1911 Encyclopædia Britannica/Wire

From Wikisource
Jump to: navigation, search

WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind), a thin long rod of metal, generally round in section. The uses of wire are multifarious and diverse beyond all enumeration. It forms the raw material of important manufactures, such as the wire-net industry, wire-cloth making and wire-rope spinning, in which it occupies a place analogous to a textile fibre. Wire-cloth of all degrees of strength and fineness of mesh is used for sifting and screening machinery, for draining paper pulp, for window screens, and for many other purposes. Vast quantities of copper and iron wire are employed for telegraph and telephone wires and cables, and as conductors in electric lighting. It is in no less demand for fencing, and much is consumed in the construction of suspension bridges, and cages, &c. In the manufacture of stringed musical instruments and philosophical apparatus wire is again largely used. Among its other sources of consumption it is sufficient to mention pin and hair-pin making, the needle and fish-hook industries, nail, peg and rivet making, and carding machinery; indeed there are few industries into which it does not more or less enter.

The physical properties requisite to make useful wire are possessed by only a limited number of metals and metallic alloys. The metals must in the first place be ductile; and, further, the wire when drawn out must possess a certain amount of tenacity, the quality on which the utility of wire principally depends. The metals suitable for wire, possessing almost equal ductility, are platinum, silver, iron, copper, aluminium and gold; and it is only from these and certain of their alloys with other metals, principally brass and bronze, that wire is prepared. By careful treatment wire of excessive tenuity can be produced. Dr W. H. Wollaston first succeeded in drawing a platinum wire 1/30000 inch in diameter by encasing a fine platinum wire within silver to ten times its diameter. The cored wire he then reduced to 1/3000 inch, and by dissolving away the silver coating the platinum wire 1/30000 inch thick only remained. By continued treatment in this way wires of platinum for spider-lines of telescopes have been obtained of such extreme tenuity that a mile length of the wire weighs not more than a grain; and it is said that platinum wire has been made which measures not more than 1/2000 mm., equal to less than the fifty-thousandth part of an inch, accompanying table shows the comparative tenacity of the of metals and metallic alloys.


 Diameter.  Strain.

 
Gold
Platinum
Silver
Copper
Iron
Copper
Brass
Steel
Phosphor Bronze 

In.
.0162
.0161
.0157
.0177
.0169
.0605
.0640
.0600
.0630

lb.
5.61 - 5.42
6.70 - 6.59
7.86 - 7.78
 10.11 - 10.20 
11.12 - 10.89
233
203
342
394


Wire was originally made by beating the metal out into plates, which were then cut into continuous strips, and afterwards rounded by beating. The art of wire-drawing does not appear to have been known till the 14th century, and it was not introduced into England before the second half of the 17th century. Wire is usually drawn of cylindrical form; but it may be made of any desired section by varying the outline of the holes in the draw-plate through which it is passed in the process of manufacture. The draw-plate or die is a piece of hard cast-iron or hard steel, or for fine work it may be a diamond or ruby. The object of utilizing precious stones is to enable the dies to be used for a considerable period without losing their size, and so producing wire of incorrect diameter. Diamond dies must be rebored when they have lost their original diameter of hole, but the metal dies are brought down to size again by hammering-up the hole and then drifting it out to correct diameter with a punch. The form of a die in section is shown by fig. I; the bell-mouthed opening receives the wire, and when it is pulled through the hole at the end its diameter becomes reduced accordingly. The action of drawing has the effect of hardening the wire and rendering it brittle, so that annealing must be done at intervals to soften it again for further drawing; the annealing is done in cast-iron pots, holding coils of wire which are raised to a red heat and then allowed to cool. Although the wire is kept air-tight as much as possible, some amount of scaling occurs, and pickling must be done to remove this scale before redrawing.


Britannica Wire.png

Fig. 1.


An important point in wire-drawing is that of lubrication to facilitate the operation and to lessen the wear on the dies. Various lubricants, such as oil, tallow, soapy water and stale beer, are employed. Another method is to immerse the wire in a sulphate of copper solution, so that a film of copper is deposited which forms a kind of lubricant, easing the drawing considerably; in some classes of wire the copper is left after the final drawing to serve as a preventative of rust.

