Steel towers are rapidly coming into use for the support of electric transmission lines that deliver large units of energy at high voltages to long distances from water-powers.

One case of this sort is the seventy-five-mile transmission of 24,000 horse-power at 60,000 volts from Niagara Falls to Toronto. Another example may be seen in the seventy-five-mile line of steel towers which carries transmission circuits of 60,000 volts to Winnipeg. Guanajuato, Mexico, which is said to have produced more silver than any other city in the world, receives some 3,300 electric horse-power over a 60,000-volt transmission line one hundred miles long on steel towers. Between Niagara Falls and Lockport the electric circuits now being erected are supported on steel towers. On a transmission line eighty miles long in northern New York, for which plans are now being made, steel towers are to support electric conductors that carry current at 60,000 volts.

For the elevations above ground at which it is common to support the conductors of transmission lines—that is, from twenty-five to fifty feet—a steel tower will cost from five to twenty times as much as a wooden pole in various parts of the United States and Canada. It follows at once from this fact that there must be cogent reasons, apart from the matter of first cost, if the general substitution of steel towers for wooden poles on transmission lines is to be justified on economic grounds. During fifteen years the electric transmission of energy from distant water-powers to important centres of population has grown from the most humble beginnings to the delivery of hundreds of thousands of horse-power in the service of millions of people, and the lines for this work are supported, with very few exceptions, on wooden poles. Among the transmissions of large powers over long distances at very high voltages that have been in successful operation during at least several years with wooden pole lines are the following: the 60,000-volt circuit that transmits some 13,000 horse-power from Electra station across the State of California to San Francisco, a distance of 147 miles, is supported by wooden poles. In the same State, the transmission line 142 miles long between Colgate power-house and Oakland, at 60,000 volts, and with a capacity of about 15,000 horse-power, hangs on wooden poles, save at the span nearly a mile long over the Straits of Carquinez. Wood is used to carry the two 55,000-volt circuits that run sixty-five miles from the 10,000-horse-power station at CaÑon Ferry on the Missouri River to Butte. Between Shawinigan Falls and Montreal, a distance of eighty-three miles, the conductors that operate at about 50,000 volts are carried on wooden poles. Electrical supply in Buffalo to the amount of 30,000 horse-power depends entirely on circuits from Niagara Falls that operate at 22,000 volts and are supported on lines of wooden poles.

In the operation of these and many other high-voltage transmissions during various parts of the past decade some difficulties have been met with, but they have not been so serious as to prevent satisfactory service. Nevertheless, it is now being urged that certain impediments that are met in the operation of transmission systems would be much reduced by the substitution of steel towers for wooden poles, and it is even suggested that perhaps the first cost, and probably the last cost, of a transmission line would be less with steel than with wood for supports. The argument for steel in the matter of costs is that while a tower requires a larger investment than a pole, yet the smaller number of towers as compared with that of poles may reduce the entire outlay for the former to about that for the latter. More than this, it is said that the lower depreciation and maintenance charges on steel supports will make their final cost no greater than that of wooden poles.

In the present state of the market, steel towers can be had at from three to three and one-half cents per pound, and the cost of a steel tower or pole will vary nearly as its weight. During the first half of 1904 the quotations on tubular steel poles to the Southside Suburban Railway Company, of Chicago, were between the limits just stated. That company ordered some poles built up of steel sections about that time at a trifle less than three cents per pound. Each of these poles was thirty feet long and weighed 616 pounds, so that its cost was about eighteen dollars (xxi, A. I. E. E., 754). For a forty-five-foot steel pole to carry a pair of 11,000-volt, three-phase circuits along the New York Central electric road the estimated cost was eighty dollars in the year last named (xxi, A. I. E. E., 753). On the 100-mile line to Guanajuato, Mexico, above mentioned, the steel towers were built up of 3 × 3 × ³/₁₆ angles for legs, and were stayed with smaller angle sections and rods. Each of these towers has four legs that come together near the top, is forty feet high, weighs about 1,500 pounds, and carries a single circuit composed of three No. 1 B. & S. gauge hard-drawn copper cables. The weight of each of these cables is 1,340 pounds per mile, and the forty-foot towers are spaced 440 feet apart, or twelve per mile, over nearly the entire length of line. At three cents per pound, the lowest figure at which these towers could probably be secured for use in the United States, the approximate cost of each would be forty-five dollars. Between Niagara Falls and Lockport each of the steel towers that is to carry a single three-phase transmission circuit has three legs built up of tubing that tapers from two and one-half inches to smaller sizes and is braced at frequent intervals. The height of these towers is forty-nine feet, and the weight of each is 2,800 pounds. At three cents per pound the cost of each tower amounts to eighty-four dollars. For a long transmission line in northern New York bids were recently had on towers forty-five feet high to carry six wires, and the resulting prices were $100 to $125 each for a tower weighing about 3,000 pounds. On the line between Niagara Falls and Toronto the standard tower holds the lowest cables 40 feet above ground at the insulators, has a weight of 2,360 pounds, and would cost $70.80 at 3 cents per pound.

