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ELECTRIC CONDUCTION

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Originally appearing in Volume V06, Page 858 of the 1911 Encyclopedia Britannica.
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ELECTRIC CONDUCTION. 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. All material substances possess in some degree, large or small, electric conductivity, and may for the sake of convenience be broadly divided into five classes in this respect. Between these, however, there is no sharply-marked dividing line, and the classification must therefore be accepted as a more or less arbitrary one. These divisions are: (1) metallic conductors, (2) non-metallic conductors, (3) dielectric conductors, (4) electrolytic conductors, (5) gaseous conductors. The first class comprises all metallic substances, and those mixtures or combinations of metallic substances known as alloys. The second includes such non-metallic bodies as carbon, silicon, many of the oxides and peroxides of the metals, and probably also some oxides of the non-metals, sulphides and selenides. Many of these sub-stances, for instance carbon and silicon, are well-known to have the property of existing in several allotropic forms, and in some of these conditions, so far from being fairly good conductors, they may be almost perfect non-conductors. An example of this is seen in the case of carbon in its'three allotropic conditions —charcoal, graphite and diamond. As charcoal it possesses a fairly well-marked but not very high conductivity in comparison with metals; as graphite, a conductivity about one-four-hundredth of that of iron; but as diamond so little conductivity that the substance is included amongst insulators or non-conductors. The third class includes those substances which are generally called insulators or non-conductors, but which are better denominated dielectric conductors; it comprises such solid substances as mica, ebonite, shellac, india-rubber, gutta-percha, paraffin, and a large number of liquids, chiefly hydro-carbons. These substances differ greatly in insulating power, and according as the conductivity is more or less marked, they are spoken of as bad or good insulators. Amongst the latter many of the liquid gases hold a high position. Thus, liquidoxygen and liquid air have been shown by Sir James Dewar to be almost perfect non-conductors of electricity. The behaviour of substances which fall into these three classes is discussed below in section I., dealing with metallic conduction. ' The fourth class, namely the electrolytic conductors comprises. all those substances which undergo chemical decomposition when they form part of an electric circuit traversed by an electric current. They are discussed in section II., dealing with electrolytic conduction. The fifth and last class of conductors includes the gases. The conditions under which this class of substance becomes possessed of electric conductivity are considered in section III., on conduction in gases. In connexion with metallic conductors, it is a fact of great interest and considerable practical importance, that, although the majority of metals when in a finely divided or powdered condition are practically non-conductors, a mass of metallic powder or filings may be made to pass suddenly into a conductive condition by being exposed to the influence of an electric wave. The same is true of the loose contact of two metallic conductors. Thus if a steel point, such as a needle, presses very lightly against a metallic plate, say of aluminium, it is found that this metallic contact, if carefully adjusted, is non-conductive, but that if an electric wave is created anywhere in the neighbourhood, this non-conducting contact passes into a conductive state. This fact, investigated and discovered independently by D. E. Hughes, C. Onesti, E. Branly, O. J. Lodge and others, is applied in the construction of the " coherer," or sensitive tube employed as a detector or receiver in that form of " wireless telegraphy " chiefly developed by Marconi. Further references to it are made in the articles ELECTRIC WAVES and TELEGRAPHY: Wireless. International Ohm.