XIII Electricity From an Old Mill
Millville is only a name or rather a reminiscence. There was once a village here, but now its population has all gone with the tide down the river, even its ghost appears to have departed. The ruins have all fallen, except the mill, which we propose to revivify.
I had built a summer cottage on the shore of the lake, about one mile from the mill. The absolute stillness of the place charmed me when worn out by the noise and heat and dirt and smell of the city. Here even the owl twittered softly as if afraid to disturb the silence.
The silence which was such a boon to me seemed to be oppressive to the younger members of the family. To prevent therefore their becoming dissatisfied with the place and wishing to go to other resorts, I planned to have some of their best friends spend much of the summer with us, and I encouraged their plans for making use of the mill. I will not offer this as an excuse for introducing electricity into a sleeping valley. Indeed, electricity has always disported itself there in the lightning, jumping from cloud to mountain peak as I have seen it nowhere else on earth.
The next time I saw the boys they had ambitious plans indeed. The penstock at the mill was to be repaired. The water-wheel was to drive an alternating current dynamo. The voltage of this current was to be stepped up by a transformer. It was to be transmitted to the cottage and there the e. m. f. was to be stepped down again by another transformer. My wife suggested that if it interfered with the simple life it would have to step down and out. Harold, however, assured his mother that they were going to simplify everything—even the subject of electricity.
Their plans were: To light the cottage by electricity; introduce a number of electric back logs, with coloured glass bottles; heat the fireless cooker by electricity; pump the water for the house by electricity; run mother’s sewing machine by electricity; run the washing machine and wringer by electricity; heat sad irons by electricity; percolate coffee, wash dishes and run the vacuum cleaner by electricity; operate the door bell and the telephone and wind the clock by electricity. I was sure that if they carried out these plans they would stay in Millville at least that summer, so I said go ahead.
We fixed the penstock. The boys estimated that 10 cubic feet of water per second would pass through it. They said that a cubic foot of water weighed 62.5 pounds and 10 cubic feet weighed 625 pounds. They said it fell at the rate of 7 vertical feet a second, making 4375 foot-pounds per second. But 550 foot-pounds per second is one horse-power, hence this is about 8 horse-power. Since one horse-power is equivalent to 746 watts of electricity, we have, if we could generate it without loss, said the boys, nearly the equivalent of 6 kilowatts of electricity, or about 54 amperes at 110 volts.
There were several things they wanted to know before they could go further with their plans.
1. How many of these electrical appliances we would be likely to use at one time.
2. How much current each device would require.
3. How much they must allow for losses in generating the current, in transmitting it, and in transforming it.
We assured them that we would never use more than twenty amperes, say, two thousand watts at one time. They might install a fuse, or circuit breaker in our line to protect their plant against a greater load from us. I told them that they might allow 20 per cent. loss of energy at the dynamo in converting water-power into electric-power.
I would suggest generating their current at 115 e. m. f. and stepping it up to 460 for transmission to us, and then stepping it down to about one hundred and ten volts for our use. They might count on about one-third loss on our supply, that is, they would need to generate about three thousand watts in order to deliver us 2000 watts.
I suggested making our line of No. 6 copper wire, which has a resistance of two ohms to the mile. The distance from the mill to the cottage is one mile, and the complete circuit therefore would require two miles of wire, or four ohms of resistance.
If we start with 3000 watts and lose 14 per cent. in transforming we shall have 2580 watts to transmit. If the voltage has been stepped up fourfold there will be about 5.6 amperes to transmit which will suffer a loss of 22.4 volts in passing through four ohms of resistance on the line. The loss in transmission will be about 5 per cent., and we shall have on arrival at the cottage about two thousand four hundred and fifty watts with a voltage of 437.6. If now in stepping this down to one fourth the voltage, viz., 109.4, we lose 14 per cent., we shall have left something over two thousand one hundred watts, or nearly twenty amperes.
Assuming that you are able to generate 4800 watts of electricity and that 3000 watts must be furnished for transmission to the cottage, you have left 1800 watts, which will give you something over fifteen amperes at 115 volts for use in your machine shop. I suggest that we get a dynamo which will generate both alternating and direct current—the alternating current you will send to the cottage, and the direct current you will have for use at the machine shop.
But how is it possible for a dynamo to generate both alternating and direct current at the same time?
