Dielectric

“ This theory of dielectrics is consistent with the laws of electricity, and may be actually true. If it is true, the specific inductive capacity of a dielectric may be greater, but cannot be less, than that of air or vacuum. No instance has yet been found of a dielectric having an inductive capacity less than that of air, but if such should be discovered, Mossotti's theory must be abandoned, although his formulae would all remain exact, and would only require us to alter the sign of a coefficient. ”

“ To store up a certain amount of electric energy in a condenser, we require a certain definite volume of dielectric, no matter how we may arrange it, and the volume required per unit of energy is determined by the dielectric strength of the material. Thus, for instance, ordinary sheet glass cannot be safely employed with a greater electric force than is represented by 20,000 volts for one-tenth of an inch in thickness, or, say, a potential gradient of 160,000 volts per centimetre. ”

“ To represent the case in which there is true conduction through the dielectric we must either make the piston leaky, or we must establish a communication between the top of the tube and the top of the tube
In this way we may construct a mechanical illustration of the properties of a dielectric of any kind, in which the two electricities are represented by two real fluids, and the electric potential is represented by fluid pressure. Charge and discharge are represented by the motion of the piston and electromotive force by the resultant force on the piston. ”

“ Remembering that E.M.I., M.I., and direction of flow of energy are connected by the right-handed screw relation, we see that the energy moves outwards from the space between A and B. As then the strain of the dielectric between A and B is gradually released by what we call a discharge current along the wire LMN, the energy thus given up travels outwards through the dielectric, following always the equipotential surfaces, and gradually converges once more on the circuit where the surfaces are cut by the wire. There the energy is transformed into heat. ”

“ These superficial electrifications will in general destroy one another when consecutive elements are considered, except where the dielectric has an internal charge, or at the surface of the dielectric. ”

“ We have seen that when electromotive force acts on a dielectric medium it produces in it a state which we have called electric polarization, and which we have described as consisting of electric displacement within the medium in a direction which, in isotropic media,, coincides with that of the electromotive force, combined with a superficial charge on every element of volume into which we may suppose the dielectric divided, which is negative on the side towards which the force acts, and positive on the side from which it acts. ”

“ Two substances may be equally good insulators, that is, they may equally well resist the spread of electrification over their surfaces, or the flow of the electric current through them, while one may be, nevertheless, a better dielectric than the other. ”

“ The excess of water in the tube may be taken to represent a positive charge of electricity on one side of the dielectric, and the excess of mercury in the tube may represent the negative charge on the other side. The excess of pressure in the tube on the side of the piston next will then represent the excess of potential on the positive side of the dielectric.
If the piston is free to move it will move back to and be in equilibrium there. This represents the complete discharge of the dielectric. ”

“ The relation between the electromotive force and the amount of electric displacement it produces depends on the nature of the dielectric, the same electromotive force producing generally a greater electric displacement in solid dielectrics, such as glass or sulphur, than in air. ”

“ It is greater, for instance, in dense air than in rare air, and greater in glass than in air, but in every case, if the electromotive force be made great enough, the dielectric gives way and its insulating power is destroyed, so that a current of electricity takes place through it. It is for this reason that distributions of electricity for which the electric resultant force becomes anywhere infinite cannot exist in nature. ”

“ In a dielectric under induction, we may conceive that the electricity in each molecule is so displaced that one side is rendered positively, and the other negatively electrical, but that the electricity remains entirely connected with the molecule, and does not pass from one molecule to another. ”

“ In the course of this research on induction, Faraday had, as we have seen, been compelled to adopt new ideas, and therefore to adopt new names to denote them. The term dielectric for the medium in or across which the electric forces operate was one of these. ”

“ If, instead of the oil, a solid plate of the best dielectric, even several times thicker than the oil intervening between the metal plates, is inserted between the latter, the air having free access to the charged surfaces, the dielectric invariably is warmed and breaks down. ”

“ In the case of conduction currents electricity flows or moves through a stationary material body called the conductor. In convection currents electricity is carried from place to place with and on moving material bodies or particles. In dielectric currents there is no continued movement of electricity, but merely a limited displacement through or in the mass of an insulator or dielectric. The path in which an electric current exists is called an electric circuit, and may consist wholly of a conducting body, or partly of a conductor and insulator or dielectric, or wholly of a dielectric. ”

“ The effect of enlarging the conducting surfaces which are opposed to each other in the act of induction, is, if the electricity be limited in its supply, to lower the intensity of action; and this follows as a very natural consequence from the increased area of the dielectric across which the induction is effected. ”

“ The conditions requisite for the production of an electric spark in its simplest form are well-known. An insulating dielectric must be interposed between two conducting surfaces in opposite states of electricity, and then if the actions be continually increased in strength, or otherwise favoured, either by exalting the electric state of the two conductors, or bringing them nearer to each other, or diminishing the density of the dielectric, a spark at last appears, and the two forces are for the time annihilated, for discharge has occurred. ”

“ The energy, by the expenditure of which electrical attractions and repulsions are produced, we suppose to be stored up in the dielectric medium which surrounds the electrified bodies, and not on the surface of those bodies themselves, which on our theory are merely the bounding surfaces of the air or other dielectric in which the true springs of action are to be sought. ”

