Encyclopaedia Britannica, 11th Edition, "Conduction, Electric" Volume 6, Slice 8

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THE ENCYCLOPÆDIA BRITANNICA

A DICTIONARY OF ARTS, SCIENCES, LITERATURE AND GENERAL INFORMATION

ELEVENTH EDITION

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VOLUME VI SLICE VIII

Conduction, Electric

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Articles in This Slice

CONDUCTION, ELECTRIC

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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.

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 substances, 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 hydrocarbons. 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, liquid oxygen 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.

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: (1) 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 10 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.

href="@public@vhost@g@gutenberg@html@files@32063@[email protected]#Footnote_4" id="FnAnchor_4" class="pginternal" tag="{http://www.w3.org/1999/xhtml}a">4   Tin (pressed) 13,178   Lead (pressed) 19,580   Antimony (pressed) 35,418   Bismuth (pressed) 130,872   Mercury (liquid) 94,8965  

Resistivity of Mercury.—The volume-resistivity of pure mercury is a very important electric constant, and since 1880 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, Phil. Trans., 1883, part i. p. 173, and R. T. Glazebrook, Phil. Mag., 1885, p. 20; “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. Glazebrook, 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 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 100 centimetres in length, taken at 0° C.; and (b) in terms of the length in centimetres of a column of mercury one square millimetre in cross-section taken at 0° C. The result of all the most careful determinations has been to show that the resistivity of pure mercury at 0° C. is about 94,070 C.G.S. electromagnetic units of resistance, and that a column of mercury 106.3 centimetres in length having a cross-sectional area of one square millimetre would have a resistance at 0° 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 compared with a known column of mercury. A column of figures is added showing the value in fractions of an international ohm of the British Association Unit (B.A.U.), formerly supposed to represent the true ohm. The real value of the B.A.U. is now taken as .9866 of an international ohm.

Table III.—Electric Volume-Resistivity of Various Metals at 0° C., or Resistance per Centimetre-cube at 0° C. in C.G.S. Units. (Fleming and Dewar, Phil. Mag., September 1893.)

Metal. Resistance at 0° C.
per Centimetre-cube
in C.G.S. Units. Mean Temperature
Coefficient between
0° C. and 100° C. Silver (electrolytic and well annealed)6 1,468     0.00400 Copper (electrolytic and well annealed)6 1,561     0.00428 Gold (annealed) 2,197     0.00377 Aluminium (annealed) 2,665     0.00435 Magnesium (pressed) 4,355     0.00381 Zinc 5,751     0.00406 Nickel (electrolytic)6 6,935     0.00618 Iron (annealed) 9,065     0.00625 Cadmium 10,023     0.00419 Palladium 10,219     0.00354 Platinum (annealed) 10,917     0.003669 Tin (pressed) 13,048     0.00440 Thallium (pressed) 17,633     0.00398 Lead (pressed) 20,380     0.00411 Bismuth (electrolytic)7 110,000     0.00433

Table IV.—Determinations of the Absolute Value of the Volume-Resistivity of Mercury and the Mercury Equivalent of the Ohm.

Observer. Date. Method. Value of B.A.U.
in Ohms. Value of 100
Centimetres of
Mercury in
Ohms. Value of Ohm
in Centimetres
of Mercury. Lord Rayleigh 1882 Rotating coil .98651 .94133 106.31 Lord Rayleigh 1883 Lorenz method .98677 .. 106.27 G. Wiedemann 1884 Rotation through 180° .. .. 106.19 E. E. N. Mascart 1884 Induced current .98611 .94096 106.33

H. A. Rowland 1887 Mean of several methods .98644 .94071 106.32 F. Kohlrausch 1887 Damping of magnets .98660 .94061 106.32 R. T. Glazebrook 1882/8 Induced currents .98665 .94074 106.29 Wuilleumeier 1890   .98686 .94077 106.31 Duncan and Wilkes 1890 Lorenz .98634 .94067 106.34 J. V. Jones 1891 Lorenz .. .94067 106.31     Mean value .98653     Streker 1885 An absolute determination   .94056 106.32 Hutchinson 1888 of resistance was not   .94074 106.30 E. Salvioni 1890 made. The value .98656   .94054 106.33 E. Salvioni .. value .98656 has been used   .94076 106.30     Mean value .94076 106.31 H. F. Weber 1884 Induced current Absolute measurements 105.37 H. F. Weber .. Rotating coil  compared with German 106.16 A. Roiti 1884 Mean effect of induced current  silver wire coils issued by 105.89 F. Himstedt 1885    Siemens and Streker 105.98 F. E. Dorn 1889 Damping of a magnet   106.24 Wild 1883 Damping of a magnet   106.03 L. V. Lorenz 1885 Lorenz method   105.93

For a critical discussion of the methods which have been adopted in the absolute determination of the resistivity of mercury, and the value of the British Association unit of resistance, the reader may be referred 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, 1890.

