After a long confab one evening at the mill we settled upon the arrangement shown in [Fig. 141]. D represents the location of the doors and W that of the windows. The equipment is designated as follows: A, saw; B, planer; C, lathe; E, emery wheel; F, grindstone; G, dynamo; H, forge; I, storage battery; J, switchboard; K and L, counter shafts suspended from the ceiling. The water-wheel is belted directly to the counter shaft L. This revolves at the rate of 240 r. p. m. A two-foot pulley on this shaft is belted to a one-foot pulley on the dynamo G, giving the dynamo a speed of 480. A 4-inch pulley on this counter shaft is belted to a 16-inch pulley on the grindstone F, giving the stone a speed of 60 r. p. m., or one revolution per second. A 32-inch pulley on shaft L is belted to an 8-inch pulley on the counter shaft K, giving a speed of 4 times 240, or 960 r. p. m. 12-inch pulleys on this shaft are belted to 6-inch pulleys on each of the machines A, B, and C, giving them a speed of 1920 r. p. m., and a 16-inch pulley on this shaft is belted to a 4-inch pulley on the emery wheel, giving it a speed of 3840 r. p. m. As soon as everything was in running order, Harold took his mother down to the machine shop and started all the machinery going at once, and while they stood in the middle of the room I heard him explaining to her how she might find out the speed of each machine. He said that she must start with the grindstone, because that goes slowly enough to count. She held her watch in hand and counted the number of revolutions in a minute, as he directed, and found them to be sixty. Then he asked her to judge how much larger the pulley on the grindstone was than the corresponding one on the counter shaft. She said that she thought it looked four times as large. He told her that she had it just right, and explained that the shaft must move four times as fast as the stone, or 240. "Now how fast do you think the emery wheel is going?" She acknowledged that she had no idea.
"Well," said he, "when you get real used to it you can tell by the tone a wheel makes just about how fast it is going."
Then he explained how she might calculate its speed by looking at the pulleys, and she found that the counter shaft was going four times as fast as the shaft L and that the emery wheel was going four times as fast as K. Hence it was going sixteen times as fast as L, or 3840 r. p. m. His mother said she thought that it was fascinating to stand in the middle of the room with the slowly moving grindstone on one hand and emery wheel moving sixty-four times as fast on the other hand and think that they were propelled by the same water-wheel. I handed Harold a speed indicator which I had just received, ([Fig. 142]), the mechanism of which was all visible. Harold looked at it for a minute and found stated upon it that the wheel B had 100 cogs, and he very quickly inferred that the axle A, whose screw threads fitted into these cogs, must revolve one hundred times each time the wheel B revolves once. The tip end of this axle had a soft rubber cap C. Without suggestion from me he soon held this rubber shoe against the end of the axle of the emery wheel and counted, not thirty-eight, but thirty-six revolutions of the wheel of the speed indicator in one minute. This puzzled him and he inquired how it happened that the emery wheel made only 3600 rather than 3840 revolutions per minute.
Fig. 142
"Well," said I, "we always have to count on belts slipping some, particularly upon very fast moving pulleys and upon very small pulleys. Here are two belts to slip, and still you are losing only the effect of one revolution of the counter shaft L in a minute. Grind something on the emery wheel and you will find that the belts will slip more. In fact, grinding upon the emery wheel will compel the water-wheel to go more slowly until its governor opens and gives it more water. The water-wheel makes fifteen revolutions per minute and the emery wheel goes 256 times as fast as that. One pound of resistance at the emery wheel is like 256 pounds of resistance at the water-wheel. You notice the same thing when you use the saw or planer, or even present a chisel to a piece of soft wood in the turning lathe.
"The only machine here that it is important to keep running at constant speed is the dynamo. We shall probably notice the dimming of our lights at the cottage every time you saw a block or grind with the emery wheel or even polish with the felt buffer, because the speed of the dynamo will slacken for a moment and the voltage will drop a little."
In addition to sending electric current to the cottage the dynamo was to keep the battery stored all the time. Each machine had an appropriate motor attached to it which could run it by drawing current directly from the battery when the water-wheel was not running. Thus if one wanted to sharpen his pocket knife he merely closed a switch at the lathe and used the small stone, or if he wished to sharpen his lead pencil he put it in the lathe and applied a chisel to it.
These motors were all adapted to the 110-volt direct current and the battery contained fifty-seven cells, each cell being rated a little under two volts.