The wire-drawing machines include means for holding the dies accurately in position and for drawing the wire steadily through the holes. The usual design consists of a cast-iron bench or table having a bracket standing up to hold the die, and a vertical drum which rotates and by coiling the wire around its surface pulls it through the die, the coil of wire being stored upon another drum or “swift” which lies behind the die and reels off the wire as fast as required. The wire drum or “block” is provided with means for rapidly coupling or uncoupling it to its vertical shaft, so that the motion of the wire may be stopped or started instantly. The block is also tapered, so that the coil of wire may be easily slipped off upwards when finished. Before the wire can be attached to the block, a sufficient length of it must be pulled through the die; this is effected by a pair of gripping pincers on the end of a chain which is wound around a revolving drum, so drawing the pincers along, and with them the wire, until enough is through the die to be coiled two or three times on the block, where the end is secured by a small screw clamp or vice ready for the drawing operation. Wire has to be pointed or made smaller in diameter at the end before it can be passed through the die; the pointing is done by hammering, filing rolling or swaging in dies, which effect a reduction in diameter. When the wire is on the block the latter is set in motion and the wire is drawn steadily through the die; it is very important that the block shall rotate evenly and that it shall run true and pull the wire in an even manner, otherwise the “snatching” which occurs will break the wire, or at least weaken it in spots.

Continuous wire-drawing machines differ from the single-block machines in having a series of dies through which the wire passes in a continuous manner. The difficulty of feeding between each die is solved by introducing a block between each, so that as the wire issues it coils around the block and is so helped on to the next die. The speeds of the blocks are increased successively, so that the elongation due to drawing is taken up and slip compensated for. The operation of threading the wire first through all the dies and around the blocks is termed “stringing-up.” The arrangements for lubrication include a pump which floods the dies, and in many cases also the bottom portions of the blocks run in lubricant. The speeds at which the wire travels vary greatly, according to the material and the amount of reduction effected; rates from 100 ft. up to 900 ft. are possible, the higher speeds being those of continuous machines.

Wires and cables for electrical purposes are covered with various insulating materials, such as cotton, silk, jute and paper, wrapped in spiral fashion and further protected with substances such as paraffin, some kind of preservative compound, bitumen or lead sheathing or steel taping. The stranding or covering machines employed in this work are designed to carry supplies of material and wind it on to the wire which is passing through at a rapid rate. Some of the smallest machines for cotton covering have a large drum, which grips the wire and moves it through toothed gears at a definite speed; the wire passes through the centre of disks mounted above a long bed, and the disks carry each a number of bobbins varying from six to twelve or more in different machines. A supply of covering material is wound on each bobbin, and the end is led on to the wire, which occupies a central position relatively to the bobbins; the latter being revolved at a suitable speed bodily with their disks, the cotton is consequently served on to the wire, winding in spiral fashion so as to overlap. If a large number of strands are required the disks are duplicated, so that as many as sixty spools may be carried, the second set of strands being laid over the first. For the heavier cables, used for electric light and power, and submarine cables, the machines are somewhat different in construction. The wire is still carried through a hollow shaft, but the bobbins or spools of covering material are set with their spindles at right angles to the axis of the wire, and they lie in a circular cage which rotates on rollers below. The various strands coming from the spools at various parts of the circumference of the cage all lead to a disk at the end of the hollow shaft. This disk has perforations through which each of the strands pass, thence being immediately wrapped on the cable, which slides through a bearing at this point. Toothed gears having certain definite ratios are used to cause the winding drum for the cable and the cage for the spools to rotate at suitable relative speeds which do not vary. The cages are multiplied for stranding with a large number of tapes or strands, so that a machine may have six bobbins on one cage and twelve on the other. In the case of submarine cables, coverings of jute-served gutta-percha are employed, upon which a protective covering of steel wires is laid, subsequently treated with jute yarns or tapes and protected with coatings of compound. Messrs Johnson & Phillips, Ltd., of Charlton, Kent, make combination machines which lay the steel wires, apply the tapes and cover with the preservative compound, in one continuous operation. The wire is carried on bobbins in two rotating cages, having twelve bobbins each, and the jute bobbins, seventy-two in number, are mounted on disks, while the compound is supplied from steam-heated tanks, through which the cable is passed by rollers. A machine of this class will turn out as much as 8 m. of finished cable in a day of twelve hours. When a supply of steel wire has been used up, the next portions are united by electric welding.

Tapes of paper, rubber or jute are served from bobbins on disks and also in some designs from independent bobbins, each mounted on its own pin, set at a suitable angle in a frame, to give the spiral lead. In some instances seventy-two layers of paper are applied to high-tension cables. These cables are subsequently put into steam-heated tanks, hermetically sealed and connected to a vacuum pump, by which the moisture is drawn off as quickly as possible. VVhen the cable is thoroughly dry a quantity of compound is admitted to the tank and so permeates the insulation. Lead is put on the outside of the paper in a press, which has dies through which the cable passes, and is covered with a uniform coating or tube of lead, forced into the dies and around the cable by hydraulic pressure. Steel tapes are in some cases used to armour cables and protect them from external injury; the tape is wound in a similar manner to the other materials already described.