“ His electrophorus in one form consisted of a disk of non-conducting material, such as pitch or resin, placed between two metal sheets, one being provided with an insulating handle. For the pitch or resin may be substituted a sheet of glass, ebonite, india-rubber or any other good dielectric placed upon a metallic sheet, called the sole-plate. To use the apparatus the surface of the dielectric is rubbed with a piece of warm flannel, silk or catskin, so as to electrify it, and the upper metal plate is then placed upon it. ”

“ Why a point should be so exceedingly favourable to the production of currents in a fluid insulating dielectric, as air, is very evident. It is at the extremity of the point that the intensity necessary to charge the air is first acquired (1374.) ; it is from thence that the charged particle recedes; and the mechanical force which it impresses on the air to form a current is in every way favoured by the shape and position of the rod, of which the point forms the termination. ”

“ The heat produced is dissipated into the surrounding medium. When there is no potential difference between two conductors, the atoms in the dielectric material between them are normal and the orbits of the electrons are circular. When there is a potential difference between two conductors, the orbits of the electrons change. The excessive negative charge on one conductor repels electrons on the dielectric toward the positive conductor and thus distorts the orbits of the electrons. A change in the path of electrons requires more energy, introducing a power loss. ”

“ The only safe dielectric for such accumulators is air, which has this inconvenience, that if any dust or dirt gets into the narrow space between the opposed surfaces, which ought to be occupied only by air, it not only alters the thickness of the stratum of air, but may establish a connexion between the opposed surfaces, in which case the accumulator will not hold a charge. ”

“ Many electricians frequently confound the theoretical dielectric of Maxwell with the dielectric bodies in use. They do not stop to think that the only perfect dielectric is ether, and that all other bodies, the existence of which is known to us, must be conductors, judging from their physical properties. ”

“ Mica and glass rank high, while air is low. The thickness of the dielectric also depends on the area of the condenser, the time of charging, and discharging in proportion to the applied potential. Yes, air can be used as a dielectric, but is of little value in its natural condition. By pumping air into a tank which holds the condenser plates, we can use it as a dielectric, because its insulating qualities increase with pressure. Arlington uses condensers insulated by air at 250 pounds pressure per square inch. If I am not mistaken, a potential of 18,000 volts is applied ”

“ If, then, we define electric displacement as that change which takes place in a dielectric when it becomes the seat of electrostatic energy, and if, further, we suppose that the change, whatever it be mechanically, satisfies certain well-known laws, and that in consequence certain pressures and tensions exist in the dielectric, electrostatic problems can be solved without reference to a charge of electricity residing on the conductors. ”

“ Using rarefied air as the dielectric, it is very easy to obtain luminous phenomena as brushes, or glow, upon both conducting balls or terminations, whilst the interval is dark, and that, when the action is so momentary that I think we cannot consider currents as effecting discharge across the dark part. ”

“ An important inquiry regarding the electrical polarity of the particles of an insulating dielectric, is, whether it be the molecules of the particular substance acted on, or the component or ultimate particles, which thus act the part of insulated conducting polarizing portions (1669.) ”

“ This true electrification must be distinguished from the apparent electrification , which is the electrification as deduced from the electrical forces in the neighbourhood of the surface, using the ordinary characteristic equation
If a solid dielectric of any form is a perfect insulator, and if its surface receives no charge, then the true electrification remains zero, whatever be the electrical forces acting on it. ”

“ The wire has capacity with respect to the earth, and it acts like the inner coating of a Leyden jar, of which the dielectric is the air and ether around it, and the outer coating is the earth's surface. When the discharge takes place, we may consider that electrons rush out of the wire and then rush back again into it. At the moment when the electrons rush out of or into the aerial wire, we say there is an electric current flowing into or out of the wire, and this electron movement, therefore, creates the magnetic flux which is distributed in concentric circles round the wire. ”

“ But inasmuch as there would be always some loss in the gap, the medium, which should be continuous should exchange through the gap at a rapid rate. In the preceding example, the glass being perfectly closed, it would mean that the dielectric in the discharge space possesses a great insulating power; the glass being cracked, it would signify that the medium in the space is a good conductor. ”

“ It seems probable, that the tension of a particle of the same dielectric, as air, which is requisite to produce discharge, is a constant quantity, whatever the shape of the part of the conductor with which it is in contact, whether ball or point ”

“ Well, the dielectric constant of space is calculable. Measure up your air condenser and see if it comes out even. Get the boys to measure the radiation resistance of this space. It should be three hundred and seventy-seven ohms. That is—if we are still in our original space. Also you might get the standing wave ratio on some of the microwave transmission lines. They depend upon the characteristic impedances of space, the permeability and dielectric constant. ”

“ According to the undulatory theory, the velocity of light in different media is inversely proportional to their indices of refraction.
There are no transparent media for which the magnetic capacity differs from that of air more than by a very small fraction. Hence the principal part of the difference between these media must depend on their dielectric capacity. According to our theory, therefore, the dielectric capacity of a transparent medium should be equal to the square of its index of refraction. ”

“ The work of Benjamin Franklin, Henry Cavendish, Michael Faraday and J. Clerk Maxwell demonstrated, however, that all electric charge or electrification of conductors consists simply in the establishment of a physical state in the surrounding insulator or dielectric, which state is variously called electric strain, electric displacement or electric polarization. ”