Wires
A
wire is a single, usually
cylindrical, string of
metal. Wires are used to bear mechanical
load and to carry
electricity and
telecommunications signals. Wire is commonly formed by
drawing the metal through a
hole in a
die or
draw plate.
Standard sizes are
determined by various
wire gauges. The
term
wire is also used more loosely to refer to a bundle
of such strands, as in 'multistranded wire', which is more
correctly termed a
wire rope in mechanics,
or a
cable in electricity.
History
In
antiquity,
jewellery often contains, in the form of chains
and applied decoration, large amounts of wire that is accurately
made and which must have been produced by some efficient, if not
technically advanced, means. In some cases, strips cut from metal
sheet were made by pulling them through perforations in stone
beads. This causes the strips to fold round on themselves to form
thin tubes.
This strip drawing technique was in use in
Egypt
by the 2nd
Dynasty. From the middle of the
2nd millennium BC most of the
gold wires in jewellery are characterized by seam lines
that follow a spiral path along the wire. Such twisted strips can
be converted into solid round wires by rolling them between flat
surfaces or the strip wire drawing method. Strip and block twist
wire manufacturing methods were still in use in
Europe in the 7th century AD, but by this time there
seems to be some evidence of wires produced by true
drawing.
Square and hexagonal wires were possibly made using a
swaging technique. In this method a metal rod was
struck between grooved metal blocks, or between a grooved punch and
a grooved metal anvil. Swaging is of great antiquity, possibly
dating to the beginning of the 2nd millennium BC in Egypt and in
the
Bronze and
Iron
Ages in Europe for
torches and
fibulae.
Twisted
square section wires are a very common filigree decoration in early Etruscan
jewellery.
In about the middle of the 2nd millennium BC a new category of
decorative wires was introduced which imitated a line of granules.
Perhaps
the earliest such wire is the notched wire which first occurs from
the late 3rd, early 2nd millennium
BC in Anatolia
and
occasionally later.
Wire was
drawn in England from the
medieval period. The wire was used to make wool cards and pins,
manufactured goods whose import was prohibited by
Edward IV in 1463.
The first wire mill in Great Britain
was established at Tintern
in about
1568 by the founders of the Company of Mineral and
Battery Works, who had a monopoly on
this. Apart from their second wire mill at nearby
Whitebrook, there were no other wire mills before the second half
of the 17th century. Despite the existence of mills, the drawing of
wire down to fine sizes continued to be done manually.
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 a
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.
Uses
Wire has many uses. It forms the raw material of many important
manufacturers, such as the wire-net
industry, wire-cloth making and wire-
rope
spinning, in which it occupies a place analogous to a
textile fiber. 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
aluminium,
copper,
nickel and
steel wire are employed for telephone and data
wires and cables, and as conductors in
electric power transmission, and
heating. It is in no less demand for fencing, and much is consumed
in the construction of suspension bridges, and cages, etc. In the
manufacture of stringed musical instruments and scientific
instruments 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 enter.
Not all metals and metallic
alloys possess the
physical properties necessary to make useful wire. The metals must
in the first place be
ductile and strong in
tension, 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 extremely thin wire can be produced.
Special purpose wire is however made from other metals (e.g.
tungsten wire for
light bulb and
vacuum
tube filaments, because of its high melting temperature).
Copper wires could be plated with other metals, such as tin,
nickel, and silver to handle different temperatures.
Production
Wire mill, 1913.
Wire is often reduced to the desired diameter and properties by
repeated
drawing through
progressively smaller dies, or traditionally holes in
draw plates. After a number of passes the wire
may be
annealed to facilitate
more drawing or, if it is a finished product, to maximize ductility
and conductivity.
Finishing, jacketing, and insulating
Electrical wires are usually covered with
insulating materials, such as plastic,
rubber-like polymers, or varnish. Insulating and jacketing of wires
and cables is nowadays done by passing them through an extruder.
Formerly, materials used for insulation included treated cloth or
paper, and various oil-based products. Since the mid-1960s,
plastic and
polymers
exhibiting properties similar to rubber have predominated.
Two or more wires may be wrapped concentrically, separated by
insulation, to form
coaxial cable. The
wire or cable may be further protected with substances like
paraffin, some kind of preservative
compound, bitumen,
lead, or aluminium
sheathing, or steel taping.
Stranding
or covering machines wind material onto wire which passes
through quickly. Some of the smallest machines for cotton covering
have a large drum, which grips the wire and moves it through
toothed gears; 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 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.
Solid versus stranded
Solid wire, also called solid-core or single-strand wire, consists
of one piece of metal wire. Stranded wire is composed of a bundle
of small-gauge wires to make a larger conductor.
Stranded wire is more flexible than solid wire of the same total
cross-sectional area. Solid wire is cheaper to manufacture than
stranded wire and is used where there is little need for
flexibility in the wire. Solid wire also provides mechanical
ruggedness; and, because it has relatively less surface area which
is exposed to attack by corrosives, protection against the
environment. Stranded wire is used whenever ease of bending or
repeated bending are required. Such situations includeconnections
between
circuit boards
inmulti-printed-circuit-board devices, where the rigidity of
solidwire would produce too much stress as a result of movement
duringassembly or servicing;
A.C. line cords forappliances; musical
instrument cables; computer mouse cables; welding electrode cables;
control cables connecting moving machine parts; mining machine
cables; trailing machine cables; and numerous others.
At high frequencies, current travels near the surface of the wire
because of the
skin effect,
resulting in increased power loss in the wire. Stranded wire might
seem to reduce this effect, since the total surface area of the
strands is greater than the surface area of the equivalent solid
wire, but in fact a simple stranded wire will have worse skin
effect than a solid wire, because of its increased average
resistivity due to inclusion of air gaps within the wire. (
The
foregoing discussion of the consequences of the skin effect has
been disputed on the .)
However, for many high-frequency applications,
proximity effect is
more severe than
skin effect, and in
some limited cases, simple stranded wire can reduce proximity
effect. For better performance at high frequencies,
litz wire, which has the individual strands
insulated and twisted in special patterns, may be used.
Number of strands
The more individual wire strands in a wire bundle the more
flexible, kink resistant, break resistant, and stronger the wire
is. But more strands cost more.
The lowest number of stands is 7. One in the middle, and 6
surrounding it.
The next level up is 19, which is another layer of 12 strands on
top of the 7. After that the number varies, but 37 and 49 are
common, then in the 70 to 100 range (the number is no longer
exact). Even larger numbers than that are typically found only in
very large wires.
For application where the wire moves 19 is the lowest that should
be used (7 should only be used in applications where the wire is
placed and then doesn't move), and 49 is much better. For
applications with constant repeated movement, such as assembly
robots, and headphone wires, 70 to 100 is mandatory.
Varieties
- Hook-up wire is small-to-medium gauge, solid or
stranded, insulated wire, used for making internal connections
inside electrical or electronic devices.
- It is often tin-plated to facilitate soldering.
- Magnet wire is solid wire, usually copper, which, to
allow closer winding when making electromagnetic coils, is
insulated only with varnish, rather than the thicker plastic or
other insulation commonly used on electrical wire.
- It is used for the winding of electric motors, transformers, inductors, generators,
speaker coils, etc.
- Resistance wire is wire
with higher than normal resistivity, often used for heating elements or for making wire-wound
resistors.
-
- Nichrome wire is the most common
type.
See also
References
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