higher than 4000. This will not suffice to supply large working
currents to a greater distance than about six or eight miles. For a
distance of 10 miles 6000 volts are desirable; for 50 miles 30,000
volts; for 100 miles 60,000 volts; for 165 miles 100,000 volts; and for
200 miles 120,000 volts. Notice that in this table the voltage rises at
the rate of 600 per mile. Since it is not desirable for the generator
itself to produce a higher voltage than 4000, we must depend upon
transformers to produce these high voltages. Let us then consider,
a little more in detail, the construction of a transformer. I have
here some drawings of one which I propose that we make in the machine
shop, and use in our central station equipment in the future. We will
procure the thinnest and softest sheet iron possible and cut out of it
a lot of pieces shaped like the letter H with the dimensions shown in
Fig. 131. These are to be piled one upon another, with strips of paper
between, until the pile is 1-1/2 inches thick. Then four pieces of
board are to be bolted to the sides of these (Fig. 132). The dimensions
of each of the four blocks, is to be 7-1/2 inches long by 3 inches
wide by 1-1/2 inches thick. Upon the cross bar of the H we will wind
400 turns of No. 12 double cotton-covered copper wire, bringing out
the ends for future attachments, and then wind on 1200 turns of No.
10 double cotton-covered copper wire. The wire will fill the space
between the blocks as indicated by the diagram in Fig. 133. We will
then cut strips of the sheet iron 6 inches long by 1-1/4 inches wide,
and bridge across the ends of the H, prying open the leaves of sheet
iron and tucking them in between as shown in Fig. 134. We shall then
drill a hole at each corner and bolt them in place. Binding posts will
be placed at _a_, _b_, _c_, and _d_ (Fig. 134), and the two ends of
the No. 12 wire will be brought to _a_ and _b_ and those of the No. 18
wire will be brought to _c_ and _d_. Going through all this detail of
construction has probably made you lose sight of the essential features
of this transformer. Let us for a moment turn back and see what they
are. We have a large coil of wire 3 inches long and 7-1/2 inches in
diameter. It is composed of a coarse winding and a fine winding, which
we may designate as the primary and secondary coils, if we choose. Of
course the only reason for having different sizes of wire is so that
we may send larger currents through one than the other. The coil has
a laminated iron core, that is, it is composed of layers of sheet
iron. These layers are insulated from one another. This is essential,
although we cannot explain why now. But perhaps the most essential
feature of the transformer is that iron extends clear around from one
pole of this electro-magnet to the other. Fig. 135 represents a section
through the coil made in the plane of _e f g_ (Fig. 134). The core of
the magnet is represented as heavily shaded. The _magnetic circuit_ is
said to be closed from one pole of this magnet to the other through
the strips of iron which pass across the ends and down the sides of
the coil. The arrows show the path of the magnetic circuit. The dotted
portion shows where the copper wire may be supposed to have been cut
across. Inasmuch as the electric current is induced in the secondary
circuit by continually varying the strength of the magnetic field as
much as possible, the alternating current is the most desirable to use
in the primary. If the direct current were used an interrupter would
be necessary, which would of course produce too much sparking when any
but low tension currents are used in the primary circuit. The most
interesting and curious fact about the transformer is that the voltages
of the primary and secondary currents are in exact proportion to the
number of turns in the wire of the two circuits.
[Illustration: Fig. 133]
[Illustration: Fig. 134]
[Illustration: Fig. 135]
In our transformer the number of turns in the coil between the binding
posts _a_ and _b_ is 400 and the number of turns between _c_ and _d_
is 1200. If now we connect a 112-volt alternating current with the
binding posts _a_ and _b_, a volt meter connected across between _c_
and _d_ will show 336 volts, and if _b_ and _c_ be connected by a short
wire, bringing in 1600 turns into the secondary circuit, a volt meter
connected across between _a_ and _d_ will show a voltage of 448. Or
if, leaving _b_ and _c_ still connected by a short wire, we connect
the 112-volt alternating current to _a_ and _d_ a volt meter connected
across between _a_ and _b_ will show 28 volts, or if connected between
_c_ and _d_ it will show 84 volts, and if finally the 112-volt current
is connected to _c_ and _d_ the pressure between _a_ and _b_ will be
37-1/3.
[Illustration: Fig. 136]
The story, then, of the central station which we have chosen as a type
is briefly this: Falling water makes dynamos revolve, generating a
4000-volt alternating current. This current is sent through the primary
windings of transformers. The secondary windings of these transformers
have twenty-five times as many turns as the primary coils. This steps
up the voltage from 4000 to 100,000, making it necessary to send only
one twenty-fifth as many amperes over the lines as would be required
at 4000 volts, and reduces the loss in transmission to nearly one
twenty-fifth. At the other end of the line the current traverses the
secondary windings of transformers, and the consumer receives his
current from primary coils which may step the e. m. f. down to any
required volts of pressure, generally 110.
Now I shall be glad to have you consider whether this suggests any
practicable problems for us here in Millville.
The sun is nearly setting and I suppose the family is expecting me home.
[Illustration: Fig. 137]
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?
[Illustration: Fig. 138]
[Illustration: Fig. 139]
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.
[Illustration: 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."
"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.
[Illustration: Fig. 141]
"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.
[Illustration: 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.
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.