A Little Prospecting
Small amount of water required for an electric plant—Exploring, on a dull day—A rough and ready weir—What a little water will do—The water wheel and the dynamo—Electricity consumed the instant it is produced—The price of the average small plant, not counting labor.
The average farmer makes the mistake of considering that one must have a river of some size to develop power of any practical use. On your next free day do a little prospecting. We have already said that 250 cubic feet of water falling 10 feet a minute will provide light, heat and small motor power for the average farm. A single water horsepower will generate enough electricity to provide light for the house and barn. But let us take five horsepower as a desirable minimum in this instance.
Measuring a small stream with a weir
In your neighborhood there is a creek three or four feet wide, toiling along day by day, at its task of watering your fields. Find a wide board a little longer than the width of this creek you have scorned. Set it upright across the stream between the banks, so that no water flows around the ends or under it. It should be high enough to set the water back to a dead level for a few feet upstream, before it overflows. Cut a gate in this board, say three feet wide and ten inches deep, or according to the size of a stream. Cut this gate from the top, so that all the water of the stream will flow through the opening, and still maintain a level for several feet back of the board.
This is what engineers call a weir, a handy contrivance for measuring the flow of small streams. Experts have figured out an elaborate system of tables as to weirs. All we need to do now, in this rough survey, is to figure out the number of square inches of water flowing through this opening and falling on the other side. With a rule, measure the depth of the overflowing water, from the bottom of the opening to the top of the dead level of the water behind the board. Multiply this depth by the width of the opening, which will give the square inches of water escaping. For every square inch of this water escaping, engineers tell us that stream is capable of delivering, roughly, one cubic foot of water a minute.
Thus, if the water is 8 inches deep in an opening 32 inches wide, then the number of cubic feet this stream is delivering each minute is 8 times 32, or 256 cubic feet a minute. So, a stream 32 inches wide, with a uniform depth of 8 inches running through our weir is capable of supplying the demands of the average farm in terms of electricity. Providing, of course, that the lay of the land is such that this water can be made to fall 10 feet into a water wheel.
Go upstream and make a rough survey of the fall. In the majority of instances (unless this is some sluggish stream in a flat prairie) it will be found feasible to divert the stream from its main channel by means of a race—an artificial channel—and to convey it to a not far-distant spot where the necessary fall can be had at an angle of about 30 degrees from horizontal.
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.
It is the purpose of these chapters to awaken the farmer to the possibilities of such small water-power as he or his community may possess; to show that the generating of electricity is a very simple operation, and that the maintenance and care of such a plant is within the mechanical ability of any American farmer or farm boy; and to show that electricity itself is far from being the dangerous death-dealing "fluid" of popular imagination. Electricity must be studied; and then it becomes an obedient, tireless servant. During the past decade or two, mathematical wizards have studied electricity, explored its atoms, reduced it to simple arithmetic—and although they cannot yet tell us why it is generated, they tell us how. It is with this simple arithmetic, and the necessary manual operations that we have to do here.
Next: How To Measure Water-power
Previous: A Working Plant