« НазадПродовжити »
CHAPTER XIV. Fa^r
PHYSICAL PROPERTIES OF CONDUCTORS.
The distribution of electrical energy from the generating plant to lamps, motors, or other devices involves problems of great scientific and technical interest. It is also a fact that in almost every electrical installation the cost of the distributing conductors is a larger item than that of the generating machinery. This is almost always true of long-distance transmission; and even in an isolated • electric-lighting plant the wiring is usually more expensive than the boilers, engines, and dynamos combined.
Substantially the same principles apply to all branches of electrical transmission and distribution, including electric lighting and power, telegraphy, telephony, etc. But the subject has been developed more thoroughly in the case of electric lighting, which requires a more nearly perfect regulation of pressure and current than the other applications.
Measuring Electrical Conductors. — Either the metric system or the ordinary English system of units can be employed to measure the length and cross-section of wires or conductors.* The former system, in spite of its many advantages, is rarely used for this purpose in America or in England; and tables or rules employing it would be practically worthless at the present time, since they must ultimately be used by common workmen. It should be made compulsory before it can be adopted generally. In English measure we can select either the mile, yard, or- foot as the unit of length. The first is too large, as it necessitates inconvenient decimal fractions; the yard is often employed in England to measure
* Tables for converting from one system to the other are given in vol. i. pp. 20-22.
electrical conductors, but it is rarely used for that purpose in America, the foot being almost universally adopted as the unit. The size of a wire may be stated either in terms of the numbers of an arbitrary gauge, or the actual diameter in fractions of an inch may be given. Unfortunately the practice of using wire gauges has existed from time immemorial, and results in much confusion because of the great number of different gauges.* This has been overcome to a certain extent in this country by the general adoption of the Brown & Sharpe, or American wire gauge; but this is quite different from the new standard British wire gauge, which is used in England. The American wire gauge will be employed in the present instance, since in this country wires are made and referred to by that gauge very generally; but in many cases it is better to specify the actual diameter or cross-section of the wire. For this purpose the word mil has been introduced, being a short name for one thousandth of an inch. That is to say, a wire 100 mils in diameter is one hundred one thousandths, or one tenth of an inch, in diameter. The cross-section of a wire one mil in diameter is called one circular mil, being the area of a circle one thousandth of an inch in diameter. Since the cross-section of any other round wire will be proportional to the square of its diameter, it follows that the cross-section in circular mils can be found by multiplying the diameter in mils by itself. We thus avoid the difficulty of converting the cross-section into square measure, which requires the square of the diameter to be multiplied by .7854 or 7r /4. This is an awkward and unnecessary calculation in the case of round wires, it being much simpler and equally definite to measure them in terms of a circular unit. For diameters greater than .46 inch (No. 0000 A.W.G.) it is customary to define the size by giving the diameter in mils or the cross-section in circular mils. For example, a solid round conductor one inch in diameter is designated as 1,000,000 circular mils. The size of a cable made up of a number of strands of wire is given as the sum of the cross-sections of the individual strands. Rectangular conductors are measured in terms of their breadth and thickness in inches or mils, or their cross-section in square mils, which is equal to the product of these two dimensions.
For measuring wires or conductors, a wire gauge or a microm» Wheeler's Chart of Wire Gauges, W. J. Johnston Co., N.Y., 1887.
eter may be employed. The former consists of a plate having slots in its edge corresponding in size to the gauge numbers. The latter is usually a screw-micrometer, which measures the diameter or thickness in mils, that is, thousandths of an inch.
Materials for Electrical Conductors. — Copper is the material employed almost universally for electrical conductors, on account of its high conductivity. The slight superiority of silver in this respect is more than offset by its higher density and cost.
There are, however, several metals which are lighter than copper for the same resistance. For example, aluminum has a conductivity about one-half that of copper, and its density is 2.7.
2 7 10
Hence, an aluminum wire weighs only x ^-g = 0.607, or a
little more than one-half as much as a copper wire having the same length and resistance. Metallic sodium is only about one-quarter as heavy as copper for equal length and resistance.
Although there are several metals that could be used as electrical conductors which are considerably lighter than copper, it is doubtful if it would be desirable to use them, except in special cases where minimum weight is of particular, importance, because their bulk would be so much greater. For example, an aluminum wire must have about twice the cross-section of an equivalent copper wire, and would, therefore, require much more insulating material to cover it, and would be more clumsy for overhead or underground conductors or interior wiring. It may be used, however, for bare conductors, such as 'bus bars, provided its cost is low enough.
In addition to metals such as aluminum, which may be employed for electrical conductors in place of copper, because they are lighter, other metals and alloys are used on account of cheapness or greater strength. The most prominent of these is iron or steel, which until a few years ago was in general use for telegraph "and telephone lines. But the most recent practice is to employ copper even for these purposes; it being found that the lower resistance, inductance, and electrostatic capacity (because smaller wires are used) more than make up for the increased first cost. Iron has rarely been utilized as a conductor in electric light, power, or other circuits that carry large currents, because its conductivity is much less, being about one-seventh that of copper. An iron wire must therefore have seven times the cross-section and - = 6.14 times the weight of an equivalent copper wire, and
would cost about as much.
Nearly all alloys have a considerable lower conductivity than pure copper, so that they are not very generally used, even for overhead conductors. In the latter case, hard-drawn copper wire is generally employed, having a conductivity about 2 to 3 per cent less than that of annealed copper, and a tensile strength about twice as great. The latter is 25,000 to 35,000 lbs. per square inch for soft copper, and 50,000 to 70,000 for hard-drawn, the lower figure being for large sizes (Nos. 0000 and 000) and the higher value for small sizes of wire (Nos. 14 and 16).
Electrical Resistance. — In electrical distribution the most important factor is resistance, from the scientific as well as from the commercial points of view. It entirely determines the flow of a direct current, and largely affects alternating-current circuits also, necessitating the use of large quantities of copper for conductors, the cost of which constitutes the chief item of expense in almost all electrical plants or systems, as already stated.
Resistance appears as a serious difficulty in electrical distribution, producing three objectionable effects. First, it causes a. drop in- voltage, so that the various lamps are not supplied with sufficient pressure or with the same pressure; second, it involves a loss of energy and efficiency; and third, it produces heating of the conductors, which may destroy the insulation or give rise to danger of fire. Each of these effects will be considered separately later.
The determination of electrical resistance can easily be made either by calculation or by actual measurement. It is not necessary here to explain the many well-known ways of measuring resistance, such as the Wheatstone bridge and the fall of potential methods. These may be found in almost any electrical work. Furthermore, it is the usual practice to determine the electrical resistance of conductors in electric light and power distribution by calculation based upon certain recognized standards. This may be verified by tests of samples of the wire or by measurements made after the conductors are laid.
The Standard Conductivity of Copper. — It is almost universally