ALUMINUM FOR ELECTRICAL CONDUCTORS.-Until within a few years copper has been the principal electrical conductor for circuits over which considerable amounts of power were to be transmitted. Owing to the recent very high prices of this useful metal unusual interest has been attracted to the development of the rival metal aluminum. The remarkable lightness of aluminum and its prevalence in nature have, since its discovery in 1827, been effective in impelling chemists to devise methods for cheaply reducing the metal from its compounds. A reasonable degree of success attended these efforts, but the electric current was needed for the complete solution of the problem and it is to the discovery of the possibility of electrolytic reduction of aluminum that the present comparatively low prices are due. These prices are now such that it is sold in direct competition with copper on the basis of equal electrical conductivity. This does not mean that the price is the same per pound, for the metals differ widely in specific gravity and in relative conductivity. Aluminum weighs almost exactly three-tenths as much as copper for the same bulk and if the current-carrying capacity of wires made of the two metals and of the same cross-section were the same, this fraction would represent their relative value as electrical conductors. Unfortunately, however, a wire made of aluminum and of the same cross-section as one of copper will not carry the same current with the same loss of power, but it must be made about 59 per cent. larger to do this. It follows that a wire of aluminum of the same conductivity as one of copper would weigh about 48 per cent. as much. In other words, if copper is worth fifteen cents per pound the corresponding price of aluminum is about thirty-one cents per pound for equivalent electrical conductors. These figures apply to pure aluminum which is not much used on account of its softness. The alloys which are generally

employed have a conductivity from 2 per cent. to 9 per cent. lower than the pure metal, but with an increased tensile strength.

Aluminum is present in large quantities in almost all soils, but it occurs in only a few materials in a form pure enough to render its reduction practicable. The best of these are cryolite (fluoride of aluminum and sodium) which is found in Greenland, and bauxite (oxides of aluminum and iron) a material found in many parts of Europe and in the southern part of the United States. The metal was discovered in 1827 by Wöhler, who recovered it from the chloride by means of potassium. In 1854, Bunsen and Deville succeeded in obtaining the metal by electrolytic decomposition, but at that time the production of the electric current was exceedingly expensive and no immediate use was made of the discovery. In the same year the reduction by the use of sodium was accomplished and for many years the efforts of chemists engaged in reducing aluminum were directed to the improvement of processes of sodium production. In the meantime progress was being made in the design of electrical machinery so that finally attention was once more turned in this direction. In America the use of the electric furnace by E. H. and A. H. Cowles of Cleveland, Ohio, in 1885, resulted in the production of aluminum at a greatly reduced cost. This was followed in 1888 by the Hall process by means of which, at the present time, a large proportion of the world's supply is produced. Due credit for progress should also be given to the European European investigators who have for some time produced large quantities of the metal.

The production of aluminum from bauxite consists of two processes: first, the preparation of pure alumina (aluminumoxide) for use in the electric furnace; and second, the electrolytic reduction of the metal from the oxide. The actual processes for preparing the alumina are secret ones,

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In the Hall process the electric furnaces, in which the metal is reduced, consist of carbon-lined metal tanks in which quantities of fused cryolite are maintained in a liquid state. The liquid condition is produced and maintained by the heat generated on account of the resistance offered to the flow of a direct electric current which passes to the carbon lining through the cryolite from carbon rods suspended in it. The cryolite forms a bath into which the alumina is fed at intervals. By the action of the current the oxide is broken up and the metal in a practically pure state is deposited on the carbon lining and falls to the bottom of the tanks, from which it is removed from time. to time and the process continues indefinitely except for the replacement of the carbon rods and lining which are consumed. A large amount of electrical energy is required to reduce a pound of the metal so that, if it were not for the possibility of obtaining very cheap power, the price would still be prohibitive for many purposes. The reason that Niagara power is used for producing a large part of the aluminum supply is at once evident.

Various reasons are advanced for the adoption of aluminum for electrical power transmission lines, one of the most convincing being that on account of the light weight the circuit is more easily supported and hence can be more cheaply installed than when copper is employed. On the other hand, if the wire must be insulated, the cost of insulation is increased over that for copper on account of the larger section required to produce a given conductivity. Objection has been made to the use of aluminum wire on account of the difficulty of soldering it, but this objection is partly overcome by the use of mechanical joints of good conductivity and tensile strength. One of the chief difficulties experienced in the erection and maintenance of aluminum wire has been the breakage due to the pres

ence of local impurities in the metal, causing it to fail when it is subjected to the great strains due to temperature changes. This evil has been overcome by making the wire in the stranded form, the effect of stranding being to spread the impurities over a considerable length. Special care, also, is now used in erecting the wire, with due consideration of the strains to which it will be subjected.

From the facts cited it is evident that aluminum is well adapted for use on circuits which are not insulated, such, for example, as the high pressure, long distance transmission lines on which insulation is useless. Numerous plants of this character are being installed in the West for the purpose of conveying power from waterfalls of great capacity to the cities where it can be utilized with profit. A very interesting example of this use of aluminum is found in the lines of the Bay Counties, California, transmission system. The Bay Counties Power Company has an aluminum line 142 miles in length between its power house, on the Yuba River, and Oakland, California. Not only is a great amount of power transmitted this distance, but the company has been selling power to another company which will have even a longer line and the total distance covered by the temporarily combined lines from the power house to San José is over 184 miles. expected that power will be transmitted shortly to San Francisco, a total distance of 222 miles. The line consists of three wires, each made of seven strands of aluminum wire twisted together into a cable, nearly three-quarters of an inch in diameter. An important test of the relative merits of hard drawn copper and aluminum will be made here, for the company has a parallel line of equal capacity made of the former metal. The conditions are ideal for this purpose.

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Other applications of aluminum as an electric conductor might be mentioned, including its use for the feeder wires of street railway systems in the form of immense stranded cables. Enough has been said, however, to show that this metal is sure to be used to an increasing extent, and while the probability of its displacing copper for electric conductors in the near future is not very great, it has a fair field for expansion in this direction.

HENRY H. NORRIS, M. E., Asst. Professor of Electrical Engineering, Cornell University.