In January, 1902, four steel towers were purchased to support transmission circuits for two spans of 132 feet each over the Chambly Canal, near Chambly Canton, Quebec. Each pair of these towers was required to support eleven No. 2-0 B. & S. gauge bare copper wires with the span of 132 feet between them. The vertical height of each of these four towers is 144 feet above the foundation, and they were designed for a maximum stress in any member of not more than one-fourth of its ultimate strength, with wires coated to a diameter of one inch with ice and under wind pressure. For these four steel towers erected on foundations supplied by the purchasers the price was $4,670, and the contract called for a weight in the four towers of not less than 121,000 pounds. On the basis of this weight the cost of the towers erected on foundations was 3.86 cents per pound.

With these examples of the cost of steel towers a fair idea may be gotten of the relative cost of wooden poles. For poles of cedar or other desirable wood thirty-five feet long and with eight-inch tops fitted with either one or two cross-arms an estimated cost of five dollars each is ample to cover delivery at railway points over a great part of the United States and Canada. This size of pole has been much used on the long, high-voltage transmission systems that involve large power units and use heavy conductors. Examples of lines where such poles are used may be seen between Niagara Falls and Buffalo, between Colgate power-house and Oakland, and between CaÑon Ferry and Butte. Of course some longer poles were used in special locations, like the crossing of steam railways, but it is also true that on the lines supported by steel towers such locations make exceptionally high towers necessary. The thirty-five-foot poles will hold the electric lines about as high above the ground level as the forty-nine-foot towers on the Niagara Falls and Toronto transmission, because the former will be set so much closer together. On the line just named the regular minimum distance of the electric cables above the ground level at the centres of spans is twenty-five feet. The standard towers on this line carry the lower electric cables forty feet above the ground at the insulators, and it was thought desirable to allow a sag of fifteen feet at the centres of the regular spans of four hundred feet each. On these towers the conductors that form each three-phase circuit are six feet apart, and lines drawn between the three cables form the sides of an equilateral triangle. With a pin fourteen and three-fourths inches long like that used on these steel towers, and one conductor at the top of a thirty-five-foot pole, where the other two are supported by a cross-arm five feet three inches below, giving six feet between cables, the lower cables are held by their insulators twenty-six feet above the ground, when the poles are set five feet deep. Between thirty-five-foot poles one hundred feet is a very moderate span, and one that is exceeded in a number of instances. Thus on the 142-mile line from Colgate power-house to Oakland the thirty-five-foot poles are 132 feet apart, and one line of these poles carries three conductors of 133,000-circular-mil copper, while the other pole line has three aluminum cables of 168,000 circular mils. On the later transmission line from Niagara Falls to Buffalo, which was designed for three-phase circuits of 500,000-circular-mil cable, the regular distance between the thirty-five-foot poles is 140 feet.