—The practical unit of electrical resistance was legally defined in Great Britain by the authority of the queen in council in 1894, as the " resistance offered to an invariable electric current by a column of mercury at the temperature of melting ice, 14.4521 grammes in mass, of a constant cross-sectional area, and a length Io6.3 centimetres." The same unit has been also legalized as a standard in France, Germany and the United States, and is denominated the " International or Standard Ohm." It is intended to represent as nearly as possible a resistance equal to to° absolute C.G.S. units of electric resistance. Convenient multiples and sub-divisions of the ohm are the microhm and the megohm, the former being a millionth part of an ohm, and the latter a million ohms. The resistivity of substances is then numerically expressed by stating the resistance of one cubic centimetre of the substance taken between opposed faces, and expressed in ohms, microhms or megohms, as may be most convenient. The reciprocal of the ohm is called the mho, which is the unit of conductivity, and is defined as the conductivity of a substance whose resistance is one ohm. The absolute unit of conductivity is the conductivity of a substance whose resistivity is one absolute C.G.S. unit, or one-thousandth-millionth part of an ohm. Resistivity is a quality in which material substances differ very widely. The metals and alloys, broadly speaking, are good conductors, and their resistivity is conveniently expressed in microhms per cubic centimetre, or in absolute C.G.S. units. Very small differences in density and in chemical purity make, however, immense differences in electric resistivity; hence the values given by different experimentalists for the resistivity of known metals differ to a considerable extent. I. CONDUCTION IN SOLIDS It is found convenient to express the resistivity of metals in two different ways: (I) We may state the resistivity of one cubic centimetre of the material in microhms or absolute units taken between opposed faces. This is called the volume-resistivity; (2) we may express the resistivity by stating the resistance in ohms offered by a wire of the material in question of uniform cross-section one metre in length, and one gramme in weight. This numerical measure of the resistivity is called the mass-resistivity, The mass-resistivity of a body is connected with its volume-resistivity and the density of the material in the following manner :—The mass-resistivity, expressed in microhms per metre-gramme, divided by so times the density is numerically equal to the volume-resistivity per centimetre-cube in absolute C.G.S. units. The mass-resistivity per metre-gramme can always be obtained by measuring the resistance and the mass of any wire of uniform cross-section of which the length is known, and if the density of the substance is then measured, the volume-resistivity can be immediately calculated. If R is the resistance in ohms of a wire of length 1, uniform cross-section s, and density d, then taking p for the volume-resistivity we have Io9R=pl/s; but lsd=M. where M is the mass of the wire. Hence t o9 R = pol2/M. If 1= too and M = t, then R = p' = resistivity in ohms per metre-gramme, and Io9p'=Io,000dp, or p=Io5p'/d, and p' = t o,000M R/12. The following rules, therefore, are useful in connexion with these measurements. To obtain the mass-resistivity per metre-gramme of a substance in the form of a unifcrm metallic wire: Multiply together 10,000 times the mass in grammes and the total resistance in ohms, and then divide by the square of the length in centimetres. Again, to obtain the volume-resistivity in C.G.S. units per centimetre-cube, the rule is to multiply the mass-resistivity in ohms by 100,000 and divide by the density. These rules, of course, apply only to wires of uniform cross-section. In the following Tables I., II. and III. are given the mass and volume resistivity of ordinary metals and certain alloys expressed in terms of the inter-national ohm or the absolute C.G.S. unit of resistance, the values being calculated from the experiments of A. Matthiessen (183t-187o) between 186o and 1865, and from later results obtained by J. A. Fleming and Sir James Dewar in 1893. (Matthiessen.) Metal Resistance at o° C. Approximate Tern- in International perature Co- Ohms of a Wire efficient near t Metre long and Weighing 2o° C. t Gramme. Silver (annealed) . . .1523 0.00377 Silver (hard-drawn) . . 