From the standpoint of cross-sectional area aluminum is inferior to copper as an electrical conductor. Comparing wires of equal sizes and lengths, the aluminum have only sixty per cent of the conductivity of the copper, so that an aluminum wire must have 1.66 times the sectional area of a copper wire of the same length in order to offer an equal electrical resistance. As round wires vary in sectional areas with the squares of their diameters, an aluminum wire must have a diameter 1.28 times that of a copper wire of equal length in order to offer the same conductivity.

The inferiority of aluminum as an electrical conductor in terms of sectional area is more than offset by its superiority over copper in terms of weight. One pound of aluminum drawn into a wire of any length will have a sectional area 3.33 times as great as one pound of copper in a wire of equal length. This follows from the fact that the weight of copper is 555 pounds while that of aluminum is only 167 pounds per cubic foot, so that for equal weights the bulk of the latter is 3.33 times that of the former metal. As the aluminum wire has equal length with and 3.33 times the sectional area of the copper wire of the same weight, the electrical conductivity of the former is 3.33 ÷ 1.66 = 2 times that of the latter. Hence, for equal resistances, the weight of an aluminum is only one-half as great as that of a copper wire of the same length. From this fact it is evident that when the price per pound of aluminum is anything less than twice the price of copper, the former is the cheaper metal for a transmission line of any required length and electrical resistance.

The tensile strength of both soft copper and of aluminum wire is about 33,000 pounds per square inch of section. For wires of equal length and resistance the aluminum is therefore sixty-six per cent stronger because its area is sixty-six per cent greater than that of a soft copper wire. Medium hard-drawn copper wire such as is most commonly used for transmission lines has a tensile strength of about 45,000 pounds per square inch, but even compared with this grade of copper the aluminum wire of equal length and resistance has the advantage in tensile strength. While the aluminum line is thus stronger than an equivalent one of copper, the weight of the former is only one-half that of the latter, so that the distance between poles may be increased, or the sizes of poles, cross-arms, and pins decreased with aluminum wires. In one respect the strain on poles that carry aluminum may be greater than that on poles with equivalent copper lines, namely, in that of wind pressure. A wind that blows in a direction other than parallel with a transmission line tends to break the poles at the ground and prostrate the line in a direction at right angles to its course. The total wind pressure in any case is obviously proportional to the extent of the surface on which it acts, and this surface is measured by one-half of the external area of all the poles and wires in a given length of line. As the aluminum wire must have a diameter twenty-eight per cent greater than that of copper wire of equal length, one-half of the total wire surface will also be twenty-eight per cent greater for the former metal. This carries with it an increase of twenty-eight per cent in that portion of the wind pressure due to wire surface. In good practice the number of transmission wires per pole line is often only three, and seldom more than six, so that the surface areas of these wires may be no greater than that of the poles. It follows that the increase of twenty-eight per cent in the surface of wires may correspond to a much smaller percentage of increase for the entire area exposed to wind pressure. Such small difference as exists between the total wind pressures on aluminum and copper lines of equal conductivity is of slight importance in view of the general practice by which some straight as well as the curved portions of transmission lines are now secured by guys or struts at right angles to the direction of the wires.

Vibration of transmission lines and the consequent tendency of cross-arms, pins, insulators, and of the wires to work loose is less with aluminum than with copper conductors as ordinarily strung, because of the greater sag between poles given the former and also probably because of their smaller weight. An illustration of this sort may be seen on the old and new transmission lines between Niagara Falls and Buffalo. The two old copper circuits consist of six cables of 350,000 circular mils section each on one line of poles, and are strung with only a moderate sag. In a strong wind these copper conductors swing and vibrate in such a way that their poles, pins, and cross-arms are thrown into a vibration that tend to work all attachments loose. The new circuit consists of three 500,000 circular mil aluminum conductors on a separate pole line strung with a large sag between poles, and these conductors take positions in planes at large angles with the vertical in a strong wind, but cause little or no vibration of their supports. One reason for the greater sag of the aluminum over that of the copper conductors in this case is the fact that the poles carrying the former are 140 feet apart while the distance between the poles for the latter is only seventy feet, on straight sections of the line.

