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after 35 years.* In this country creosoted railway ties last about 20 years on the average. Cresoting also protects timber from the attacks of the teredo navalis and the limnoria terebrans.
4. Carbolining consists in treating timber at a temperature of 250° F. with an oil called carbolineum avenarius (invented by Captain Avenarius).
5. Vulcanizing is accomplished by heating timber in closed cylinders from 8 to 12 hours at 300° to 500° F., and under a pressure of 150 to 200 lbs. per square inch. A circulation of heated and dried compressed air removes moisture and any water that does not take part in the chemical reaction, and combine with the woody constituents. This process changes the character of the sap so that it does not ferment, and seals up the pores. Tests at Columbia University showed an average increase in strength of 18.9 per cent, in addition to preservative effect.
6. Applying pitch or tar to the butt of a pole may do more harm than good, as it confines the sap, hastening fermentation and decay. But, after the pole has been standing two or three years, it might be treated in this way, by digging around it.
Poles are 35 to 60 feet long, but are sometimes 100 feet or even longer. Those of 50 feet or more are usually set about one-tenth of their length in the ground, but for shorter poles or in soft earth they are sometimes buried to the extent of one-eighth or one-sixth of their total length. In soft ground they should be surrounded with a grouting of Portland cement, sand, and broken stone, tamped around the bottom of the pole, or the butt of the pole may be set in a barrel filled with sand or firm earth. The standard practice is to put from 40 to 50 poles per mile, making spans from 132 to 106 feet each. About every tenth pole should
* N. W. L. Brown in Elec. Railway Gazette, October 19, 1895.
be guyed laterally, to prevent wind pressure from overthrowing them. This is quite likely to happen ; and if one pole falls it is likely to drag down the next one, and so on for a long distance, unless they are supported by side guys at reasonably frequent intervals. The guys usually consist of several strands of No. 6 or
Figs. 187 and 188. Arrangement of Guys for Turning a Corner.
8 iron or steel wire, which is more easily handled than the larger wire or rods that are sometimes used. They may be made simple, as in Fig. 183, or for high poles they have the Y form (Fig. 184).
Fig. 185 shows wire guy and pole brace. When a pole is to be made very secure it is guyed in two directions, or double guyed. This adds greatly to the stability of the pole. See Fig. 186.
On curves or at corners the guys should be more frequent and stronger, being placed on the outer side of the curve. Methods of guying suitable for lines that turn a right angle at street corners are shown in Figs. 187 and 188. In such cases, or where lines come to an end, as in front of an electric light station,
Fig. 189. Guying of Terminal or Corner Pole.
the last two or three poles should be stronger and more firmly set than the others, and may be guyed as indicated in Fig. 189. It is also well if the last one or two spans, A B and B C, are left somewhat more slack than usual, in order not to bring too much strain on the terminal pole.
Cross-Arms are of yellow pine or oak, being usually about 31 x 44, or 32 x 44 inches for smaller sizes, and as much as 43 x 52 inches for the Niagara transmission line. A cross-arm about 3 feet long is used for two insulators, about 5 or 6 feet for 4 insulators, and so on. The spacing of the pins is about 4 to 6 inches from the ends, 24 to 30 inches in the middle, and 12 to 18 inches for the rest, depending upon the size of insulators and other conditions.
The "gains” or flat spots on which the cross-arms are placed should be cut in the pole before it is set up. Ordinarily these are placed about 24 inches, center to
Fig. 190. Bracing a Cross-Arm. center. The cross-arms should be fastened to the pole by two bolts or lag screws, placed diagonally in order not to split the wood, and are braced by two iron strap braces also attached by bolts or lag screws, as represented in Fig. 190. The cross-arms should be put alternately on opposite sides of the poles so that they cannot be pulled off successively.
Guard Wires. — Where one set of overhead electrical wires pass
under another set, the former should be protected by guard wires. An arrangement of this kind is represented in Fig. 191, ABC being the galvanized iron or steel guard wires attached directly without insulation to a cross-arm or to the top of the pole. These guard wires serve to prevent any wire that may fall from coming in contact with the electrical conductors carried on the insulators DE.
Guard Hooks. — A hook of stout iron wire or a hoop, as indicated in Fig. 192, is often attached to the cross-arm to catch an overhead conductor, and prevent it from falling in case the insulator, insulator pin, or tie-wire should happen to break. They are required especially on the inside of curves or angles in the line.
In this connection it may be stated that electric light or other conductors carrying high voltage or heavy current should, if possible, be put over telegraph and telephone wires, because the latter are more likely to fall, not being so well laid or so carefully watched,
and being more numerous. Another reason for this is the risk of requiring telegraph and telephone linemen to pass up through the more dangerous wires with which they may not be familiar; whereas, electric light linemen would not be injured by telegraph or telephone wires.
Insulators. — The problem of supporting overhead wires is somewhat difficult, since those materials having sufficiently high insulating qualities are not very strong mechanically. Glass and porcelain are employed almost universally for the purpose, but neither is possessed of the great strength that is very desirable in order to enable the insulators to stand the heavy stresses to which they are subjected. Other materials, such as hard rubber and various compositions of vegetable or mineral matter, have been tried ; but they are rarely used except that the latter are commonly employed to support overhead trolley wires.
The advantages of porcelain over glass are that it is less brittle and generally stronger than glass, and it is less hygroscopic. On the other hand, glass is cheaper than porcelain, and the fact that it is transparent enables an internal defect to be detected more readily. It also makes the cavities in the insulator less likely to invite the building of nests by insects. Another difference, which is much more serious than it sounds, is the fact that white porcelain insulators more often attract the eye of a boy or hunter, and frequently are made to serve as targets for stones or bullets.
Glass or porcelain insulators for electric Fig. 193. "Deep Groove, light and power lines have been developed Double
glass Insulator. directly from those that are employed for telegraph and telephone service. In fact, there is no substantial difference, the only modifications being an increase in size and
strength to suit the heavier conductors, and improvement in insulation by lengthening the path for leakage of current, secured by adopting the double and triple in place of the single petticoat form.
Types of Insulators. — The deep grooved double petticoat pattern of screw-glass insulator is the ordinary standard, being used with insulated wires for lines of 2000 volts. This type is shown in Fig. 193. For higher potentials the use of porcelain or special forms of glass begins.
The present increasing employment of high voltages, and the tendency to raise the voltage still higher, has brought into use new form of insulators.
The oil insulator, shown in Fig. 194, is Fig. 194. Porcelain “Oil
nted mounted upon an iron pin, and provided with on Iron Pin.
a recess that is filled with an insulating oil. There is a “built-up” type of porcelain insulator, being made in parts as is shown in Fig. 195, and the parts burned together with a vitreous cement.
Type" Insulator, mounted