1657 Copper (annealed). . '1421 0.00388 Copper (hard-drawn) . • 1449 (Matthiessen's Standard) Gold (annealed) •402 0.00365 Gold (hard-drawn) .4044 Aluminium (annealed) .0757 . . Zinc (pressed) . •4013 Platinum (annealed) . 1.9337 Iron (annealed) . . •765 . . Nickel (annealed) . . I.0581 Tin (pressed) . . . .9618 0.00365 Lead (pressed) . .I 2.2268 0.00387 Antimony (pressed) . 2.3787 0.00389 Bismuth (pressed . . 12.85541 0.00354 Mercury (liquid) . . 12.8852 0.00072 The data commonly used for calculating metallic resistivities were obtained by A. Matthiessen, and his results are set out in the Table II. which is taken from Cantor lectures given by Fleeming Jenkin in 1866 at or about the date when the researches were made. The figures given by Jenkin have, however, been reduced to inter-national ohms and C.G.S. units by multiplying by (ir/4)X0.9866X 105=77,485. Subsequently numerous determinations of the resistivityof various pure metals were made by Fleming and Dewar, whose results are set out in Table III. Resistivity of Mercury.-The volume-resistivity of pure mercury is a very important electric constant, and since 188o many of the most competent experimentalists have directed their attention to the determination of its value. The experimental process has usually been to fill a glass tube of known dimensions, having large cup-like extensions at the ends, with pure mercury, and determine the absolute resistance of this column of metal. For the practical details of this method the following references may be consulted:-" The Specific Resistance of Mercury," Lord Rayleigh and Mrs Sidgwick, Pkil. Trans., 1883, part i. p. 173, and R. T. Glazebrook; Phil. Mag., 1885, p. 2o; " On the Specific Resistance of Mercury," R. T. Glazebrook and T. C. Fitzpatrick, Phil. Trans., 1888, p. 179, or Proc. Roy. Soc., 1888, p. 44, or Electrician, 1888, 21, p. 538; " Recent Determinations of the Absolute Resistance of Mercury," R. T. Glaze-brook, Electrician, 1890, 25, pp. 543 and 588. Also see J. V. Jones, " On the Determination of the Specific Resistance of Mercury in Absolute Measure," Phil. Trans., 1891, A, p, 2. Table IV. gives the values of the volume-resistivity of mercury as determined by ' The values for nickel and bismuth given in the table are much higher than later values obtained with pure electrolytic nickel and bismuth. 2 The value here given, namely 12.885, for the electric mass-resistivity of liquid mercury as determined by Matthiessen is now known to be too high by nearly %. The value at present accepted is 12.789 ohms per metre-gramme at o° C. or Resistance Per Centimetre-cube in C.G.S. Units at o° C. Metal, Volume-Resistivity at o° C. in C.G.S. Units. Silver (annealed) . 1,502 Silver (hard-drawn) . 1,629 Copper (annealed) . 1,594 Copper (hard-drawn) I,63o ' Gold (annealed) 2,052 Gold (hard-drawn) 2,090 Aluminium (annealed) 3,006 Zinc (pressed) 5,621 Platinum (annealed) 9,035 Iron (annealed) 10,568 Nickel (annealed) 12,429 2 Tin (pressed) 13,178 Lead (pressed) . 19,580 Antimony (pressed) . 35,418 Bismuth (pressed) . . 130,872 Mercury (liquid) 94,896' various observers, the constant being expressed (a) in terms of the resistance in ohms of a column of mercury one millimetre in cross-section and loo centimetres in length, taken at o° C.; and (b) in terms of the length in centimetres of a column of mercury one square milli-metre in cross-section taken at o° C. The result of all the most careful determinations has been to show that the resistivity of pure mercury at o° C. is about 94,070 C.G.S. electromagnetic units of resistance, and that a column of mercury Io6.3 centimetres in length having a cross-sectional area of one square millimetre would have a Metal Resistance at o° C. Mean Temperature per Centimetre- Coefficient between cube in C.G.S. Units. o° C. and too° C. Silver (electrolytic and 1,468 0.00400 well annealed)' Copper. (electrolytic 1,561 0.00428 well annealed'tic . and Gold (annealed) 2,197 0.00 Aluminium (annealed) 2,665 0.00435 Magnesium (pressed) . 