A maximum sag of twenty-four inches between poles 100 feet apart under the conditions named above brings the lowest points of the wire twenty-four feet above the ground. The steel towers on the line to Guanajuato being only forty feet in length, and spaced 440 feet apart, it seems that the distance of conductors from the ground at the centres of spans is probably no greater than that just named. Particular attention is called to this point because it has been suggested that the use of steel towers would carry cables so high that wires and sticks could not be thrown onto them. It thus appears that thirty-five-foot wooden poles set one hundred feet apart will allow as much distance between conductors, and still keep their lowest points as far above the ground, as will forty- to forty-nine-foot towers placed four hundred feet or more apart. The two lines that have their conductors further apart perhaps than any others in the world are the one from CaÑon Ferry to Butte, on thirty-five-foot wooden poles, and the one to Guanajuato, on steel towers. In each of these cases the cables are seventy-eight inches apart at the corners of an equilateral triangle. With steel towers four hundred feet or wooden poles one hundred feet apart, four of the latter must be used to one of the former. At $5 per pole this requires an investment of $20 in poles as compared with at least $45 for a tower like those on the Guanajuato line, $84 for a tower like those on the line from Niagara Falls to Lockport, or $70 for one of the towers on the Niagara and Toronto line. Each of the towers on the line to Toronto carries two three-phase circuits, and the least distance between cables is six feet. To reach the same result as to the distance between conductors with the two circuits on poles, it would be desirable to have two pole lines, so that $40 would represent the investment in the poles to displace one tower for two circuits. The older pole line between Niagara Falls and Buffalo carries two three-phase circuits on two cross-arms, and the 350,000-circular-mil copper cables of each circuit are at the angles of an equilateral triangle whose sides are each three feet long. In this case, however, the electric pressure is only 22,000 volts.

The costs above named for poles and towers include nothing for erection. Each tower has at least three legs and more commonly four, and owing to the heights of towers and to the long spans they support it is the usual practice to give each leg a footing of cement concrete. It thus seems that the number of holes to be dug for a line of towers is nearly or quite as great as that for a line of poles, and considering the concrete footings the cost of erecting the towers is probably greater than that for the poles. With wooden poles about four times as many pins and insulators are required as with steel towers, or say twelve pins and insulators on poles instead of three on a tower. For circuits of 50,000 to 60,000 volts the approximate cost of each insulator with a steel pin may be taken at $1.50, so that the saving per tower reaches not more than $13.50 in this respect. In the labor of erecting circuits there may be a small advantage in favor of the towers, but the weight of the long spans probably offsets to a large extent any grain of time due to fewer points of support.

An approximate conclusion from the above facts seems to be that a line of steel towers will probably cost from 1.5 to twice as much as a line or lines of wooden poles to support the same number of conductors the same distance apart, even when the saving of pins and insulators is credited to the towers. This conclusion applies to construction over a large part of the United States and Canada. It is known that wooden poles of good quality retain enough strength to make them reliable as supports during ten or fifteen years, and it is doubtful whether steel towers will show enough longer life to more than offset their greater first cost. It may be noted here that any saving in the cost of insulators or other advantage that there may be in spans four hundred feet or more long can be as readily secured with wooden as with steel supports. With these long spans the requirements are greater height and strength in the line supports, and these can readily be obtained in structures each of which is formed of three or four poles with cross-braces. Such wooden structures have long been in use at certain points on transmission lines where special long spans were necessary or where there were large angular changes of direction. In those special cases where structures 75 to 150 or more feet in height are necessary to carry a span across a waterway, as at the Chambly Canal above mentioned, steel is generally more desirable than wood because poles of such lengths are not readily obtainable. Neither present proposals nor practice, however, contemplates the use of steel towers having a length of more than forty to fifty feet on regular spans.

Much the strongest argument in favor of steel towers for transmission lines is that these towers give a greater reliability of operation than do wooden poles. It is said that towers will act as lightning-rods and thus protect line conductors and station apparatus. As to static and inductive influences from lightning, it is evident that steel towers can give no protection. If each tower has an especial ground connection it will probably protect the line to some extent against direct lightning strokes, but there is no reason to think that this protection will be any greater than that given by well-grounded guard wires, or even by a wire run from a ground plate to the top of each pole or wooden tower. If a direct lightning stroke passes from the line conductors to a wooden support it frequently breaks the insulator on that support, and the pole is often shattered or burned. Such a result does not necessarily interrupt the transmission service, however, as the near-by poles can usually carry the additional strain of the line until a new pole can be set. Quite a different result might be reached if lightning or some other cause broke an insulator on a steel tower, and thus allowed one of the electric cables to come into contact with the metal structure, as the conductor would then probably be burned in two. To repair a heavy cable thus severed where the spans were as much as 400 feet long would certainly require some little time. Where a conductor in circuits operating at 20,000 to 35,000 volts has in many cases dropped onto a wooden cross-arm, it has often remained there without damage until discovered by the line inspector, but no such result could be expected with steel towers and cross-arms (xxi, A. I. E. E., 760). Where steel towers are employed it would seem to be safer to use wooden cross-arms, for the reasons just stated. This is, in fact, the practice on the steel towers before named that support 25,000-volt circuits over the Chambly Canal, and also on the steel towers that carry the 60,000-volt circuits from Colgate power-house over the mile-wide Straits of Carquinez.

