Electrical conductivity

“ Extremely fine metallic powder, as a rule, offers almost perfect resistance to the passage of a current. But if we take a sufficiently short column and exert a sufficiently great pressure a point is soon reached when the electrical influence will effect a sudden increase in the conductivity. ”

“ The relative power of conducting the electric current possessed by
different substances. A path for the current through the ether is opened
by the presence of a body of proper quality, and this quality, probably
correlated to opacity, is termed conductivity. There is no perfect
conductor, all offer some resistance, q. v., and there is hardly any
perfect non-conductor. It is the reverse and reciprocal of resistance. ”

“ In such a medium, the higher the frequency and potential difference, the greater wilt be the current which will find its way through the surrounding medium, and the smaller the part which will pass through the central portion of the wire: In the case of a wire immersed in air and traversed by a high-frequency current, the facility with which the energy is dissipated may be considered as the equivalent of the conductivity ”

“ From the point of view of the field-concept, conductivity consists in the property of a body not to allow any change of potential along its surface. Such a surface, therefore, is always an equipotential. In the language of alchemy, conductivity is a mercurial property. In the presence of such a body, therefore, no Salt-Sulphur contrasts can obtain. In view of what we found above as the mean position of the metals in the alchemic triad, it is significant that they, precisely, should play so outstanding a role as electrical conductors. ”

“ If the medium, instead of being a perfect insulator, is a conductor whose conductivity per unit of volume is , the disturbance will consist not only of electric displacements but of currents of conduction, in which electric energy is transformed into heat, so that the undulation is absorbed by the medium. ”

“ The best conductors offer some resistance to the passage of the current and the best insulators conduct to some extent. If we make a comparison of electric conductors we find that the metals that conduct heat best also conduct electricity best. This, it seems to me, is a confirmation of the atomic theory of electricity so far as it means anything. If a good conductor, as silver, is subjected to intense cold by putting it into liquid air, its conductivity is greatly increased. It is well known that [Pg 48] heating a conductor ordinarily diminishes its power to conduct electricity. ”

“ Nature knows no hard lines, so that we find that even the worst conductors will permit the escape of some electricity, while the very best conductors oppose a measurable resistance to its passage. Between the limits of good conductors, on the one hand, and non-conductors (or insulators) on the other, we have bodies possessing varying degrees of conductivity. ”

“ If we eliminate the polarization at the electrodes, it can be shown that an electrolyte possesses a definite electric resistance and therefore a definite conductivity. The conductivity gives us the amount of electricity conveyed per second under a definite electromotive force. On the view of the process of conduction described above, the amount of electricity conveyed per second is measured by the product of the number of ions, known from the concentration of the solution, the charge carried by each of them, and the velocity with which, on the average, they move through the liquid. ”

“ To make matters worse, the same cause that creates a path of relatively good conductivity along wooden pins and cross-arms often materially lowers the resistance offered to the leakage of current by the insulator surfaces. Thus an increase of the rate at which energy passes from wire to wire of a circuit, and the concentration of this energy in certain parts of the wooden path, are sometimes brought about at the same time. ”

“ CONDUCTION, ELECTRIC. The electric conductivity of a substance is that property in virtue of which all its parts come spontaneously to the same electric potential if the substance is kept free from the operation of electric force. Accordingly, the reciprocal quality, electric resistivity, may be defined as a quality of a substance in virtue of which a difference of potential can exist between different portions of the body when these are in contact with some constant source of electromotive force, in such a manner as to form part of an electric circuit. ”

“ The fact that the conductivity of the gas is removed by filtering shows that it is due to something mixed with the gas which is removed from it by filtration, and since the conductivity is also removed by an electric field, the cause of the conductivity must be charged with electricity so as to be driven to the sides of the tube by the electric force. ”

“ We can show this by replacing the tube C by a metal tube with an insulated wire passing down the axis of the tube. If there is no potential difference between the wire and the tube then the electroscope will leak when a current of air is drawn through the vessel, but the leak will stop if a considerable difference of potential is maintained between the wire and the tube: this shows that a strong electric field removes the conductivity from the gas. ”

“ The figures given as indicating the relative resistance of the above bodies to the passage of electricity must be taken as approximate only, since the conductivity of all these bodies varies very largely with their purity, and with the temperature. Metals become worse conductors when heated ”

“ A number of rather long transmissions is carried out in Europe with continuous, constant current, and in such systems the line voltage varies directly with the load. As the loss of power in an electrical conductor depends entirely on its ohms of resistance, which are constant at any given temperature, and on the amperes of current passing through it, the line loss in a constant-current system does not change during the period of operation, no matter how great may be its changes of load. ”

“ In very large systems it is worth noting that the so-called “ skin effect " in alternating current conductors may become conspicuous. In the transmission circuits themselves the wires are, in practice, never large enough to produce any sensible difference in conductivity for continuous and for alternating currents. ”

“ It should be observed that the phenomenon of suddenly increased conductivity occurs, even if the sensitive substance is not in circuit with a battery at the time it is influenced. Thus, the metallic filings, after having been placed in circuit with a Daniell cell, and its high resistance observed, may then be completely insulated and submitted in this condition to the action of a distant spark, or of a charged rod, or of induced currents. ”

