Taking the tensile strength of soft copper at 34,000 pounds per square inch, hard-drawn copper at 45,000 to 70,000, silicon-bronze at 60,000 to 100,000, phosphor-bronze at 100,000, iron at 55,000, steel at 100,000, and aluminum at 30,000 pounds, the relative strengths of wires with equal sectional areas compared with the soft copper are, for hard-drawn copper, 1.32 to 2.06; silicon-bronze, 1.76 to 2.94; phosphor-bronze, 2.94; iron, 1.62; steel, 2.94; and for aluminum, 0.88.
Comparing wires on the basis of equal resistances for equal lengths, with soft copper again the standard, the tensile strength of each as to it is as follows: A hard-drawn copper line has 1.02 × 1.32 = 1.34 to 1.02 × 2.06 = 2.10 times the strength of a line of soft copper. With silicon-bronze the strength of line wire would range between 1.02 × 1.76 = 1.79 and 2.17 × 2.94 = 6.38 times that of copper. Iron would give the line a strength as to soft copper represented by 7.14 × 1.62 = 11.56. Steel of 100,000 pounds tensile strength per square inch will give a line 9.09 × 2.94 = 26.70 times as strong as it would be if composed of soft copper. With aluminum the strength of the line would be 1.66 × 0.88 = 1.46 times that of copper. For phosphor-bronze the figures are 3.84 × 2.94 = 11.29.
From the foregoing it may be shown how many times the price of soft copper per pound may be paid for each of the other metals to form a line of given length and resistance at a cost equal to that of a soft copper line. These prices per pound for the several metals relative to that of soft copper are as follows: Taking the price of soft copper as one, the price for hard-drawn copper must be 1 ÷ 1.02 = 0.98. For silicon-bronze the price may be as high as 1 ÷ 1.02 = 0.98, or as low as 1 ÷ 2.17 = 0.46 of the price of soft copper wire. Phosphor-bronze may have a price represented by only 1 ÷ 3.84 = 0.26 that of copper. The price of iron wire should be 1 ÷ 6.21 = 0.16 of that of copper, and for steel wire of the quality stated the price can only be 1 ÷ 8.01 = 0.12. Aluminum wire alone may have a higher price per pound than soft copper for the same resistance and cost of line, the figure for the relative cost of this metal being 1 ÷ 0.5 = 2.