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Printers provide information in a permanent, human-readable form. They are the most commonly used output devices and are components of almost all computer systems. Printers vary greatly in performance and design. Vfe will classify printers as character printers, line printers and page printers in order to identify three different approaches to printing, each with a different speed range. In addition, printers can be described as either impact or nonimpact. Printers that use electromechanical mechanisms that cause hammers to strike against a ribbon and the paper are called impact printers. Nonimpact printers do not hit or impact a ribbon to print.
Character printers print only one character at a time. A typewriter is an example of a character printer. Character printers are the type used with literally all microcomputers as well as on computers of all sizes whenever the printing requirements are not large. Character printers may be of several types. A letter-quality printer is a character printer which produces output of typewriter quality. Letter-quality printers typically have speeds ranging from 10 to 50 characters per second. Dot-matrix printers form each character as a pattern of dots. These printers have a lower quality of type but are generally faster printers than the letter-quality printers — in the range of 50 to 200 characters per second. One of the newest types of character printer is the ink-jet printer. It sprays small drops of ink onto paper to form printed characters. The ink has a high iron content, which is affected by magnetic fields of the printer. These magnetic fields cause the ink to take the shape of a character as the ink approaches the paper.
Line printers are electromechanical machines used for high-volume paper output on most computer systems. Their printing speeds are such that to an observer they appear to be printing a line at a time. They are impact printers. Trie speeds of line printers vary from 100 to 2500 lines per minute. Line printers have been designed to use many different types of printing mechanisms. Two of the most common print mechanisms are the drum and the chain. Drum printers use a solid, cylindrical drum, rotating at a rapid speed. Speeds of dram printers vary from 200 to over 2000 lines per minute. Chain printers have their character set on a rapidly rotating chain called a print chain. Speeds of chain printers range from 400 to 2400 lines per minute.
Page printers are high-speed nonimpact printers. Their printing rates are so high that output appears to emerge from the printer a page at a time. A variety of techniques are used in the design of page printers. These techniques, called electrophotographic techniques, have developed from the paper copier technology. Laser-beam printers use a combination of laser beam and electrophotographic techniques to create printer output at a rate equal to 18000 lines per minute.
2. When I (came) to his house, they (told) me that he (had left) an hour before.
3. On checking up his answers he (found) out that he (had made) several mistakes.
4. When I (left) home, the snow (had already stopped), but a strong wind (was blowing).
5. (Have you read) this book? — Yes, I (have read) it. I (think) it (is) very interesting.
6. What (are the children doing) now? — Oh, they (are playing) the new table game which I (bought) for them the day before yesterday.
7. They (have reached) the corner of the street by now and (are standing) at the bus stop.
8. After we (had been walking) about two hours, we arrived at a picturesque glade covered with fresh grass.
9. We could not go out because it (had been raining) hard since early morning.
10. She (has been teaching) at our school for twenty years now.
11. Ring me up as soon as you (come) home.
12. He (began) to write his composition at three o'clock. It is already eleven, and he (is still writing) it. He says he (will have finished) it by twelve.
13. We (have been helping) our librarian to put the books in the right order for already three days, but we (have arranged) only half the books.