Question 10-19
Barbed wire, first patented in the United States in 1867, played an important part in the development of American farming, as it enabled the settlers to make effective fencing to enclose their land and keep cattle away from their crops. This had a considerable effect on cattle ranching, since the herds no longer had unrestricted use of the plans for grazing, and the fencing led to conflict between the farmers and the cattle ranchers.
Before barbed wire came into general use, fencing was often made from serrated wire, which was unsatisfactory because it broke easily when under strain, and could snap in cold weather due to contraction. The first practical machine for producing barbed wire was invented in 1874 by an Illinois farmer, and between then and the end of the century about 400 types of barbed wire were devised, of which only about a dozen were ever put to practical use.
Modern barbed wire is made from mild steel high-tensile steel, or aluminum. Mild steel and aluminum barbed wire have two strands twisted together to form a cable which is stronger than single-strand wire and less affected by temperature changes. Single-strand wire, round or oval, is made from high-tensile steel with the barbs crimped or welded on . The steel wires used are galvanized - coated with zinc to make them rustproof. The two wires that make up the line wire or cable are fed separately into a machine at one end. They leave it at the other end twisted-together and barbed. The wire to make the barbs is fed into the machine from the sides and cut to length by knives that cut diagonally through the wire to produce a sharp point. This process continues automatically, and the finished barbed wire is wound onto reels, usually made of wire in length of 400 meters or in weights of up to 50 kilograms.
A variation of barbed wire is also used for military purposes. It is formed into long cojis or entanglements called concertina wire.
【題組】18. According to the passage, concertina wire is used for
(A) livestock management
(B) international communications
(C) prison enclosures
(D) military purposes.
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