“ In the first place, in moving a current of electricity to and from a coil of wire, or any other conductor, or (which is the same thing, since action and reaction are equal and opposite) in moving a coil of wire or any other conductor to and from a current of electricity, a sense of resistance will be experienced, and energy will have to be spent upon the process; in the second place, an electrical current will be generated in the conductor, and the conductor will be heated in consequence. ”

“ The accumulator if charged and left insulated appears to lose its charge by conduction, but it is found that the proportionate rate of loss is much greater at first than it is afterwards, so that the measure of conductivity, if deduced from what takes place at first, would be too great. ”

“ How quickly a spoon inserted into a hot cup of tea becomes uncomfortably hot, though the hand can be held very close to the liquid without feeling more than a gentle warmth. And we all have noticed that the very least air-gap in an electric circuit effectively breaks a current capable of traversing miles of wire. If the current is so intense that it insists on passing the gap, it leaps across with a report, making a spark that is at once intensely bright and hot. Metal wires are to electricity what speaking tubes are to sound; they are as it were electrical tubes through the air and ether. ”

“ If the tube be exhausted of air, a non electric lining in contact with the wire is not necessary; for in vacuo, the electrical fire will fly freely from {78}the inner surface, without a non-electric conductor: but air resists its motion; for being itself an electric per se, it does not attract it, having already its quantity. ”

“ It is absolutely certain that the electric force will make its way through materials, termed good conductors, which allow it free passage, and avoid those of the opposite class, or bad conductors, the character of every substance on earth being well known as regards these qualifications, although it would not be easy to draw sharp lines of demarcation, all conductivity being relative and not absolute. ”

“ It is also, by Art. 93, equal to , under the condition that the potentials of the conductors are constant. If the conductors are insulated, they tend to move so that their energy is diminished, and the work done by the electrical forces during the displacement is equal to the diminution of energy. ”

“ All metals conduct electricity to some extent, but certain ones are very much better than others. For example, lead, platinum, brass, and iron are poor conductors, which is equivalent to saying that they heat up readily on the passage of an electric current. On the other hand, silver, copper, and aluminum are good conductors. In making a lightning rod, the best all-round conductor should be used, when cost and conductivity are the basis for the selection. As an example, take the metals iron, copper, and aluminum for comparison. ”

“ External disturbances or stimuli impinging on a substance induce a molecular upset, the effectiveness of the stimulus depending on the rapidity of the onset of the disturbance. The molecular upset is attended by changes induced in the properties of the substance. The most sensitive means for its detection is electrical: by the method of conductivity or of electromotive variation (p. 170) .
Stimulation of the substance can be produced by electric radiation, by visible light, and by mechanical vibration. ”

“ Suppose such a sphere to be charged and discharged rapidly with electricity from any source, such as a battery. It would take electricity from the source at a certain rate, and would in fact act like a resistance in permitting the passage through it or by it of a certain quantity of electricity per unit of time. If K is the capacity and n is the number of discharges per second, then nK is a quantity of the dimensions of an electric conductivity, or of the reciprocal of a resistance. ”

“ With these adjustments and the use of iodide of potassium on paper, chemical action is sometimes a more delicate test of electrical currents than the galvanometer (273.) . Such cases occur when the bodies traversed by the current are bad conductors, or when the quantity of electricity evolved or transmitted in a given time is very small. ”

“ As the solid becomes a fluid, it loses almost entirely the power of conduction for heat, but gains in a high degree that for electricity; but as it reverts hack to the solid state, it gains the power of conducting heat, and loses that of conducting electricity. ”

“ 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. ”

“ It has long been the custom of professional engineers to represent graphically on sheets of paper ruled in squares, various properties of matter under regularly-varying conditions. For instance, to show the electric conductivity of wire at various temperatures, horizontal strips represent degrees of temperature, and vertical strips, degrees of conductivity; and if a line connecting points corresponding to conductivity at various temperatures be convexly curved above, it shows that conductivity decreases more rapidly than temperature increases. ”

“ The existence of currents of electricity passing through the earth's crust and on its surface along the lines of least resistance has long been an established fact. Experiments conducted at Harvard, U.S.A., by Professor Trowbridge have proved beyond a doubt that, by means of such delicate apparatus as the telephone and microphone, it is possible for the observer to state in which direction, from a given point, the best line of conductivity runs. ”

“ In fact, the conductivity of a pure metal for heat is almost independent of the temperature; while the electric conductivity varies in inverse proportion to the absolute temperature, so that a pure metal as it approaches the absolute zero of temperature tends to assume the character of a perfect conductor. ”

“ In later telephones the current is actually transmitted [81] through the contacting points, but this only became practicable after the carbon microphone transmitter was invented, in which the essential undulations of the electric current were produced in another way, i. e., by the application of the important discovery that the varying of the pressure on carbon, by vibration, varied its conductivity, and in this way produced the same result of undulating a current without breaking it. ”

“ Electrical energy may be transmitted around the world if the line voltage is unlimited. This follows from the law that a given power may be transmitted to any distance with constant efficiency and a fixed weight of conductors, provided the voltage is increased directly with the distance.
The physical limits of electric-power transmission are thus fixed by the practicable voltage that may be employed. The effects of the voltage of transmission must be met in the apparatus at generating and receiving stations on the one hand, and along the line on the other. ”