On the 40,000-volt transmission line between Gromo and Nembro, Italy, where timber is scarce and steel is cheap, both the poles and cross-arms are of wood. It is thought that the comparatively small number of insulators used where a line is supported at points about four hundred feet apart should contribute to reliability in operation, but insulators now give no more trouble than other parts of the line, and the leakage of energy over their surfaces is very small in amount, as was shown in the Telluride tests. Whatever benefits are to be had from long spans are as available with wooden as with steel supports, and at less cost.

One advantage of steel towers over wooden poles or structures is that the former will not burn and are probably not subject to destruction by lightning. Where a long line passes over a territory where there is much brush, timber or long grass, the fact that steel towers will not burn may make their choice desirable. In tropical countries where insects rapidly destroy wooden poles the use of steel towers may be highly desirable even at much greater cost, and such a case was perhaps presented on the line to Guanajuato, Mexico.

Mechanical failures of wooden insulator pins have been far more common than those of poles, both as a direct result of the line strains and because such pins are often charred and weakened by the leakage of energy from the conductors. For these reasons the general use of iron or steel pins for the insulators of long lines operating at high voltages seems desirable. Such pins are now used to support the insulators on a number of lines with wooden poles and cross-arms, among which may be mentioned the forty-mile, 30,000-volt transmission between Spier Falls and Albany and the forty-five-mile 28,000-volt line from Bear River to Ogden, Utah. Iron or steel pins add very little to the cost of a line, and materially increase its reliability. One of the cheapest and best forms of steel pins is that swaged from a steel pipe and having a straight shank and tapering stem with no shoulder. A pin of this sort for the 400-foot spans of 190,000-circular-mil copper cable on the line from Niagara Falls to Toronto measures three and one-quarter inches long in the shank, eleven and one-half inches in the taper, and has diameters of two and three-eighths inches at the larger and one and one-eighth inches at the smaller end. On spans under 150 feet between wooden poles pins of this type but with a much smaller diameter could be used to advantage.

On long transmission lines where the amount of power involved is very large the additional reliability to be had with steel towers is probably great enough to justify their use. For the great majority of power transmissions, however, it seems probable that wooden poles or structures will long continue to be much the cheaper and more practicable form of support.

The line of steel towers on a private right of way seventy-five miles long, carrying two circuits for the transmission of 24,000 horse-power at 60,000 volts from Niagara Falls to Toronto, is one of the most prominent examples of this type of construction.

Eventually there will be two rows of steel towers along the entire length of the line.

On the straight portions of the line the steel towers are regularly erected 400 feet apart, but on curves the distances are less between towers, so that their total number is about 1,400 for each line. Standard curving along the line requires towers placed 50 feet apart, and a change in the direction of not more than ten degrees at each tower, except at the beginning and end of the curve, where the change in direction is three degrees. When the change in the direction of the line is not more than six degrees, the corresponding spans allowed with each change are as follows:

At some points along the line conditions require a span between towers of more than 400 feet, the regular distance for straight work. One example of this sort occurs at Twelve-Mile Creek, where the stream has cut a wide, deep gorge in the Erie plateau. At this point the lines make a span of 625 feet between towers.

