The use of one circuit of larger wire instead of two circuits of smaller wire has the obvious advantage of greater mechanical strength in each conductor, saves the cost of one pole line and of the erection of the second circuit. With voltages above 40,000 to 50,000 on long transmission lines there is a large loss of energy by leakage directly through the air from wire to wire. To keep this loss within desirable limits it may be necessary to give each wire of a circuit a greater distance from the others of the same circuit than can readily be had if all the wires of each circuit are mounted on one line of poles. If there is only one three-wire circuit to be provided for, three lines of poles or two lines with a long crosspiece between them may be set with any desired distance between the lines so that the leakage through the air with one wire on each pole will be reduced to a small quantity. On a line built in this way it would be practically impossible for an arc to start between the wires by any of the usual means.
Distances from pole to pole in the same line vary somewhat with the number, size, and material of the conductors to be carried. On ordinary construction in a straight line poles are often spaced from 100 to 110 feet apart--that is, about fifty poles per mile. On curves and near corners the spacing of poles should be shorter. Poles for the 80.3 miles, mentioned in New Hampshire, are regularly located 100 feet apart. Of the two pole lines between Niagara Falls and Buffalo, the older was designed to carry twelve copper cables of 350,000 circular mils each, and its poles were spaced only 70 feet apart. The newer line is designed to carry six aluminum cables of 500,000 circular mils each and its poles are 140 feet apart. Poles in each of the lines between Cañon Ferry and Butte are regularly spaced 110 feet apart and each pole carries three copper cables of 106,500 circular mils.
[Illustration: FIG. 81.--Chambly-Montreal Line Crossing the Chambly Canal.]
[Illustration: FIG. 82.--Special Wooden Structures on Line Between Spier Falls and Schenectady.]
The two 142-mile lines between Colgate and Oakland are each made up of poles 132 feet apart, and one line of poles carries the three copper conductors and the other line of poles the aluminum conductors already named. As aluminum wire has only one-half the weight of copper wire of equal conductivity, the length of span between poles carrying aluminum wire may be greater than that where copper is used. Only a part of the strain on poles is due to the weight of wires carried, however. Where a body of water must be crossed, a very long span, with special supports for the wires at each side, may be necessary. A case of this sort was met where the Colgate and Oakland line crosses the Carquinez Straits at a point where the waterway is 3,200 feet wide. It was necessary to have the lowest part of the cables across these straits at least 200 feet above the surface of the water so that vessels with the tallest masts could pass underneath. To secure the necessary elevation for the cables a steel tower was built on each bank of the straits at such a point that the distance between the points for cable support on the two towers is 4,427 feet apart. As the banks rise rapidly from the water level, one steel tower was given a height of only 65 feet, while the height of the other was made 225 feet. Between these two towers four steel cables were suspended, each cable being made up of nineteen strands of galvanized steel wire, having an outside diameter of seven-eighths inch and weighing 7,080 pounds for the span. The breaking strain of each cable is 98,000 pounds, and it has the electrical conductivity of a No. 2 copper wire. The cables are simply supported on the towers by steel rollers, and the pull of each cable, amounting to twelve tons, is taken by an anchorage some distance behind each tower, where the cable terminates. Each anchorage consists of a large block of cement deeply embedded in the ground, and with anchor bolts running through it. Each cable is secured to its anchorage through a series of strain insulators, and the regular line cables of copper and aluminum are connected with the steel cables just outside of the shelter built over the strain insulators of each anchor. Steel cables were used for the long span across the straits because of the great tensile strength that could be had in that metal. This span is, no doubt, the longest and highest that has ever been erected for electrical transmission at high voltage.