Modern usage in the matter of type of alter nating current employed leans largely toward three-phase. (See ELECTRIC ALTERNATING RENT MACHINERY). For each circuit this re quires three conductors which are arranged on the poles and cross-arms usually in an equilat eral triangle, the wires being separated from one another by a distance of six or eight feet. The wires are in some cases transposed on the poles, to form in effect a long horizontal spiral. This is generally done to prevent inductive effects on the telephone wires used for signaling on the same or adjacent poles, although some engineers also think that spiraling the con ductors diminishes the impedance of the cir cuit. So far as the telephone line belonging to the transmission company is concerned the sim plest way to avoid inductive effects is to spiral the telephone circuit. While, as just intimated, the long-distance transmission of energy is car ried on chiefly by means of the alternating cur rent, transformers, etc., high potential trans mission in at least one instance in Europe has been effected with continuous current. In this instance the line pressure is 60,000 volts, which is generated directly on the line by six dynamo machines in series, each generating 10,000 volts. At the point of distribution six motors are con nected in series and each motor is caused to drive a generator which in turn develops elec trical energy of a desired potential and current output.
For the supports of the transmission line the choice is practically between the use of wood poles or steel towers. The kind of wood employed for the poles varies somewhat with the locality, cedar being used in the northern States and redwood in the Pacific Coast States. These poles must be of sufficient height to afford ample clearance from ground and sufficiently strong to withstand wind strains, etc. Poles fitted to meet these requirements and to carry two three-wire circuits should be at least 35 to 40 feet in height, set 5 to 6 feet in the earth and be 12 to 14 inches in diameter at the butt and at least 8 inches at top. The use of steel towers permits longer spans and conse quently diminishes the total number of insu lators necessary. With wooden poles the maxi mum length of span is 180 feet; minimum 80 feet. With steel towers using 12 to the mile the span between towers is 440 feet. On one long-distance transmission the towers are made up of four galvanized angle iron posts 40 feet in length, 3 inches by 6 inches with three-six teenths inch angles, the posts being stayed with suitable angles and cross rods.
For the very high potentials used on long distance transmission lines extra precautions as to insulation are requisite, both where the wires leave the power-houses and on the poles or towers. For insulating the conductors from the poles or towers large porcelain discs are now commonly used, in a series of four or more (arranged somewhat like Japanese dinner gongs), termed suspension insulators, the upper disc of which is attached to the pole or tower.
The conductor is attached to the lowest disc. Much care is required in the manufacture of these insulators. This series arrangement of insulators has largely increased the amount of electro-motive force that can be successfully employed in electrical transmission of power as the electric potential is divided between the individual discs. Other types of insulators for high tension service consist of large petticoat insulators about 12 inches in diameter across the top, 12 inches in height and weigh 18 to 20 pounds. They are supported on the cross arms or on the pole itself by wooden or iron pins. For pressures up to about 25,000 volts wooden pins are found fairly satisfactory, but above that pressure they are found to char by a peculiar action of the current, and it is ad visable on this account, as well as for mechanical reasons, to employ cast iron or metal composi tion pins. These pins are from 15 to 17 inches in height and they maintain the insulator about 12 inches from the pole or cross-arm. (See illustration, which shows an iron tower, a three wire circuit, with cross-arms, pins and porcelain insulators; also the openings in the gable of power-house by which the high tension con ductors pass out).
The choice of metal for the conductors in this service is virtually confined to copper and aluminum. It is known that there is a tendency to a brush discharge, termed corona, in the air between conductors conveying currents at high pressures that leads to a waste of electric energy when with wires of given diameter a critical electromotive force is reached. The critical electromotive force also varies with the dis tance between the wires. It was at one time thought that this effect would constitute the limiting factor in the long-distance transmission of electric power, but in Prof. Harris J. Ryan's paper,