Insulator

“ Faraday showed by experiment that if the charged sphere is immersed in an insulating fluid, say an insulating mineral oil, or in a solid insulator like rubber, or even if a piece of an insulator is brought near it, then the reacting force for a given charge on the sphere is smaller than when the sphere is surrounded by a vacuum; or, in other words, liquid and solid insulators are more permeable to the electrical lines of force (that is, to electricity) than a vacuum is. ”

“ An insulator is a non-conducting substance, impervious, so to speak, to electricity. It is the theoretical antipodes of a conductor. While the conductor is a substance through or over which electricity flows freely, the insulator will neither{282} permit electricity to pass through its mass nor over its surface. It can therefore be used as a means of confining electricity to a conductor, and preventing it from escaping to other conductors in the neighbourhood. In short it can be made to insulate or isolate the particular conductor from all other conductors. ”

“ If mechanical displacement of particles or transportation of conducting bodies seem inadmissible, it is probable that there is a modification of the insulator itself, the modification persisting for some time by virtue of a sort of “coercive force.” An electric current of high potential, which would be completely stopped by a thick insulating sheet, may be supposed to gradually traverse the very thin dielectric layers between the conducting particles, the passage being effected very rapidly if the electric pressure is great, and more slowly if the pressure is less. ”

“ Such suspension is necessary, for if we were to handle the spheres or lay them on to the table their charges would leak away; if we wish a body to preserve its charge for a sensible time we must support it by an insulator—such as silk, glass, mica, ebonite, which does not allow electricity to flow along or through it. Metals offer a very easy path for the flow of electricity, and are therefore called conductors. There is no sharp line of demarcation between insulators and conductors. Dry wood, for instance, is not a perfect insulator; and when damp it is not a perfect conductor. ”

“ Ordinary insulators display the characteristics of the dielectric of a capacitor formed by the wire and ground. As the frequency increases, the overall impedance decreases. A better high-frequency insulator is a quarter-wave section of transmission line shorted at one end. Such an insulator is shown in figure 3-20. The impedance of a shorted quarter-wave section is very high at the open-end junction with the two-wire transmission line. This type of insulator is known as a METALLIC INSULATOR and may be placed anywhere along a two-wire line. ”

“ To the end of a sprit is attached an insulator of some kind, which may be a simple ebonite rod, or sometimes a more elaborate arrangement of oil insulators, and to the lower end of this insulator is attached the aerial wire. As at the top of the aerial we have to deal with potentials capable sometimes of giving sparks several feet in length, the insulation of the upper end of the aerial is an important matter. ”

“ A conductor is attracted by a charged body more strongly than an insulator, because the latter is not easily polarized. A neutral body, then, is really no longer neutral when it is in the electric field. Polarization precedes attraction.
EXPERIMENT 78. To find whether electric induction will act through an insulator.
Apparatus. Small bits of carbon (Exp. 58) ; bits of moist tissue-paper, T P; one-half of the flat box, T F B (No. 41) ; sheet of glass, G (No. 38) ; electrophorus (Exp. 68) . Place the carbon and T P into T F B (Fig. 45) , and cover with the glass.
[64] ”

“ When the outgoing and incoming wires are both placed upon or underground the strain upon the insulation is very much increased, for then instead of the two lines being separated by fifteen or twenty feet of pole, which is a very fair insulator, they are separated by only a few inches of earth or perhaps metal, the first of which is a fairly good conductor, while the last is a nearly perfect one. It is evident, therefore, that the insulation proper in an underground or surface system must be of the highest order. ”

“ When two bodies are rubbed together to produce electricity in the usual way, one at least must be an insulator. During the act of rubbing, the particles of opposite kinds must be brought more or less closely together, the few which are most favourably circumstanced being in such close contact as to be short only of that which is consequent upon chemical combination. At such moments they may acquire by their mutual induction (1740.) ”

“ The insulation of lightning-rods, says the Journal, is a grave error, because the insulators to some extent arrest the flow of currents of rarefied electricity, which it is the true function of the lightning-rod to facilitate. On the other hand, the insulator amounts to nothing as a barrier against a discharge of lightning, which can either pass through it or leap the short distance between the rod and the building. ”

“ In pole and over-house lines, the wires are kept from each other and the current from escaping to the earth by insulators of glass. In Europe white porcelain and brown stoneware insulators are used, and the former substance, when of good quality, well glazed and well burned, is perhaps the most perfect of all insulating materials, and does not deteriorate with age. The fewer the poles on which the wires are suspended, the better is the insulation and the less the cost, but the liability to accident is greater. ”

“ One end of the stem is firmly attached to the pole, and the other bears the wire. The best idea of a single telegraphic insulator may be got from a common umbrella, with its stem of insulating substance attached upright to the top of a pole and bearing the wire supported in a notch on the top outside. The umbrella may be either of the same substance as the stem—all glass or all glazed earthenware, for instance—or of a stronger material, such as iron, with an insulating stem fitted to it to support it below. Very good insulators may be made of continuous glass ”

“ If the dielectric is not a good insulator, it is difficult to measure its inductive capacity, because the accumulator will not hold a charge for a sufficient time to allow it to be measured; but it is certain that inductive capacity is a property not confined to good insulators, and it is probable that it exists in all bodies. ”

