according to which the electromagnetic actions are always transmitted from place to place in a continuous medium with a finite velocity. This medium is the ether of space, and the velocity is the velocity of light. The laws of variation of an electromagnetic field in the ether are expressed at each point by the equations of Maxwell, and the causes which produce the electromagnetic field are sought in the existence of positive and negative atoms of electricity and in the motions of these atoms. We are thus returning to a conception which recalls the old idea of two electrical fluids, only that we distinguish clearly the atomic structure of these fluids, and we understand better the relations which exist between the atoms of electricity and matter, a relation which is the most important aspect of the problem.
An atom of electricity in motion produces around itself an electromagnetic field which accompanies the movement of the particle, and which represents a certain quantity of energy whose amount is greater the higher the velocity of the charged projectile. It is not possible to increase this velocity without the expenditure of energy, and in consequence the charged projectile is endowed with a certain inertia. In mechanics inertia is used as a measure of the mass, and we may say that the atom of electricity possesses mass on account of its charge. Computation shows that the mass depends upon the velocity. It remains constant when the velocity of the projectile is small (about one one-hundredth the velocity of light), but for increasing velocities it augments very rapidly and tends toward an infinite value when the velocity approaches that of light, so that this is a limiting velocity which cannot be realized.
It may be imagined that a group of atoms of electricity, both positive and negative, whose total charge is zero, possesses, nevertheless, inertia in consequence of the constituent electrical charges. This group might serve as a model of a material atom. Thus may be proposed a more general form of mechanics than that customarily considered, which is based on the constancy of mass. The latter would be no more than a first approximation to the truth, and holds good only for cases of motion where the velocity is not extremely great. Preliminary attempts have been made to explain universal gravitation between atoms constituted as above proposed. Altogether these studies tend toward an intimate fusion of the idea of electricity and the idea of matter, so that these two conceptions may yet be actually identified.
This proposed constitution of the atoms serves as an excellent foundation for a theory of the emission of light or radiation by a body. Such emission may be regarded as consisting of electro-magnetic waves of short period, emitted by an atom whose constituent ions are in a state of vibration. The same atomic structure serves also very well in the case of radioactive atoms. These atoms are in fact emitting corpuscles, some of which are electrons, others positively charged particles having a size comparable with that of atoms.
But we will not now penetrate further the domain of these theories, but turn rather to examine some of the phenomena which have served as a foundation for their development. It is well known that gases in their ordinary state, when exposed to a weak electrical field have so insignificant a conductibility that they are regarded as remarkably good insulators. But it is not the same when the gases are under the influence of certain exterior conditions, as, for example, the Roentgen rays, for in such conditions a gas becomes conducting. A charged electroscope in connection with a metallic plate in ordinary circumstances loses its charge but slowly. If, however, a stream of Roentgen rays penetrates the air around the plate, the discharge proceeds rapidly. It is not necessary for the Roentgen rays actually to strike the plate, but suffices that the air be traversed within a distance where the electric field is still sensible. This is shown by constraining the Roentgen rays to follow a tube impenetrable to them, and thus shielding the plate from their path, so that it is certainly the gas which is modified and rendered conducting. We say that the gas is ionized, some of its molecules having been decomposed by the rays, and that each of these has given rise to the formation of two ions laden with equal electric charges having opposite signs. The ions are put in motion under the influence of the electric field with a velocity which increases with the strength of the field. If the electroscope is charged positively, the negative ions are drawn toward and discharge it, while the positive ions go in the opposite direction and neutralize the charge found at the extremities of the lines of force which emanate from the plate.
If the gas which has been under the influence of the rays is left to itself without the action of any electric field to move the ions, its conductivity disappears spontaneously, and we say that the ions have recombined to form neutral molecules.
There appear to be in the gas movable charged centers, which travel toward the plate of the electroscope. These centers may be intercepted by means of a screen of paraffin. The screen should not itself be charged, as may be tested by means of a second electroscope. The positive charged plate of the first electroscope may now be covered with the screen, and the Roentgen rays then allowed to act for a time. Negative ions moving toward the charged plate are arrested by the paraffin, and they charge the screen negatively. This may be verified by again bringing the paraffin screen near the second electroscope.
It may be shown that under the action of the Roentgen rays the number of ions produced in a gas in given time is definitely limited.
The rate of discharge of the electroscope is measured by the rate of fall of the gold leaves; and it increases with the electric intensity of the charge, as may be easily understood. Therefore, the stronger the electric field and the greater the velocity the less is the chance that the opposite ions draw together. But for a charge sufficiently great, the rate of the discharge no longer depends on the amount of the charge and does not increase as it augments. Under these circumstances there are no longer any recombinations of ions; they are all utilized for conducting the current, which cannot exceed what they can carry. Such a current is called a current of saturation. It is constant for a given intensity of radiation independent of the sign of the electric charge.
An important difference shows itself between the properties of positive and negative ions. This difference is easily shown by the gases of flames. These gases are ions and conductors, and the approach of flame promotes the electric discharge. Contact with the flame is not necessary. It is sufficient that the ions are produced within the region covered by the electric field. The attraction of the charge of the electroscope suffices to draw from the flame the ions of contrary sign, which neutralize it, and this phenomenon takes place, whatever the sign of the charge. But an isolated flame placed between the two plates of a charged condenser inclines toward the negative field; hence we conclude that the flame is then charged positively. This is because the negative ions produced in the flame are smaller and by far more active than the positive ions, so that they are more easily drawn from the flame, and thus there is left with it an excess of positive electricity. In a cold gas the positive and negative ions have a nearly equal mobility, which is less than that found in a warm gas. They are thought to be in this case formed by the agglomeration of molecules grouped by electrostatic attraction about the charged centers. The dissimilarity between positive and negative ions manifests itself in certain cases even in their formation. This is shown, for example, in what is called the phenomena of Hertz: Certain negatively charged metals, such as zinc, lose their charge when illuminated by ultraviolet light, but if the charge is positive the illumination produces no discharge. It seems to be proved now that zinc and some other easily oxidizable metals have the property of spontaneously giving off electrons under the action of ultraviolet rays. If the emission is given off in a vacuum the electrons are able to acquire a very high velocity in an electric field, and they comport themselves then like the cathode rays of Crookes tubes. If the emission takes place in the air at ordinary pressure the electrons surround themselves with agglomerations of neutral molecules, and form ions of little activity, like those ions which are formed in the air by the Roentgen rays. But in either case the discharge is non-reversible and takes place only if the metal is negatively charged, for the metal is not able to emit negative electrons if the departure of them is obstructed by the attraction of a positive charge residing upon the metal. (To be continued.)