Larger elevations and plan (70 kB)

The regular steel tower used in this transmission measures 46 feet in vertical height from its foot to the tops of the lower insulators, and 51 feet 3 inches to the tops of the higher insulators. The lower six feet of this tower are embedded in the ground, so that the tops of the insulators measure about 40 feet and 45 feet 3 inches respectively above the earth. At the ground the tower measures 14 feet at right angles to the transmission line and 12 feet parallel with it. The width of each tower at the top is 12 feet at right angles to the line, and the two sides having this width come together at points about 40 feet above the ground. Between the two L bars thus brought nearly together, at each side of a tower a piece of extra heavy 3-inch steel pipe is bolted so as to stand in a vertical position. Each piece of this pipe is about 3¹/₂ feet long and carries a steel insulator pin at its upper end. The two pieces of pipe thus fixed on opposite sides of the top of a tower carry the two highest insulators. For the other four insulators of each tower, pins are fixed on a piece of standard 4-inch pipe that serves as a cross-arm, and is bolted in a horizontal position between the two nearly rectangular sides of each tower, at a point two feet below the bolts that hold the vertical 3-inch pipes, already named, in position. Save for the two short vertical and one horizontal pipe, and the pins they support, each tower is made up of L-shaped angle-bars bolted together. Each of the two nearly rectangular sides of a tower consists of two L bars at its two edges, three L bars for cross-braces at right angles to the edges, and four diagonal braces also formed of L bars. The lower halves of the L bars at the edges of each side of a tower have sections of 3 × 3 × ¹/₄, and the upper halves have sections of 3 × 3 × ³/₁₆. This last-named cross-brace and the other two cross-braces have a common section of 2 × 1¹/₂ × ¹/₈. For the lower set of diagonal braces the common section is 2¹/₂ × 2 × ¹/₈, and the upper set has a section of 2 × 1¹/₂ × ¹/₈ in each member. At the level of the lowest cross-braces the two rectangular sides of a tower are tied together by one member of 2 × 1¹/₂ × ¹/₈ of L section and at right angles to the sides, and by two diagonal braces of ⁵/₈ round rod between the corners of the tower. On each of its two triangular sides a tower has four horizontal braces and three sets of diagonal braces. The two upper horizontal braces are of 2 × 1¹/₂ × ¹/₈ L section, and the lowest is the same, but the remaining horizontal brace has a section of 2¹/₂ × 2 × ¹/₈. Bars of 2 × 1¹/₂ × ¹/₈ L section are used for the two upper sets of diagonal braces, and bars of 2¹/₂ × 2 × ¹/₈ for the lower set. In addition to the cross-braces named, each triangular side of a tower near the top of the corner bars has two short cross-pieces with the common L section of 3¹/₂ × 3¹/₂ × ⁵/₈, one just above and the other just below the cross-arm of 4-inch pipe to hold it in place. At the bottom of each of the four corner bars of a tower a foot is formed by riveting a piece of 3 × ¹/₄ L section and 15 inches long at right angles to the corner bar. On one corner bar of each tower there are two rows of steel studs for steps, one row being located in each flange of the L section. On the same flange these steps are two feet apart, but taking both flanges they are only one foot apart. Every part of each steel tower is heavily galvanized.

The labor of erecting these steel towers was reduced to a low figure by the method employed, as shown in the accompanying illustration. Each tower was brought to the place where it was to stand with its parts unassembled. For erecting the towers a four-wheel wagon with a timber body about thirty feet long was used. When it was desired to raise a tower, two of the wheels, with their axle, were detached from the timber body of the wagon, and this body was then stood on end to serve as a sort of derrick. This derrick was guyed at its top on the side away from the tower, and a set of blocks and tackle was then connected to the top of the derrick and to the tower at a point about one-fourth of the distance from its top. A rope from this set of blocks ran through a single block fixed to the base of the derrick and then to a team of horses. On driving these horses away from the derrick the steel tower was gradually raised on the two legs of one of its rectangular sides until it came to a vertical position. The next operation was to bring the legs of the tower into contact with the extension pieces that were fixed in the earth and then bolt them together.