4,355 0.00381 Zinc . .. 5,751 0.00406 Nickel (electrolytic)' . 6,935 o•oo6t8 Iron (annealed) 9,065 0.00625 Cadmium . . 10,023 0.00419 Palladium . 10,219 0.00354 Platinum (annealed) 10,917 o•003669 Tin (pressed) . 13,048 0.00440 Thallium (pressed) 17,633 0.00398 Lead (pressed) . 20,380 0.00411 Bismuth (electrolytic) 5 I10,000 0.00433 resistance at o° C. of one international ohm. These values have accordingly been accepted as the official and recognized- values for the specific resistance of mercury, and the definition of the ohm. The table also states the methods which have been adopted by the different observers for obtaining the absolute value of the resistance of a known column of mercury, or of a resistance coil afterwards 1 The value (163o) here given for hard-drawn copper is about } % higher than the value now adopted, namely, 1626. The difference is due to the fact that either Jenkin or Matthiessen did not employ precisely the value at present employed for the density of hard-drawn and annealed copper in calculating the volume-resistivities from the mass-resistivities. 2 Matthiessen's value for nickel is much greater than that obtained in more recent researches. (See Matthiessen and Vogt, Phil. Trans., 1863, and J. A. Fleming, Proc. Roy. Soc.. December 1899.) Matthiessen's value for mercury is nearly t % greater than the value adopted at present as the mean of the best results, namely 94,070. ' The samples of silver, copper and nickel employed for these tests were prepared electrolytically by Sir J. W. Swan, and were exceedingly pure and soft. The value for volume-resistivity of nickel as given in the above table (from experiments by J. A. Fleming, Proc. Roy. Soc., December 1899) is much less (nearly 40 %) than the value given by Matthiessen's researches. 5 The electrolytic bismuth here used was prepared by Hartmann and Braun, and the resistivity taken by J. A. Fleming. The value is nearly 20% less than that given by Matthiessen. Observer. Date. Method. Value of Value of Value of B.A.U. in Too Centi- Ohm in metres of Centi- Ohms. Mercury metres of in Ohms. Mercury. Lord Rayleigh . . 1882 Rotating coil .98651 .94133 106.31 Lord Rayleigh . . 1883 Lorenz method .98677 .. 106.27 G. Wiedemann . . 1884 Rotation throught8o° . .. 1o6.19 E. E. N. Mascart . 1884 Induced current •98611 •94096 106.33 H. A. Rowland . . 1887 Mean of several .98644 '94071 106.32 F. Kohlrausch . . 1887 methods .9866o •94061 106.32 Damping of magnets R. T. Glazebrook 1882 Induced currents •98665 .94074 106.29 1888 Wuilleumeier 1890 .98686 .94077 106.31 Duncan and Wilkes 1890 Lorenz .98634 •94067 106.34 J. V. Jones . . . 1891 Lorenz .. .94067 106.31 Streker . 1885 Mean value •98653 .94056 106.32 An absolute determin- Hutchinson 1888 ation of resistance •94074 106.30 E. Salvioni 1890 was not made. The .94054 106.33 E. Salvioni . . . . . value .98656 has .94076 106.30 been used Mean value .94076 106.31 H. F. Weber . . 1884 Induced current 105'37 H. F. Weber Rotating coil Absolute measure- To6.16 A. Roiti . . . . 1884 Mean effect of in- ments compared 105.89 F. Himstedt . . . 1885 duced current with German silver 105.98 wire coils issued by F. E. Dorn . . 1889 Damping of a magnet Siemens and Streker 106.24 Wild . . . . 1883 Damping of a magnet 106.03 L. V. Lorenz . . 1885 Lorenz method 105'93 Alloys. Resistivity Tempera- Composition in per ture Co- at 0° C. efficient at cents. 15° C. Platinum-silver . . 31,582 .000243 Pt 33 %, Ag 66 % Platinum-iridium . 30,896 •000822 Pt 8o %, Jr 20 % Platinum-rhodium . 21,142 .00143 Pt 90%, Rd to% Gold-silver . . . 6,280 .00124 Au 90 %, Ag to % Manganese-steel 67,1¢8 .00127 Mn 12 %, Fe 78 % Nickel-steel . . 29,452 •00201 Ni 4•J5%, remain- German silver . . 29,982 •000273 ing percentage chiefly iron, but uncertain Cu5Zn3Ni2 Platinoid 2 . . . 41,731 .00031 Manganin 46,678 •0000 Cu 84 %, Mn 12 %, Aluminium-silver 4,641 .00238 Ni 4 % Al %, Ag 6 Aluminium.co pper . 2,904 •00381 q¢ Al 94 % Cu 6 % Copper-aluminium . 