“ It may therefore be confidently asserted that no insulator can possibly be made that would be capable of preventing the electric discharge leaving the lightning conductor provided it could find an easier path leading to the earth. Mr. Phin, in his work on ‘Lightning-Rods’ says very pertinently:—But not only are insulators worthless—they are positively dangerous if the principle upon which they are adopted is fully carried out, which, however, is but rarely done. ”

“ When the condenser is not subjected to a very high potential difference, the insulator separating its two surfaces or coatings need not be glass, but may be a cheaper material, such as paraffined paper. ”

“ Wooden insulator pins are among the weakest elements in electric transmission systems. As line voltages have gone up it has been necessary to increase the distances between the outside petticoats of insulators and their cross-arms and to lengthen the insulators themselves in order to keep the leakage of current between the conductors within permissible limits. ”

“ No. 4 is a heavy electroid which will not break or burn. No. 5 is an extra heavy insulator which insures a good spark in case the outer porcelain insulator [187] becomes broken or cracked. No. 6 is a bushing which holds the insulator firmly in place from the top. No. 7 is a high compression washer which allows for upward expansion and makes an even seat for the bushing which holds the insulator in position. No. 8 is a massive porcelain insulator designed to withstand a high temperature without cracking. No. 9 is a copper asbestos washer that allows for the downward expansion of the insulator. ”

“ Mechanical failures of wooden insulator pins have been far more common than those of poles, both as a direct result of the line strains and because such pins are often charred and weakened by the leakage of energy from the conductors. For these reasons the general use of iron or steel pins for the insulators of long lines operating at high voltages seems desirable. ”

“ Earth-conductor.
is run through the hook on the top of the insulator, and stretched tight by a man holding it over his shoulder, who keeps it in this position until the next pole is ready to receive it. Wherever there are trees or walls near the line, the work is still further lightened by dispensing with the poles, and merely attaching the wire to the insulators specially constructed for this purpose. ”

“ Conductors and insulators are necessary to the efficient and economic flow of a current, the insulator preventing the escape of electricity and lessening the danger of fire, and the conductor carrying the current. ”

“ Porcelain insulators are much stronger mechanically than are those of glass, and are not liable to crack because of unequal internal expansion, a result sometimes met with where glass insulators are exposed to a hot morning sun. ”

“ Fig. 206.—The Battery.
It should be observed that the degree of value as a conductor or non-conductor depends somewhat upon the atmosphere. For instance, glass is an excellent insulator, or non-conductor, when dry, but when wet it changes to a conductor. So insulators are at times covered with a solution of shell-lac, or fat, to keep away moisture. We may reasonably conclude that bodies [Pg 204] which are good and bad conductors are good and bad conductors of electricity. ”

“ In many animals the insulation is increased by non-conducting hair, wool, fur, etc. And naked man finds it expedient to reinforce his own insulation by clothing of silk, satin, hair, wool, flannel and other non-conducting materials. We are exhilarated by a dry atmosphere: depressed by a damp one, because the moist air, being a conductor, carries off some of our electricity to the earth, while dry air is a more complete insulator and prevents this leakage. ”

“ It is a well-ascertained fact that the insulator, gutta-percha, is, when kept under water, practically imperishable, so that it is only the original strength of the sheathing wires and the deterioration allowable in them that have to be considered. Cables have frequently been picked up showing after many years of submergence no appreciable deterioration in this respect, while in other cases ends have been picked up which in the course of twelve years had been corroded to needle points, the result probably of metalliferous deposits in the locality. ”

“ All insulators, whether made of glass or porcelain, should be tested electrically to determine their ability to resist puncture, and to hold back the surface leakage of current, before they are put into practical use on high-tension lines. Experience has shown that inspection alone cannot be depended on to detect defective glass insulators. Electrical testing of insulators serves well to determine the voltage to which they may be subjected in practical service with little danger of puncture by the disruptive passage of current through their substance. ”

“ It is not known whether there is such a thing as a perfect insulator, even if we presuppose ideal circumstances. Materials as they exist must be regarded merely as of high specific resistance, that is if we allow ourselves to use such a term in connection with substances, conduction through which is neither independent of electromotive force per unit length, nor of previous history. ”

“ If we go into a science laboratory and examine the great brass machines for holding electrical charges, we find that they are all mounted on glass feet. These are the insulators, and if it were not for them, no electricity would remain on the surface; as it is, electricity is hard enough to keep in charge, even with the best insulators. And we know sometimes what it is to have life and power pass into us from above, but we don't know how to retain it, because we have never learnt true retirement of heart and insulation of life. ”

“ As a medium for the discharge space, a solid, or even a liquid, would be preferable therefor. It is somewhat difficult to conceive of a solid body which would possess the property of closing instantly after it has been cracked. But a liquid, especially under great pressure, behaves practically like a solid, while it possesses the property of closing the crack. Hence it was thought that a liquid insulator might be more suitable as a dielectric than air. ”

“ The distinction between a conductor and an insulator or dielectric is supposed to be that in the former the particles of electricity can pass with more or less freedom from molecule to molecule. In the latter such transference is impossible, the particles can only be displaced within the molecule with which they are connected; the cells or vortices of the medium are supposed to be elastic, and to resist by their elasticity the displacement of the particles within174 them. ”

“ In general my experience tends to show that bodies which possess the highest specific inductive capacity, such as glass, afford a rather inferior insulation to others, which, while they are good insulators, have a much smaller specific inductive capacity, such as oils, for instance, the dielectric losses being no doubt greater in the former. ”