The modes of determining the thickness of wire, and the improve merits in graduation suggested by Holtzapffel and Whitworth for this purpose, are noticed under GAGE.
In drawing wire by hand the draw-plate is laid against two upright pillars fixed on a bench or table, and, the extreme end of the wire to be drawn being so reduced as to enable it readily to pass through the hole, a small portion is drawn through by a lever apparatus. When a sufficient length of wire has thus been brought through the plate, it is secured to a conical or cylindrical drum, which is mounted upon a vertical axis opposite to the hole in the draw-plate. The workman then takes in one hand the coil of thick wire to be reduced, and in the other a lever handle attached to the drum ; and while he turns the drum so as to wind the wire upon its circumference, and consequently to draw it through the plate, he Imparts a kind of twist to the wire which enters the plate, by a peculiar motion of the band in which the coil is supported. In drawing coarse wire, which requires considerable power, the workman walks round the bench at each revolution of the drum, carrying the lever round with him; but for finer wire the apparatus is much lighter, and requires very little labour. In factories where inanimate power is used the winding-cylinders are turned by bevil-gearing underneath the bench, and the toll of undrawn wire is placed on a reel. This apparatus is accompanied by an ingenious contrivance which allows the drum to fall out of connection with the gearing, and consequently to cease to revolve, as soon as the piece of wire is drawn completely through the plate. In some cases, as for pinion wire, which would be injured by winding upon a drum, wire is drawn upon a long draw bench in a straight line, the power, which is equalised by the use of a fly-wheel, being applied to a winch-handle which, by means of spur gear, imparts motion to a horizontal rack.
Between the repeated drawings which are requisite to reduce wire, especially of the finer or smaller sizes, to the required degree of tenuity, it is necessary frequently to heat and anneal it, by which processes the fibrous character imparted by the drawing is In some degree removed before a fresh extension takes place. The annealing
oven should be so contrived as to avoid oxidation as much as possible, and to heat all sizes of wire with tolerable uniformity, the thickest being placed in such a situation as to receive most heat ; and after leaving it the wire must be scoured or washed to free it from whatever oxide may have formed upon its surface. In order more perfectly to remove the oxide (which, if left ou the surface, not only Impairs the appearance and the strength of the wire, 'but also injures the draw plate) the coil of wire is sometimes immersed in starch-water or stale beer-grounds during the operation of drawing. A curious and im portant discovery was made some years since at an wire manufacto7, where, in order to heat the acid liquor in which the annealed wire was steeped, some ingote of brass which happened to be at hand were made red-hot and quenched in it. It was sub sequently found that, owing to the action of the acid upon the brass, the iron wire bad become coated with a thin film of copper, which greatly facilitated its passage through the draw-plate, acting, it would appear, like a lubricating medium. So important was the advantage gained, that the practice has been universally adopted in that factory of using a weak solution of copper in the acid liquor in which iron or steel wire is washed. The film of copper is entirely removed by the last annealing process. The operation of drawing is also facilitated by the free use of grease, or, for the finer descriptions of wire, wax, to lubricate the wire as it passes through the plate. The repeated annealings of steel-wire would deprive it of too much of its carbon, but for the practice, which is not pursued with iron wire, of surround ing it with charcoal-dust while in the furnace. The rapidity of the drawing process must vary with the quality of the metal, the hardest steel wire requiring the slowest motion ; but as each eucccssivo draw ing increases the fibrous or filamentous character of the metal, the rapidity of the extension may be safely increased as the wire becomes more and more attenuated.