Resistivity of Copper.—In connexion with electro-technical 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 1860 and 1864 (see Phil. Trans., 1860, p. 150), 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 0° C. For many purposes it is more convenient to express temperature in Fahrenheit degrees, and the recommendation of the 1899 committee on copper conductors8 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 metre long, weighing one gramme which at 60° F. is 0.153858 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 1¼% 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 0° C. is 0.1421 international ohms, or at 60° 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 60° F. Hence the volume-resistivity of pure annealed copper at 0° C. is 1.594 microhms per c.c., or 1594 C.G.S. units, and that of pure hard-drawn copper at 0° 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 1890. (Brit. Assoc. Report, 1890, 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 0° C. of a number of well-known alloys, with their chemical composition.

rb">Platinum-silver 31,582  .000243 Pt 33%, Ag 66% Platinum-iridium 30,896  .000822 Pt 80%, Ir 20% Platinum-rhodium 21,142  .00143 Pt 90%, Rd 10% Gold-silver 6,280  .00124 Au 90%, Ag 10% Manganese-steel 67,148  .00127 Mn 12%, Fe 78% Nickel-steel 29,452  .00201 Ni 4.35%, remaining percentage        chiefly iron, but uncertain German silver 29,982  .000273 Cu5Zn3Ni2 Platinoid9 41,731  .00031   Manganin 46,678  .0000 Cu 84%, Mn 12%, Ni 4% Aluminium-silver 4,641  .00238 Al 94%, Ag 6% Aluminium-copper 2,904  .00381 Al 94%, Cu 6% Copper-aluminium 8,847  .000897 Cu 97%, Al 3% Copper-nickel-aluminium 14,912  .000643 Cu 87%, Ni 6.5%, Al 6.5% Titanium-aluminium 3,887  .00290  

Generally speaking, an alloy having high resistivity has poor mechanical qualities, that is to say, its tensile strength and ductility are small. It is possible to form alloys having a resistivity as high as 100 microhms per cubic centimetre; but, on the other hand, the value of an alloy for electro-technical purposes is judged not merely 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 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 5½ 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.:—

Table VI.—Electric Volume-Resistivity in Microhms per Centimetre-cube of Various Forms of Carbon at 15° C.

Substance. Resistivity. Arc lamp carbon rod 8000 Jablochkoff candle carbon 4000 Carré carbon 3400 Carbonized bamboo 6000 Carbonized parchmentized thread 4000 to 5000 Ordinary carbon filament from glow-lamp    “treated” or flashed 2400 to 2500 Deposited or secondary carbon  600 to  900 Graphite  400 to  500

The resistivity of liquids is, generally speaking, much higher than that of any metals, metallic alloys or non-metallic conductors. Thus fused lead chloride, one of the best conducting liquids, has a resistivity in its fused condition of 0.376 ohm per centimetre-cube, or 376,000 microhms per centimetre-cube, whereas that of metallic alloys only in few cases exceeds 100 microhms per centimetre-cube. The resistivity of solutions of metallic salts also varies very largely with the proportion of the diluent or solvent, and in some instances, as in the aqueous solutions of mineral acids; there is a maximum conductivity corresponding to a certain dilution. The resistivity of many liquids, such as alcohol, ether, benzene and pure water, is so high, in other words, their conductivity is so small, that they are practically insulators, and the resistivity can only be appropriately expressed in megohms per centimetre-cube.

In Table VII. are given the names of a few of these badly-conducting liquids, with the values of their volume-resistivity in megohms per centimetre-cube:—

Table VII.—Electric Volume-Resistivity of Various Badly-Conducting Liquids in Megohms per Centimetre-cube.

Substance. Resistivity
in Megohms
per c.c. Observer. Ethyl alcohol 0.5 Pfeiffer. Ethyl ether 1.175 to 3.760 W. Kohlrausch. Benzene 4.700   Absolutely pure water approximates probably to 25.0 at 18° C. Value estimated by F. Kohlrausch      and A. Heydweiler. All very dilute aqueous salt solutions having a 1.00 at 18° C. From results by F. Kohlrausch  concentration of about 0.00001 of an equivalent    and others.  gramme molecule10 per litre approximate

to    

The resistivity of all those substances which are generally called dielectrics or insulators is also so high that it can only be appropriately expressed in millions of megohms per centimetre-cube, or in megohms per quadrant-cube, the quadrant being a cube the side of which is 109 cms. (see Table VIII.).

Table VIII.—Electric Volume-Resistivity of Dielectrics reckoned in Millions of Megohms (Mega-megohms) per Centimetre-cube, and in Megohms per Quadrant-cube, i.e. a Cube whose Side is 109 cms.

Substance. Resistivity. Temperature
Cent. Mega-megohms
per c.c. Megohms per
Quadrant-cube. Bohemian glass 61    .061 60° Mica 84    .084 20° Gutta-percha 450    .45 24° Flint glass 1,020    1.02 60° Glover’s vulcanized indiarubber 1,630    1.63 15° Siemens’ ordinary pure vulcanized indiarubber 2,280    2.28 15° Shellac 9,000    9.0 28° Indiarubber 10,900   10.9 24° Siemens’ high-insulating fibrous material 11,900   11.9 15° Siemens’ special high-insulating indiarubber 16,170   16.17 15° Flint glass 20,000   20.0 20° Ebonite 28,000   28. 46° Paraffin 34,000   34. 46°

Effects of Heat.—Temperature affects the resistivity of these different classes of conductors in different ways. In all cases, so 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 a maximum 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-manganese. An alloy formed of 80% copper and 20% manganese in an annealed condition has a nearly zero temperature-variation of resistance between 20° C. and 100° C. In the case of non-metals the action of temperature is generally to diminish the resistivity as temperature rises, though this is not universally so. The interesting observation has been recorded by J. W. Howell, that “treated” carbon filaments and graphite are substances which have a minimum resistance corresponding to a certain temperature approaching red heat (Electrician, vol. xxxviii. p. 835). At and beyond this temperature increased heating appears to increase their resistivity; this phenomenon may, however, be accompanied by a molecular change and not be a true temperature variation. In the case of dielectric conductors and of electrolytes, the action of rising temperature is to reduce resistivity. Many of the so-called insulators, such as mica, ebonite, indiarubber, and the insulating oils, paraffin, &c., decrease in resistivity with great rapidity as the temperature rises. With guttapercha a rise in temperature from 0° C. to 24° C. is sufficient to reduce the resistivity of one-twentieth part of its value at 0° C., and the resistivity of flint glass at 140° C. is only one-hundredth of what it is at 60° C.