Recall that all dynamos are generators of alternating current. If the brushes rest upon rings upon the axle they send forth alternating current—but if the brushes rest upon commutator bars they send forth direct current. Now we will have two sets of brushes, one pair of which shall rest upon the rings on the axle, and they will collect alternating current for the cottage, while the other pair will slide over the commutator bars and collect direct current for the machine shop. I have constructed a model which will make it plain. Here is a piece of a broom handle (Fig. 138), one foot long, which shall represent the axle of an armature. a b c d is a stout wire which represents the coil of the armature. In this case it has no iron at its centre. Nevertheless it will serve as an armature having one loop of its coil left. e and f are rings, sawed from a piece of brass pipe, which fit snugly upon the axle. Another ring of the brass pipe was sawed lengthwise, as shown in Fig. 139. These two halves are also fastened upon the axle and one end of the wire loop, c, is fastened to one of these, and the other end of the loop, b, is fastened to the other half of the ring. These two halves of the piece of brass pipe are placed so that their edges are near to each other but do not touch on either side of the axle. The two ends of this wire loop are also connected with the rings e and f. A short wire connects b and e and another connects c and f passing through the wood of the axle, as shown by the dotted line. We will now revolve this loop slowly about its axle in a strong magnetic field. To produce this field I will send two amperes of electricity through the coils of wire (Fig. 140), which surround two iron pole pieces that are screwed into an iron base. Between the poles N and S of this electro-magnet we will thrust this wire loop and revolve it as an armature very slowly. Meanwhile I connect two wires to my sensitive ammeter and let their free ends brush along on the rings e and f. The needle of the ammeter swings to and fro for each half revolution of the armature, showing an alternating current of.01 amperes. If this armature had many turns of wire instead of this one loop, if it had an iron core, and if it should revolve at high speed, the results would differ in degree but not in kind.
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.”
“I don’t like the idea,” drawled Erg, “that this machinery has got to be kept running all the time. When will a fellow get a chance to sleep or go a-fishing or have any vacation, with this central-station machine shop on his hands all the time?”
I had inquired how the last two boys won their nicknames of Dyne and Erg and had been informed that one was very keen about dining and the other had a great aversion for work. They had doubtless seen these terms somewhere in their reading of physics, but they appeared to have forgotten their significance by a too familiar use of them. I told them that these were sacred terms, the first being a name for the unit of force, while the second designated the unit of work. Both boys said that under those circumstances they would like to shed the names. The names, however, stuck and the boys themselves might, I think, be said to exercise a maximum of power with the least waste of energy.
This idea of running the plant continuously had evidently received no attention hitherto and it bid fair to quench all the enthusiasm until I came to the rescue by proposing a storage battery.
If we can procure a battery in which we may store energy, which shall always be on draught by merely pushing a button, one which “is not injured by overcharging nor too rapid discharging, nor even by complete discharge”; one which is not injured by standing idle for any length of time, either charged or discharged; and finally one which “is practically foolproof”—we want to try it. I propose that you appoint a committee to look into it. But at any rate this enterprise must go on even if I have to hire a man to live in the loft of the mill and keep the machinery going.
“No man in the loft,” said Dyne, “while I have my rations.”
“There will be no need for him so long as I can store energy here,” said Erg, “so let the job of equipping the establishment go on in the regular fashion.”
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.”
“Yes,” said Erg, as he rolled over, “they will have the right voltage when they use 20 amperes, but what will happen if they simply turn on one lamp. The drop in voltage then will be (.5 amperes × 4 ohms =) 2 volts; 105 cells at 1.8 volts a cell will send out there 189 volts minus the drop of 2 volts, leaving 187 volts upon a lamp adapted to 110 volts, and it will immediately burn out. The same thing would happen to any single piece of apparatus if the current was turned upon it alone. The only thing they could do if they wanted to light a lamp, say in the middle of the night to take a dose of medicine, would be to start up all together, all their lamps, sewing machine, wringer, dishwasher, fireless cooker, vacuum cleaner, etc., etc., to keep down the voltage so that one lamp would not burn out.”
“I have read,” said Ernest, “that they use rectifiers, which convert the alternating into direct current, for storing batteries. These are much used over the country. Electric automobiles run by storage batteries, and in the great majority of communities there is no other electric current than the alternating. So they would be helpless without the rectifier. We should then get another battery of fifty-five cells for the cottage and keep it stored by using a rectifier with our alternating current.
“But all their equipment up there,” said Ernest, “is adapted to the alternating current. Of what use would a direct current be to them?”
“Well,” said Harold, “it is all the same whether you have alternating or direct current on lamps, cooking apparatus, etc., and I have understood that they have motors which run on both alternating and direct currents. If so, that would fix them up all right.”
The boys now turned to me for the first time to inquire whether motors could be obtained which would run on both alternating and direct current, and I replied that small motors for running sewing machines, vacuum cleaners, etc., were made which would serve us, perhaps not economically, but as they were the only solution to our problem we could get along with them.
“Why don’t they have alternating current batteries?” inquired Erg.
“Well, it is time that we learned about the nature of batteries,” said I, “if you boys are going to have two storage batteries to care for.”