As may be seen from the foregoing facts, the choice of copper or aluminum for a transmission line should turn mainly on the cost of conductors of the required length and resistance in each metal. So nearly balanced are the mechanical and electrical properties of the two metals that not more than a very small premium should be paid for the privilege of using copper. As already pointed out, the costs of aluminum and copper conductors of given length and resistance are equal when the price per pound of aluminum wire is twice that of copper. During most of the time for several years the price of aluminum has been well below double the copper figures, and the advantage has been with aluminum conductors. With the two metals at the same price per pound aluminum would cost only one-half as much as equivalent copper conductors. When the price of aluminum is fifty per cent greater per pound than that of copper, the use of the former metal effects a saving of twenty-five per cent. For the new Niagara and Buffalo line, completed early in 1901, aluminum was selected because it effected a saving of about twelve per cent over the cost of copper. All of the aluminum lines here mentioned, except the short one near Hartford, were completed during or since 1900. Most of the facts here stated as to the line between Niagara Falls and Buffalo are drawn from vol. xviii., A. I. E. E., at pages 520 and 521.
The greater diameter of aluminum over equivalent copper conductors has advantages in transmission with alternating current at very high voltages. At high voltages, say of 40,000 or more, the constant silent loss of energy from one conductor to another of the same circuit through the air tends to become large and even prohibitive in amount. This loss is greater, other factors being constant, the smaller the diameter of the conductors in the line. It follows that this loss is more serious the smaller the power to be transmitted, because the smaller the diameter of the line wires. The silent passage of energy from wire to wire increases directly with the length of line and thus operates as a limit to long transmissions.
VOLTAGE AND LOSSES ON TRANSMISSION LINES.
The voltage on a transmission line may be anything up to at least 60,000, and the weight of conductors varies inversely with the square of the figures selected, the power, length and loss being constant. Whatever the total line pressure, the weight of conductors varies inversely with the percentage of loss therein.