[Electric engineering.] . ires to be carried, andtherefore the number of cross-arms, determines to someextent the general height of the pole to be used. 5. Spacing of Poles.—Practice varies as to the spacingof poles. Of course, the number and sizes of the Avires tobe carried is the most important consideration in determi-ning this point, but the climatic conditions, especially withregard to heavy wind and sleet storms, should also beconsidered. In general, it may be said that the best linescarrying a moderate number of wires use 40 poles to themile, while for exceptionally heavy lines, the use
[Electric engineering.] . ires to be carried, andtherefore the number of cross-arms, determines to someextent the general height of the pole to be used. 5. Spacing of Poles.—Practice varies as to the spacingof poles. Of course, the number and sizes of the Avires tobe carried is the most important consideration in determi-ning this point, but the climatic conditions, especially withregard to heavy wind and sleet storms, should also beconsidered. In general, it may be said that the best linescarrying a moderate number of wires use 40 poles to themile, while for exceptionally heavy lines, the use of 52 polesto the mile, or one pole every hundred feet, is not uncommonpractice. As a general rule, which it is safe to follow inthe majority of cases 35 or 40 poles to the mile should beused. For city work, the poles should be set on an averagenot farther apart than 125 feet, 15 ELECTRIC TRANSMISSION. CROSS-ARMS. 6. The cross-arms should be made of well-seasoned,straight-grained, Norway pine, yellow pine, or creosoted. Fig. 1. white pine. Cross-arms are made in standard sizes, thelength of the arm depending on the number of pinsit is intended to hold. The standard cross-arm is3i X 4£ inches, and varies in length usually from 3 to8 feet. They are usually bored for l^--inch pins andprovided with holes for two ^--inch bolts. The armsare generally braced by flat, iron braces, about1|- inches wide by \ to f inch thick. These bracesare shown in Fig. 1, which gives a view of an ordi-nary pole top provided with two 4-pin cross-arms. PINS. 7. The pins by which insulators are mountedupon cross-arms are shown in Fig. 2. They may bemade of locust, chestnut, or oak (the woods being K Flu ELECTRIC TRANSMISSION. 15 preferred in the order named), and are turned with a coarsethread on the end on which the insulator is to be shank K is turned 1^ inches in diameter. The pin should be secured in the hole by driving a nailthrough the arm and the shank of the pin. This renders itdif
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