Part 20

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.

"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.

The boys frequently discussed possible combinations in this system. I spent a great deal of time loafing around among them in a comatose condition, and they talked quite as freely when I was around as when they were alone among themselves. One day I heard Dyne say, "Suppose we should store in a reservoir the water which comes down the penstock during a day and store all the electricity it will generate in a day in a storage battery, then at night let the battery run the dynamo backward as a motor, and that turn the water-wheel backward as a rotary pump, we should have the water in the upper reservoir to begin work with the next morning and the problem of perpetual motion would be solved.

"Aw, why do you want to do all that," said Erg, "when nature is doing it for us?"

Ernest said he had a better scheme than that. He would turn the battery current on to all the motors in the room and they would run the counter shafts forward and the counter shafts would run the dynamo forward and the dynamo would charge the battery, and so you could keep up the motion perpetually if you wanted to.

"Get out your pencils," said Harold, as he took down a copy of Houston and Kennelly. "Let us see how we would come out if we tried Dyne's proposition for, say, twenty hours, storing the energy from the falling water for ten hours in the battery and then using this energy during the next ten hours for re-storing the water in the upper pond. We will say that the water-wheel furnishes eight horse-power for ten hours--eighty horse-power hours."

I notice it is stated in this book that small dynamos are usually unable to deliver more than 75 per cent. of the energy impressed upon them, and storage batteries and motors deliver about 80 per cent. of the energy impressed upon them. The accounts would, therefore, stand as follows:

Dynamo Horse-power Hours Dr. Cr.

To energy impressed by water-wheel 80 By energy delivered to storage battery 60 By loss in heat 20 --------- 80 80

(Assuming that the battery was able to receive all the dynamo could give.)

STORAGE BATTERY ACCOUNT

To energy impressed by dynamo 60 By energy delivered back to dynamo running as motor 48 By loss in heat 12 --------- 60 60

Dynamo Running as Motor Horse-power Hours Dr. Cr.

To energy impressed by battery 48 By energy delivered back to water-wheel 36 By loss in heat 12 --------- 48 48

(This dynamo being a particularly inefficient motor.)

We cannot give the account of a water-wheel acting as a pump, because such a machine has not yet been perfected. It is evident however that if a water-wheel could be devised that should be a perfect pump, the losses in this chain of machinery are more than half; indeed, the accounts show them to be 60 per cent. We should, therefore, be able to return less than half the water drawn from the lake each day, and we should rapidly move toward bankruptcy.

"Well," said Ernest, "my proposition is more successful than that, because it sets out to be a fool proposition."

It was first suggested by the snake who undertook to swallow himself. Suppose the account does taper down from eighty to one, so does the snake, but he still remains "wise as a serpent." Our account would stand as follows:

Dynamo Battery Motors

36 27 27 20 20 15 15 12 12 9 9 7 7 5 5 4 4 3 3 2 2 1 1 .8 .8 .6 .6 .48 .48 .36 .36 .27 .27 .20 .20 .15 .15 .12 .12 .09 .09 .07 .07 .05 .05 .04 .04 .03 .03 .02 .02 .01 .01 .003

It is evident that while our energy would dwindle continually we should never quite come out of the little end of the horn, since any number may diminish by 20 per cent. of itself indefinitely.

"Let us get at something practical," said Erg. "How are we going to furnish electricity to the cottage when the dynamo is not running? If we put a storage battery at the cottage, how are we going to store it having nothing but alternating current up there; and if we attempt to transmit current from our central station battery, how are we going to get along with the drop in the voltage?"

"I'll tell you how to do that," said Dyne. "They want 20 amperes and the line offers 4 ohms of resistance. That means a drop of 80 volts. We have simply to provide a subsidiary battery of 48 cells, which we may throw in series with our 57 cells when we supply electricity to the cottage, and then they will have the right voltage for use out there."