8,847 000897 Cu 97 %, Al 3 % CTitanium-aluminium 14,912 .0oo64J Cu 87 %, Ni 6.5 %, Copper-nickel-aluminium 3,887 .00290 Al 6.5 % by its resistivity, but also by the degree to which its resistivity varies with temperature, and by its capability of being easily drawn into fine wire of not very small tensile strength. Some pure metals when alloyed with a small proportion of another metal do not suffer much 2 Platinoid is an alloy introduced by Martino, said to be similar in composition to German silver, but with a little tungsten added. It varies a good deal in composition according to manufacture, and the resistivity of different specimens is not identical. Its electric properties were first made known by J. T. Bottomley, in a paper read at the Royal Society, May 5, 1885. Mercury and the Mercury Equivalent of the Ohm. metre long, weighing one gramme which at 6o° F. is o•153858 international ohms." Matthiessen also measured the mass-resistivity of annealed copper, and found that its conductivity is greater than that of hard-drawn copper by about 2.25 % to 2.5% As annealed copper may vary considerably° in its state of annealing, and is always somewhat hardened by bending and winding, it is found in practice that the resistivity of commercial annealed copper is about i % less than that of hard-drawn copper. The standard now accepted for such copper, on the recommendation of the 1899 Committee, is a wire of pure annealed copper one metre long, weighing one gramme, whose resistance at o° C. is •1421 international ohms, or at 6o° F., 0.150822 international ohms. The specific gravity of copper varies from about 8.89 to 8'95, and the standard value accepted for high conductivity commercial copper is 8.912, corresponding to a weight of 555 lb per cubic foot at 6o F. Hence the volume-resistivity of pure annealed copper at o° C. is 1.594 microhms per c.c., or 1594 C.G.S. units, and that of pure hard-drawn copper at o° C. is 1.626 microhms per c.c., or 1626 C.G.S. units. Since Matthiessen's researches, the most careful scientific investigation on the conductivity of copper is that of T. C. Fitzpatrick, carried out in 189o. (Brit. Assoc. Report, 189o, Appendix 3, p. 120.) Fitzpatrick confirmed Matthiessen's chief result, and obtained values for the resistivity of hard-drawn copper which, when corrected for temperature variation, are in entire agreement with those of Matthiessen at the same temperature. The volume resistivity of alloys is, generally speaking, much higher than that of pure metals. Table V. shows the volume resistivity at o° C. of a number of well-known alloys, with their chemical composition. compared with a known column of mercury. A column of figures Generally speaking, an alloy having high resistivity has poor is added showing the value in fractions of an international ohm of mechanical qualities, that is to say, its tensile strength and ductility the British Association Unit (B.A.U.), formerly supposed to represent are small. It is possible to form alloys having a resistivity as high the true ohm. The real value of the B.A.U. is now taken as .9866 as too microhms per cubic centimetre; but, on the other hand, the of an international ohm, value of an alloy for electro-technical purposes is judged not merely For a critical discussion of the methods which have been adopted in the absolute determination of the TABLE V.-Volume-Resistivity of Alloys of known Composition at o° C. in C.G.S. resistivity of mercury, and the value of the British Units per Centimetre-cube. Mean Temperature Coefficients taken at 15° C. Association unit of resistance, the reader may be re- (Fleming and Dewar.) ferred to the British Association Reports for 1890 and 1892 (Report of Electrical Standards Committee), and to the Electrician, 25: p. 456, and 29, p. 462. A discussion of the relative value of the results obtained between 1882 and' 1890 was given by R. T. Glazebrook in a paper presented to the British Association at Leeds, 189o. Resistivity of Copper.