Rubber covering of wires and cables is done by passing them through grooved rollers simultaneously with rubber strips above and below, so that the rubber is crushea on to the wires, the latter emerging as a wide band. The separate wires are parted forcibly, each retaining its rubber sheathing. Vulcanizing is afterwards done in steam-heated drums.

Many auxiliary machines are necessary in connexion with wire- and cable-covering, as plant for preparing the rubber and paper, &c., cutting it into strips, winding it, measuring lengths, &c.

Wire Gauges. — In commerce, the sizes of wire are estimated by gauges which consist of plates of circular or oblong form having notches of different widths round their edges to receive wire and sheet metals of different thicknesses. Each notch is stamped with a number, and the wire or sheet, which just fits a given notch, is stated to be of, say, No. 10, 11, 12, &c., of the wire gauge. But it is always necessary to state what particular gauge is used, since, unfortunately, uniformity is wanting. Holtzapffel investigated the subject, and published a valuable collection of facts relating thereto in 1846. A more exhaustive report was published by a committee of the Society of Telegraph Engineers in 1879 (Journ. Soc. Tel. Eng. viii. p. 476), a result of which was the sanctioning by the Board of Trade, in 1884, of the New Imperial Standard Wire Gauge. That report stated: “The different gauges in use might be counted by hundreds. . . . Every wire-drawer has gauges adjusted to suit special objects. When competition is keen, wire is commonly drawn by one gauge and sold by another; half sizes and quarter sizes are in constant use among the dealers, the wire being sold as whole sizes. Sometimes four or five different gauge plates have been made by one maker — some by which the workmen are paid, and others by which the wire is sold. . . . The whole system is in confusion, and lends itself to those who desire to use fraudulent practices.” Thomas Hughes (The English Wire Gauge, London, 1879) stated that, “In the same town some use Stubs, some the Warrington, some the Lancashire, some the Yorkshire, some the Birmingham, some the iron wire gauge and some their own made wire gauge, all maintaining the gauge in their own possession to be the correct one.”

Gauges may be broadly divided into two groups, the empirical and the geometrical. The first include all the old ones, notably the Birmingham (B.W.G.) and the Lancashire or Stubs. The origin of the B.W.G. is lost in obscurity. The numbers of wire were in common use earlier than 1735. It is believed that they originally were based on the series of drawn wires No. 1 being the original rod, and succeeding numbers corresponding with each draw, so that No. 10, for example, would have passed ten times through the draw plate. But the Birmingham and the Lancashire gauge, the latter being based on an averaging of the dimensions collated from a large number of the former in the possession of Peter Stubs of Warrington, have long held the leading position, and are still retained and used probably to a greater extent than the more recent geometrical gauges. There is no need, therefore, to give an account of the other and less known gauges which have been used by manufacturers. In no case is there any regular increment of dimensions from which a regular curve could be drawn.

The first attempt to adopt a geometrical system was made by Messrs Brown & Sharpe in 1855. They established a regular progression of thirty-nine steps between the English sizes, No. 0000 (460 mils) and No. 36 (5 mils). Each diameter was multiplied by 0.890522 to give the next lower size. This is now the American gauge, and is used to a considerable extent in the U.S.A.

The Imperial Standard Wire Gauge, which has been sanctioned by the British Board of Trade, is one that was formulated by J. Latimer Clark. Incidentally, one of its recommendations is that it differs from pre-existing gauges scarcely more than they differ among themselves, and it is based on a rational system, the basis being the mil. No. 7/0, the largest size, is 0.50 in. (500 mils) in diameter, and the smallest, No. 50, is 0.001 in. (1 mil) in diameter. Between these the diameter, or thickness, diminishes by 10.557%, and the weight diminishes by 20%.

But the fact remains that a large number of gauges are still in common use, and that gauges of the same name differ and are therefore not authoritative. Sheet iron wire gauge differs from Stubs' steel wire gauge. Gauges for wire and plate differ. Accuracy can only be secured by specifying precisely the name of the gauge intended, or, what is generally better, the dimensions in decimals, which can always be tested with a micrometer. A decimal gauge has been proposed. Tables of decimal equivalents of the wire gauges have been prepared, and are helpful.

The circular forms of gauge are the most popular, and are generally 3¾ in. in diameter, with thirty-six notches; many have the decimal equivalents of the sizes stamped on the back. Oblong plates are similarly notched. Rolling mill gauges are also oblong in form. Many gauges are made with a wedge-like slot into which the wire is thrust; one edge being graduated, the point at which the movement of the wire is arrested gives its size. The graduations are those of standard wire, or in thousandths of an inch. In some cases both edges are graduated differently to serve for comparison between two systems of measurement. A few gauges are made with holes into which the wire has to be thrust. All gauges are hardened and ground to dimensions.