[已经解决 ] 1.2 EARLY TELEGRAPH LINES
P: 2016-10-31 09:16:47
1.2 EARLY TELEGRAPH LINES
In 1812, Baron Schilling detonated a mineunder the Neva River at St. Petersburg, Russia, by using an electrical pulsesent through a cable insulated with strips of India rubber. This is probablythe earliest use of a continuously insulated conductor on record. One of theearliest experiments with an underground cable was carried out by FrancisRonalds in 1816. This work was in conjunction with a system of telegraphy consistingof 500 feet of bare copper conductor drawn into glass tubes, joined togetherwith sleeve joints, and sealed with wax. The tubes were placed in a creosotedwooden trough buried in the ground. Ronalds was very enthusiastic over thesuccess of this line, predicting that underground conductors would be widelyused for electrical purposes and outlining many of the essentialcharacteristics of a modern distribution system.
The conductor in this case was firstinsulated with cotton saturated with shellac before being drawn into the tubes.Later, strips of India rubber were used. This installation had many insulationfailures and was abandoned. No serious attempt was made to develop the ideacommercially.
In 1837, W. R. Cooke and Charles Wheatstonelaid an underground line along the railroad right-of-way between London’sEuston and Camden stations for their five-wire system of telegraphy. The wireswere insulated with cotton saturated in rosin and were installed in separategrooves in a piece of timber coated with pitch. This line operatedsatisfactorily for a short time, but a number of insulation failures due to theabsorption of moisture led to its abandonment. The next year, Cooke andWheatstone installed a line between Paddington and Drayton stations in London,but iron pipe was substituted for timber to give better protection frommoisture. Insulation failures also occurred on this line after a short time,and it was also abandoned.
In 1842, S. F. B. Morse laid a cableinsulated with jute, saturated in pitch, and covered with strips of Indiarubber, between Governor’s Island and Castle Garden in New York harbor. Thenext year, a similar line was laid across a canal in Washington, DC. Thesuccess of these experiments induced Morse to write to the Secretary of theTreasury that he believed “telegraphic communications on the electro-magneticplan can with a certainty be established across the Atlantic Ocean.”
In 1844, Morse obtained an appropriationfrom the U.S. Congress for a telegraph line between Washington and Baltimore.An underground conductor was planned and several miles were actually laidbefore the insulation was proved to be defective. The underground project wasabandoned and an overhead line erected. The conductor was originally planned tobe a #16 gage copper insulated with cotton and saturated in shellac. Fourinsulated wires were drawn into a close-fitting lead pipe, which was thenpassed between rollers and drawn down into close contact with the conductors.The cable was coiled on drums in 300-foot lengths and laid by means of aspecially designed plow. Thus, the first attempts at underground constructionwere unsuccessful, and overhead construction was necessary to ensuresatisfactory performance of the lines. After the failure of Morse’s line, noadditional attempts were made to utilize underground construction in the UnitedStates until Thomas A. Edison’s time. Gutta-percha—a natural, thermoplasticrubber—was introduced in Europe in 1842 by Dr. W. Montgomery, and in 1846 wasadopted upon the recommendation of Dr. Werner Siemens for the telegraph linethat the Prussian government was installing. Approximately 3,000 miles of suchwire were laid from 1847 to 1852. Unfortunately, the perishable nature of thematerial was not known at the time and no adequate means of protecting it fromoxidation was provided. Insulation troubles soon began to develop andeventually became so serious that the entire installation was abandoned.
However, gutta-percha provided a verysatisfactory material for insulating telegraph cables when properly protectedfrom oxidation. It was used extensively for both underground and submarineinstallations.
In 1860, vulcanized rubber was used for thefirst time as insulation for wires. Unvulcanized rubber had been used on severalof the very early lines in strips applied over fibrous insulation for moistureprotection. This system had generally been unsatisfactory because ofdifficulties in closing the seam. Vulcanized rubber proved to be a much betterinsulating material, but did not become a serious competitor of gutta-perchauntil some years later.
P: 2016-11-07 14:07:07