Fig. 140
We will now move the wires which are acting as brushes over to the metal pieces b and c. When now we revolve the armature the needle swings to the right, and just as the needle is about to swing back each brush slides from the plate on which it is rubbing to the opposite one and the needle gets another impulse forward. If the armature is turned rapidly the pulses disappear and the needle stands constantly at about .015 amperes. This then is both an alternating and a direct current dynamo. It simply needs more iron, more copper wire, and more rapid motion, to give us the 4800 watts of electrical energy we are seeking.
"But how shall we produce the current which we wish to send around the spools of the field?" inquired the boys.
"Connect the field with the brushes which rub upon the commutator," I replied. "It will magnetize its own field."
As good luck would have it, we found that the ledge of rock which furnished the basis for the mill dam was immediately underneath the floor at the north end of the machine shop. Upon this we built up a solid foundation for the dynamo. Our water-wheel gave a speed of 240 revolutions per minute to the counter shaft. A pulley of two feet in diameter upon this counter shaft was belted to the pulley of one foot in diameter upon the dynamo—thus giving its armature a speed of 480 revolutions per minute. We had to fix a governor upon the water-wheel to keep this speed constant at varying loads. The voltage is very sensitive to slight changes in the speed of the generator.
We had next to plan what equipment we should need for the machine shop and to decide where to locate each machine. The first machine we installed was a lathe adapted for use both with metals and wood. Among the adjuncts of this were all sorts of drills, chisels, circular saws, grinding and burnishing tools, etc. The second machine located was a small forge with an electric fan to furnish the blast. These were followed by a small band saw and a small planer. The fifth machine was a big grindstone and the sixth was an emery wheel. The boys had a long discussion, running through several days, on the question whether these machines should be belted to the counter shaft, and thus get power directly from the water-wheel, or whether each machine should be operated by an electric motor attached to it.
Harold said: "Suppose I want to saw a piece of wood requiring a horse-power, I must start an eight horse-power water-wheel which will run a six-horse-power dynamo which will operate a two-horse-power motor that will revolve the saw. There is a loss in each machine, and the lighter the load the greater the loss. In order that the motor may deliver one horse-power to the saw, it must receive from the dynamo, say, one and one-half horse-power, and in order that the dynamo may deliver to the motor one and one-half horse-power, it must receive from the water-wheel, say, two horse-power. What is the matter with my saving time and energy by sawing off the block with my own right arm?"
"But," said Ernest, "you forget that this water-wheel and the dynamo must run all the time by the terms of our agreement with the cottage, and they will run fairly well loaded, so that the starting of the saw will not entail any such losses as you reckon. Furthermore the water-power is running to waste, anyway. You simply divert its channel when you start all this machinery. That's all. And lastly, if the saw requires a horse-power, as you say, your right arm could not furnish it."
"Oh," interposed Dyne, "it would take a horse-power to do it as quickly as the machine does, but Harold simply proposes to take more time in sawing the block and less in running the machinery. An infant can do the work of a horse if you give him proportionally more time."