The tops of the three pins that carry the insulators for each three-phase circuit are at the corners of an equilateral triangle (Fig. 100), each of whose sides measures six feet. The six steel insulator pins used on each tower are exactly alike, and each is swaged from extra heavy pipe. Each finished pin is 2³/₈ inches in diameter for a length of 3¹/₄ inches, and then tapers uniformly to a diameter of 1¹/₈ inch at the top through a length of 11¹/₂ inches. This gives the pin a total length of 14³/₄ inches. In the larger part there are two ⁹/₁₆-inch holes from side to side, and within two inches of the top there are three circular grooves each ³/₁₆ inch wide and ¹/₁₆ inch deep. Forged steel sockets of two types are employed to attach the steel pins with the pipes. Each socket is made in halves, and these halves are secured to both the pipe and the pin by through bolts. Like all other parts of the towers, these steel pins and sockets are heavily galvanized. On each of the four corner bars of a tower the lower six feet of its length is secured to the upper part by bolts or rivets. This lower six feet of each corner bar is embedded in the earth, and the construction just named makes it easy to replace the bars in the earth when corrosion makes it necessary.

Footings for each tower are provided by digging four nearly square holes with their sides at approximately 45 degrees with the direction of the transmission line, and the shortest side of each hole at least two feet long. Centres of these holes are 14 feet 3 inches apart in a direction at right angles to the line, and 13 feet 9 inches apart parallel with the line. In hard-pan each one of the holes was filled to within 2 feet 6 inches of the top with stones, after the leg of the tower was in position, and then the remainder of the hole was filled with cement grouting mixed four to one.

At the bottom of each hole in marsh land a wooden footing 3 feet × 6 inches × 24 inches was laid flat beneath the leg of the tower, and then the hole was filled to within 2¹/₂ feet of the surface with the excavated material. Next above this filling comes a galvanized iron gutter-pipe, four inches in diameter, and filled with cement about the leg of the tower for a length of two feet. Outside of this pipe the hole is made rounding full of cement grouting.

At some points along the transmission line exceptionally high towers are necessary, a notable instance being found at the crossing over the Welland Canal, where the lowest part of each span must not be less than 150 feet above the water. For this crossing two towers 135 feet high above ground are used, as seen in Fig. 101. Each of these towers is designed to carry all four of the three-phase power circuits that are eventually to be erected between Niagara Falls and Toronto. For this purpose there was used a special design of tower with a width of about 48 feet at right angles to the direction of the line below the top truss, and a width of about 68.5 feet at that truss where the two lower conductors of each circuit are attached.

With all spans longer than 400 feet, a tower of heavier construction than the standard type is used, and this tower provides three insulators for the support of each conductor. A tower of this type that supports the lowest conductors about 40 feet above the ground level has its corner bars made up of 4 × 4 × ³/₈ and 4 × 4 × ⁵/₁₆ L sections, has three cross-arms of extra heavy 4-inch pipe, and a 6-inch vertical standard pipe to support each group of three insulators for the highest conductor of each circuit. Each of the lower conductors of a circuit on this tower is supported by an insulator on each of the three parallel cross-arms. On some of these towers, for long spans, the two outside insulators for the support of each conductor are set a little lower than the insulator between them.

Angle towers, used where the line makes a large change in direction at a single point, have three legs on each rectangular side, a width of 20 feet on each of these sides for some distance above the ground, and a width of 27 feet 2 inches at the top. In these towers the two legs on[321]
[322] the triangular side that is in compression are each made up of four 3 × 3 × ¹/₄ L sections joined by 1¹/₂ × ¹/₄ lattices and rivets. Towers of this sort are used near the Toronto terminal-station, where the line changes 35 degrees at a single point, and near the crossing of Twelve-Mile Creek, where the angular change of the line on a tower is 45 degrees. Close to each terminal-station and division-house the transmission line is supported by terminal towers. These towers differ from the others in that each carries insulators for only three conductors, and these insulators are all at the same level. Each terminal tower has nine insulators, arranged in three parallel rows of three each for the conductors of a single circuit, and each conductor thus has its strain distributed between three pins. All three wires of a circuit are held 40 feet above the ground by a terminal tower, and pass to their entries in the wall of a station at the same level. As these terminal towers must resist the end strain of the line, they are made extra heavy, the four legs each being made up of 4 × 4 × ⁵/₁₆ and 4 × 4 × ³/₈ L sections. For the three cross-arms on one of these towers three pieces of 4-inch pipe, each 15 feet 9 inches long, are secured at its top with their parallel centre lines 30 inches apart in the same plane. Each of these pipes carries three insulator pins with their centres 7 feet 4¹/₂ inches apart. On the bottom of each leg of a terminal tower there is a foot, formed by riveting on bent plates, that measure 15 and 18 inches, respectively, on the two longer sides. Each foot of this tower is set in a block of concrete 5 feet square that extends from 3.5 feet to 7.5 feet below the ground level.