-In connexion with electrotechnical work the determination of the conductivity or resistivity values of annealed and hard-drawn copper wire at standard temperatures is a very important matter. Matthiessen devoted considerable attention to this subject between the years 186o and 1864 (see Phil. Trans., 186o, p. 15o), and since that time much additional work has been carried out. Matthiessen's value, known as Matthiessen's Standard, for the mass-resistivity of pure hard-drawn copper wire, is the resistance of a wire of pure hard-drawn copper one metre long and weighing one gramme, and this is equal to 0.14493 international ohms at o° C. For many purposes it is more convenient to express temperature in Fahrenheit degrees, and the recommendation of the 1899 committee on copper conductors i is as follows:-" Matthiessen's standard for hard-drawn conductivity commercial copper shall be considered to be the resistance of a wire of pure hard-drawn copper one i In 1899 a committee was formed of representatives from eight of the leading manufacturers of insulated copper cables with delegates from the Post Office and Institution of Electrical Engineers, to consider the question of the values to be assigned to the resistivity of hard-drawn and annealed copper. The sittings of the committee were held in London, the secretary being A. H. Howard. The values given in the above paragraphs are in accordance with the decision of this committee, and its recommendations have been accepted by the General Post Office and the leading manufacturers of insulated copper wire and cables. change, in resistivity, but in other cases the resultant alloy has a much higher resistivity. Thus an alloy of pure copper with 3 % of aluminium has a resistivity about 5z times that of copper; but if pure aluminium is alloyed with 6 % of copper, the resistivity of the product is not more than 20 % greater than that of pure aluminium. The presence of a very small proportion of a non-metallic element in a metallic mass, such as oxygen, sulphur or phosphorus, has a very great effect in increasing the resistivity. Certain metallic elements also have the same power; thus platinoid has a resistivity 30% greater than German silver, though it differs from it merely in containing a trace of tungsten. The resistivity of non-metallic conductors is in all cases higher than that of any pure metal. The resistivity of carbon, for instance, in the forms of charcoal or carbonized organic material and graphite, varies from 600 to 6000 microhms per cubic centimetre, as shown in Table VI.: Centimetre-cube of Various Forms of Carbon at 15° C Substance. Resistivity. Arc lamp carbon rod 800o Jablochkoff candle carbon 4000 Caere carbon 3400 Carbonized bamboo . . 6000 Carbonized parchmentized thread . 4000 to 5000 Ordinary carbon filament from glow-lamp 2400 to 2500 " treated " or flashed . . . . Deposited or secondary carbon 600 to 900 Graphite 400 to 500 far as is yet known, the resistivity of a pure metal is increased if its temperature is raised, and decreased if the temperature is lowered, so that if it could be brought to the absolute zero of temperature (– 273° C.) its resistivity would be reduced to a very small fraction of its resistance at ordinary temperatures. With metallic alloys, however, rise of temperature does not always increase resistivity; it sometimes diminishes it, so that many alloys are known which have amaximurn resistivity corresponding to a certain temperature, and at or near this point they vary very little in resistance with temperature: Such alloys have, therefore, a negative temperature-variation of resistance at and above fixed temperatures. Prominent amongst these metallic compounds are alloys of iron, manganese, nickel and copper, some of which were discovered by Edward Weston, in the United States. One well-known alloy of copper, manganese and nickel, now called manganin, which was brought to the notice of electricians by the careful investigations made at the Berlin Physikalisch - Technische Reichsanstalt, is characterized by having a zero temperature coefficient at or about a certain temperature in the neighbourhood of 15° C. Hence within a certain range of temperature on either side of this critical value the resistivity of manganin is hardly affected at all by temperature. Similar alloys can be produced from copper and ferro-
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