The necessity for greater sag in aluminum than in copper conductors, even where the span lengths are equal, arises from the greater coefficient of expansion possessed by the former metal. Between 32° and 212° Fahrenheit aluminum expands about 0.0022, and copper 0.0016 of its length, so that the change in length is 40 per cent greater in the former than in the latter metal. The conductors in any case must have enough sag between poles to provide for contraction in the coldest weather, and it follows that the necessary sag of aluminum wires will be greater than that of copper at ordinary temperature.

In pure air aluminum is even more free from oxidation than copper, but where exposed to the fumes of chemical works, to chlorine compounds, or to fatty acids the metal is rapidly attacked. For this reason aluminum conductors should have a water-proof covering where exposed to any of these chemicals. The aluminum line between Niagara Falls and Buffalo is bare for most of its length, but in the vicinity of the large chemical works at the former place the wires are covered with a braid treated with asphaltum. Aluminum alloyed with sodium, its most common impurity, is quickly corroded in moist air, and should be carefully avoided. All of the properties of aluminum here mentioned relate to the pure metal unless otherwise stated, and its alloys should not, as a rule, be considered for transmission lines. As aluminum is electropositive to most other metals the soldering of its joints is quite sure to result in electrolytic corrosion, unless the joints are thoroughly protected from moisture, a result that is hard to attain with bare wires. The regular practice is to avoid the use of solder and rely on mechanical joints. A good joint may be made by slipping the roughened ends of wires to be connected through an aluminum tube of oval section, so that one wire sticks out at each end, then twisting the tube and wires and giving each of the latter a turn about the other. Aluminum may be welded electrically and also by hammering at a certain temperature, but these processes are not convenient for line construction. Especial care is necessary to avoid scarring or cutting into aluminum wires, as may be done when they are tied to their insulators. Aluminum tie wires should be used exclusively. To avoid the greater danger of damage to solid wires and also to obtain greater strength and flexibility, aluminum conductors are most frequently used in the form of cables. The sizes of wires that go to make up these cables commonly range from No. 6 to 9 B. & S. gauge for widely different cable sections. Thus the 183,708 circular mil aluminum cable between Shawinigan Falls and Montreal is made up of seven No. 6 wires, and the 471,034 circular mil cable between Electra and Mission San José contains thirty-seven No. 9 wires. From the Farmington River to Hartford each 336,420 circular mils cable has exceptionally large strands of approximately No. 3 wire. It appears from the description of a 43-mile line in California (vol. xvii., A. I. E. E., p. 345) that a solid aluminum wire of 294 mils diameter, or No. 1 B. & S. gauge, can be used without trouble from breaks. This wire was tested and its properties reported as follows:

This wire also stood the test of wrapping six times about its own diameter and then unwrapping and wrapping again. It was found in tests for tensile strength that the wire in question took a permanent set at very small loads, but that at points between 14,000 and 17,000 pounds per square inch the permanent set began to increase very rapidly. From this it appears that aluminum wires and cables should be given enough sag between poles so that in the coldest weather the strains on them shall not exceed about 15,000 pounds per square inch. This rather low safe working load is a disadvantage that aluminum shares with copper. From the figures just given it is evident that the strains on aluminum conductors during their erection should not exceed one-half of the ultimate strength in any case, lest their sectional areas be reduced.

Aluminum Cables in Transmission Systems.

Locations.Number
of
Cables.		Miles
of
Each.		Circular
Mils
of Each.		Strands
per
Cable.		Size of
Strands.
B. & S. G.
Approx-
imate.
Niagara Falls to Buffalo320 500,000....
Shawinigan Falls to Montreal385 183,70876
Electra to Mission San José3100 471,034379
Colgate to Oakland3144 211,00075-6
Farmington River to Hartford311 336,42073
Lewiston, Me.33.5144,68878
Ludlow, Mass.64.5135,24777