Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/61

RESISTANCE.] E L E C T R I C I T Y 51 On Resistance in General. We have drawn no distinction between statical and dynamical electricity in our application of Ohm s law, and no such essential distinction has ever been proved to exist. In proportion as a body is a good conductor for galvanic electricity, it is a bad insulator for statical electricity. In sr of general, however, bodies which are good enough insulators Luc- to retain a charge of statical electricity are so bad conduc- y- tors that it is with difficulty that we can compare then- conductivities by means of the voltaic current. On the other baud, it is difficult by means of statical electricity to compare satisfactorily the conductivities of very good conductors. Determinations of the last-mentioned kind have, however, been made by Riess (vide infra, Heating Effects), and the results agree with those obtained by other methods. The insulating power of a substance depends practically to a great extent on the nature of its surface. The dissipation of statical electricity by insu lating supports is due, in most cases, almost entirely to the conducting power of a thin surface layer of mois ture condensed from the atmosphere, or of some product of chemical decomposition caused by exposure to the air, or of dust or other foreign matter accidentally de posited. As far as high specific resistance is concerned, paraffin, shellac, ebonite, and glass at ordinary tempera tures would all be about equally good insulators ; but in practice they stand in the order in which we have named them. Paraffin and shellac surpass the other two in their power of preserving for a long time a clean dry surface ; ebonite is very good for a time, but ultimately its surface becomes covered with a layer of sulphuric acid, arising from the decomposition of the material ; glass, again, is very hygroscopic, although white flint glass, when kept dry by artificial means, is said to be one of the best insulators known, ils. Highest in the order of conductivity stand the metals and their alloys. In this class of bodies the passage of the electric current is unattended by chemical decomposition, and the conductivity decreases as the temperature increases. Along with the metals may ba ranked a few other bodies, which have anomalous conductivity, but are not decomposed; such as graphite, red phosphorus, chloride and oxide of lead under the melting-point, various sulphides and selenides, tellurium, and selenium. In the great majority of the bodies included in this supplementary class the conduc tivity increases with the temperature ; the last two present several anomalies, to which we shall refer farther on. tro- A second class of bodies is formed by those which are " decomposed by the electric current. The specific conduc tivity of these is much lower than that of the metals, and it increases when the temperature is raised. To this class belong, when in solution or in the melted state, most simple binary compounds composed of equal equivalents of two elements, and compounds derived from these by " double decomposition" (see, however, art. ELECTROLYSIS); also some sulphides which have an anomalous conductivity, and glass and some bodies like it, which in the melted state, and in the soft state preceding fusion conduct as electrolytes. con- Non-conductors, on the other hand, are : All gases and ors, vapours, whether at ordinary pressures or in what is called a vacuum, diamond, sulphur, amorphous phosphorus, amorphous selenium, fluid chlorine, bromine, solid and melted iodine, bichloride and biniodide of tin, sulphuric anhydride, solid silicic acid, oxide of iron, oxide of tin; most compounds that are not binary, that is, do not consist of an equal number of equivalents of two components, e.g., many organic compounds etheric oils, resins, wood fibre, caoutchouc ; also " binary compounds " in the solid state. To these may be added pure water, pure hydro chloric acid, &c., which are very bad conductors, if not absolute non-conductors. Before leaving this part of our subject, it will be inte resting to throw together a few of the general principles that have been arrived at, and to give a few numerical results, which will convey to the reader an idea of the posi tion of the different classes of bodies in the scale of con ducting power. For farther details we refer to Wiede- maun s Galvanismus. Metals. (1.) It was remarked by Forbes that the order of con- Specific ductivity is the same for electricity as for heat. The measurements resist- of "VViedemann and Franz have established that the ratio of the ance and conductivities for heat and for electricity is very nearly constant, tempera- not only for pure metals, but also for alloys. (See "Wiedernaim s ture co- Galvanismus, bd. i. 194.) efficient (2.) The conductivity of the pure metals decreases as the teni- of metals. perature rises from to 100 C., the rate of decrease becoming smaller towards the upper limit, Matthiessen expresses the con ductivity by the formula k = k (l- aG + ftd-), where k denotes the conductivity at C., 6 the temperature, and a and ft constants. He found that a and ft had nearly the same value for all pure metals in the solid state, with the exception of thallium and iron, and gives as the mean values for pure metals o = 00376470, /3 = 0000083402. The values fo^ iron are a = 0051182, ft = 0-000012915; for mercury, a= 0007443, = 0000008263. Although there can be no doubt about the general agreement iu the fornmloe for the different pure metals, yet the actual formula arrived at is purely empirical and must be used only between 0" and 100 C. If we earned its application beyond, it would give a minimum conductivity for pure metals about 300 C. The direct experiments of Miiller and Siemens give no indication of such a minimum. The latter represents the results of his experiments (extending in some cases as far as 1000 C. ) by means of the for mula r = aV T + /3T- 7, when, / is the specific resistance, T the air solute temperature, a, ft, y constants. Relying on a formula ot this kind for platinum, Siemens has constructed a pyrometer for determining the temperature of furnaces by means of resistance measurements. (3.) As we have seen, the specific resistance of pure metals goes on increasing continuously as the temperature rises. At the melt ing-point there is a sudden rise in the resistance, and after that the resistance goes on increasing with a smaller temperature coefficient than before. This is in accordance with the fact, that both the specific conductivity and temperature coefficient of mercury are smaller than those of the other metals in the solid state. Bismuth and antimony are exceptions to this rule, in that there is a sudden decrease of resistance at the melting-point. According to the re sults of L. de la Rive, the resistance of metals in general is about doubled in passing the melting-point. We should therefore expect the specific conductivity of froze:? mercury to be about 3*31, that of silver being 100. Alloys. (1.) Matthiessen fouii. 1 that the metals could be divided Alloys. into two classes, according to th conducting properties of their alloys; o. Lead, Tin, Cadmium, and Zinc. ft. Most of the other metals Bismuth, Antimony, Plati num, Palladium, Iron, Aluminium, Sodium, Gold, Copper, Silver. Let v, v be the volumes, s, sf the specific gravities, k, k 1 the con ductivities of the two components of any alloy ; and let 7- 7-, and k =V , v + v called the mean specific gravity, and mean conductivity of the alloy. Then alloys of any one metal of class a, with any other of the same class, have very nearly the mean specific gravity and condnctivity calculated by the above formulae. Alloys of a metel a with a metal ft have specific gravity and con ductivity always less than the mean. If a metal a is alloyed with a considerable percentage of ft, the conductivity is not much altered, but if a metal ft be alloyed with even a very small quantity of a, the conductivity is greatly reduced. Alloys of the metals ft among themselves have in general a con ductivity much inferior to that of either component. The con ductivity remains constant through a considerable range of per centage, but rises very quickly as the percentage of either metal approaches 100. This property is very marked iu an alloy of gold and silver. Matthiessen recommended an alloy of two parts by weight of gold to one of silver for the reproduction of the standard of resistance. The resistance of such an alloy would be very slightly affected by small variations in its composition. Mercury, and melted metals generally, are not subject to the foregoing laws. A very small percentage of another even worse

conducting metal raises the conductivity of mercury, but the