If you find there is twice as much water as you need for the amount of power you require, a five-foot fall will give the same result. Or, if there is only one-half as much water as the 250 cubic feet specified, you can still obtain your theoretical five horsepower if the means are at hand for providing a fall of twenty feet instead of ten. Do not make the very common mistake of figuring that a stream is delivering a cubic foot a minute to each square inch of weir opening, simply because it fills a certain opening. It is the excess water, falling over the opening, after the stream has set back to a permanent dead level, that is to be measured.
This farmer who spends an idle day measuring the flow of his brook with a notched board, may say here: "This is all very well. This is the spring of the year, when my brook is flowing at high-water mark. What am I going to do in the dry months of summer, when there are not 250 cubic feet of water escaping every minute?"
There are several answers to this question, which will be taken up in detail in subsequent chapters. Here, let us say, even if this brook does flow in sufficient volume only 8 months in a year—the dark months, by the way,—is not electricity and the many benefits it provides worth having eight months in the year? My garden provides fresh vegetables four months a year. Because it withers and dies and lies covered with snow during the winter, is that any reason why I should not plow and manure and plant my garden when spring comes again?
A water wheel, the modern turbine, is a circular fan with curved iron blades, revolving in an iron case. Water, forced through the blades of this fan by its own weight, causes the wheel to revolve on its axis; and the fan, in turn causes a shaft fitted with pulleys to revolve.
The water, by giving the iron-bladed fan a turning movement as it rushes through, imparts to it mechanical power. The shaft set in motion by means of this mechanical power is, in turn, belted to the pulley of a dynamo. This dynamo consists, first, of a shaft on which is placed a spool, wound in a curious way, with many turns of insulated copper wire. This spool revolves freely in an air space surrounded by electric magnets. The spool does not touch these magnets. It is so nicely balanced that the weight of a finger will turn it. Yet, when it is revolved by water-power at a predetermined speed—say 1,500 revolutions a minute—it generates electricity, transforms the mechanical power of the water wheel into another form of energy—a form of energy which can be carried for long distances on copper wires, which can, by touching a button, be itself converted into light, or heat, or back into mechanical energy again.
If two wires be led from opposite sides of this revolving spool, and an electric lamp be connected from one to the other wire, the lamp will be lighted—will grow white hot,—hence incandescent light. The instant this lamp is turned on, the revolving spool feels a stress, the magnets by which it is surrounded begin to pull back on it. The power of the water wheel, however, overcomes this pull. If one hundred lights be turned on, the backward pull of the magnets surrounding the spool will be one hundred times as strong as for one light. For every ounce of electrical energy used in light or heat or power, the dynamo will require a like ounce of mechanical power from the water wheel which drives it.
The story is told of a canny Scotch engineer, who, in the first days of dynamos, not so very long ago, scoffed at the suggestion that such a spool, spinning in free air, in well lubricated bearings, could bring his big Corliss steam engine to a stop. Yet he saw it done simply by belting this "spool," a dynamo, to his engine and asking the dynamo for more power in terms of light than his steam could deliver in terms of mechanical power to overcome the pull of the magnets.
Electricity must be consumed the instant it is generated (except in rare instances where small amounts are accumulated in storage batteries by a chemical process). The pressure of a button, or the throw of a switch causes the dynamo instantly to respond with just enough energy to do the work asked of it, always in proportion to the amount required. Having this in mind, it is rather curious to think of electricity as being an article of export, an item in international trade. Yet in 1913 hydro-electric companies in Canada "exported" by means of wires, to this country over 772,000,000 kilowatt-hours (over one billion horsepower hours) of electricity for use in factories near the boundary line.
This 250 cubic feet of water per minute then, which the farmer has measured by means of his notched board, will transform by means of its falling weight mechanical power into a like amount of electrical power—less friction losses, which may amount to as much as 60% in very small machines, and 15% in larger plants. That is, the brook which has been draining your pastures for uncounted ages contains the potential power of 3 and 4 young horses—with this difference: that it works 24 hours a day, runs on forever, and requires no oats or hay. And the cost of such an electric plant, which is ample for the needs of the average farm, is in most cases less than the price of a good farm horse—the $200 kind—not counting labor of installation.