The transformer is an important adjunct in an electrical transmission system. It is based on the principle of the induction coil (q.v.). The essential parts are a core of laminated sheet iron wound with two coils of copper wire, one coil consisting of numerous turns of fine wire and the other coil of a few turns of coarse wire. If the fine wire coil is con nected to the line a much lower pressure cur rent can be drawn from the coarse wire or secondary coil; if the connections are reversed and the fine wire coil is used for a secondary, a current of much increased pressure is gener ated. Thus the transformer is used, as its name implies, to °step up° or °step down° the pressure. The relative number of turns in the primary and secondary coils determines the ratio of the steps obtained.
To distribute hydro-electric power economic ally, it has been found best to first create a very high potential for the alternating current, and to send this out to a station, where there are transformers, and it can be stepped down for use, and by means of rotary converters can be changed into direct current when desired. It is not many years since it was considered an achievement to send out 40,000 volts on a wire, but now 150,000 voltage is used, and still higher pressures are contemplated.
To develop the water power, a stream has to be dammed, a reservoir formed to maintain a flow of water and a power-house with water wheels must be installed. The head or fall of water determines the sort of water wheel to be employed. All heads under 300 feet are now classed as low heads, and all over about 700 as high heads; the wheel accepted for low heads is the turbine, and for high heads the impulse or Felton wheel. For intermediate heights of head, conditions have to determine which form of wheel is best. For a fuller description of water wheels see HYDRAULICS.
Water-power plants are usually supplied with two sorts of dams, storage reservoirs and diverting dams. These are now mostly con structed of masonry and concrete. Conduits for conveying the water are made of both wood and steel; the wood may be rectangular or box-shaped, or of circular cross-section, made like a continuous barrel with staves and steel bands. These wood stave pipes are exceedingly strong, and will carry a 200 foot head, and last for years. But for greater pressure and durability steel pipe is requisite. Lap-welded galvanized sheet steel is the favorite material, and it is made of a tensile strength of 50,000 to 65,000 pounds per square inch of section. It is lapped and triple-riveted for carrying heavy pressures. Such steel pipes are often three to
five feet in diameter, and have been made up to seven feet. When of considerable length the construction has to allow for contraction and expansion of the metal with changes in temperature. One engineer reported that a long section of metal pipe °crawled°— that is, ex panded and moved its position lengthwise— seven inches between the extremes of heat and cold of a single 24 hours.
Where the water is not under pressure, as in a tail-race; it may be carried in an open channel, and such are frequently built of con crete. It is often necessary to tunnel through the rock to find a proper outlet. To use a high head of water a power station must oc cupy a low site, which means standing in a valley. After using the water here the next problem is to get rid of it, which means lead ing it to some lower locality where it can flow away. To find such lower level it is sometimes necessary to tunnel for miles.
Complete statistics of the extent of hydro electric power development are lacking. The United States census of 1910, covering returns made by manufacturing establishments in 1909, shows only a very small percentage of water power in use, and it is known to be far larger than the official figures show. To illustrate: In 1869 the census reported a total of 2,346,000 horse power used in manufacturing, of which 1,130,000 or nearly a half was water power. In 1889 the total power recorded as used in manu facturing was only 10,000,000 horse power, of which 1,454,000, or 14.4 per cent, was water power, and nearly all the rest steam power. This was the date at which hydro-electric de velopment really began, yet 20 years later, in 1909, in spite of the tremendous development of plants, as recorded farther on, the increase of water power is given at only a little more than 1 per cent a year. The steam power is credited in 1909 with 14,199,000 horse power, electric power is credited with 1,749,000 horse power and the water wheels get credit for only 1,823,000, or less than 10 per cent of the total. This despite the known development of tremendous horse power at Niagara, Big Creek, Tallulah River, Las Plumas, Butte, Keokuk, etc. It is true than the census figures quoted do not include the use for mines and quarries, which is reported separately at the insignificant figure of 97,460 water power as against 255 699 electric and 4,608,000 steam Morse power. this 1909 census does credit New England with 27.9 per cent of her horse power as being derived from water, but only allows 5 to 8 per cent for far Western States.