Insulators for the transmission line, which are illustrated in Fig. 104, are of brown, glazed porcelain, made in three parts, and cemented together. The three parts consist of three petticoats or thimbles, each of which slips over or into one of the others, so that there are three outside surfaces and three interior or protected surfaces between the top of an insulator and its pin.

From top to bottom the height of each insulator is 14 inches, and this is also the diameter of the highest and largest petticoat. The next or middle petticoat has a maximum diameter of 10 inches and the lowest petticoat one of 8 inches. Cement holds the lowest petticoat of the insulator on one of the steel pins previously described, and in this position the edge of the lowest petticoat is about 2¹/₂ inches from the steel support. At the top of each insulator the transmission conductor is secured, and the shortest distance from this conductor to any of the steel parts through the air is about 17 inches.

From the step-up transformer house at Niagara Falls to the terminal-station at Toronto, a distance of seventy-five miles, each three-phase, 60,000-volt, 25-cycle circuit on the steel towers is made up of three hard-drawn copper cables with a cross section of 190,000 circular mils each, and is designed to deliver 12,000 electric horse-power with a loss of ten per cent, on a basis of 100 per cent power factor. Six equal strands of copper make up each cable, and this wire has been specially drawn with an elastic limit of more than 35,000 pounds and a tensile strength of over 55,000 pounds per square inch. This cable is made in uniform lengths of 3,000 feet, and these lengths are joined by twisting their ends together in copper sleeves, and no solder is used. No insulation is used on these cables.

Instead of a tie-wire, a novel clamp is employed to secure the copper cable on each insulator. This complete clamp is made up of two separate clamps that grasp the cable at opposite sides of each insulator and of two half-circles of hard-drawn copper wire of 0.187 inch diameter. Each half-circle of this wire joins one-half of each of the opposite clamps, and fits about the neck of the insulator just below its head. Two bronze castings, one of which has a bolt extension that passes through the other, and a nut, make up each separate clamp. When the combined clamp is to be applied, the sides are separated by removing the nut that holds them together, the half-circles are brought around the neck of the insulator, and each of the side clamps is then tightened on to the cable by turning the nut that draws its halves together. This complete clamp can be applied as quickly as a tie-wire, is very strong, and does not cut into the cable.

Each of the regular steel towers is designed to withstand safely a side strain of 10,000 pounds at the insulators, or an average of 1,666 pounds per cable. With the 190,000-mil cable coated to a depth of ¹/₂ inch with ice and exposed to a wind blowing 100 miles per hour, the estimated strains on each steel pin for different spans and angular changes in the direction of the line are given in the accompanying table:

Pounds Strain on Pins, ¹/₂-inch Sleet, 100 Miles Wind.

The copper cables were so strung as to have a minimum distance from the ground of 25 feet at the lowest points of the spans. In order to do this the standard steel towers that hold the lower cables 40 feet above the ground level at the insulators are spaced at varying distances apart, according to the nature of the ground between them. At each tower the upper cable of each circuit is 5 feet 3 inches higher than the two lower cables, and this distance between the elevations of the upper and the lower cables is maintained whatever the amount of sag at the centre of each span. If there is a depression between two standard towers on a straight portion of the line, the sag in the centre of a span 400 feet long may be as much as 18 feet. Where a rise and fall in the ground between towers make it necessary to limit the sag to 14 feet in order to keep the lowest cables 25 feet above the highest point of earth, the length of span is limited to 350 feet. If the rise and fall of ground level between towers allow a sag of only 11 feet with the lowest cable 25 feet above the earth, the length of span with 40-foot towers is reduced to 300 feet; and if for a like reason the sag is limited to 8 feet, the span may only be 250 feet.

Fig. 105.—Take-up Arrangement on Terminal Tower.