coils. Let us connect _b_ with _c_ so as to make one continuous circuit
of 300 turns. Let us then put a branch upon _b_ to connect with the
battery, thus having 100 turns for the primary circuit, and put a
branch upon _a_ to connect with the lamp, thus having 300 turns upon
the lamp, (Fig. 119). When now we rub _b_ upon the file, as before,
the lamp lights up more brightly than before, indicating that we have
stepped up the voltage still higher. Varying the strength of the
magnetic field induces a secondary current and the voltage of the
induced current is determined, in part, by the number of turns in the
secondary circuit. If what we have been saying is true we ought to be
able to get these same results from an electric bell. To test this we
connected wires with _a_ and _c_, (Fig. 120), and since I knew that
the secondary current at _S_ would be too severe for the tongue we
decided to feel it with the hands. For this purpose we want a larger
surface than the wires themselves offer for contact with the hands,
and so I twisted the bare end of each wire around an iron spike. The
four boys then arranged themselves in line, joining hands, and the
boy at each end of the line held a spike in his free hand. Thus we
had put the enormous resistance of four human bodies joined in series
in the secondary circuit. When now I connected two dry cells with _a_
and _b_ (_P_, Fig. 120) the hammer of the bell acted, like the file in
the former case, as interrupter of the primary circuit. As it rapidly
made and broke the primary circuit, it produced rapid changes in the
strength of the magnetic field and thus induced a secondary current
which the boys all felt. The fact that it forced its way through four
bodies shows that its voltage was high. The high voltage was also
indicated by the spark which always occurred in the bell. The primary
circuit in this case has not more than three volts while the secondary
has more than a hundred. We have it in our power to give the secondary
current almost any voltage we choose, with this limitation _each
increase in voltage necessitates a proportional sacrifice of quantity_.
The watt power induced in the secondary circuit cannot exceed that
contributed to the primary circuit--indeed cannot quite equal it since
there is some loss in heat.
[Illustration: Fig. 120]
Suppose we operate a bell on a primary current having three volts
and .25 ampere, that is, .75 watt. Suppose then the voltage of the
secondary current is stepped up to fifty times three, or 150 volts. The
quantity of secondary current will be found to be somewhat less than
one fiftieth of .25 or .005 ampere. The 150-volt alternating current
from the bell is more tolerable than that from a 150-volt dynamo,
because the quantity is limited in the former case.
Our spark coil has a vibrator which acts precisely like the hammer of
the bell to make and break the primary circuit and thus make rapid
changes in the magnetic field produced by the primary coil. The primary
coil of the spark coil is many times larger than the coil of the bell,
that is, it contains many more turns of wire. It has much more iron
in the core. We use upon it five cells instead of the two cells upon
the bell. The result of all this is that we have a much more powerful
magnetic field than that in the bell and many more watts of energy
from which to induce a secondary current. Now the number of turns
employed in the secondary circuit of our spark coil is very great,
stepping its voltage up to thousands where the bell induced hundreds.
[Illustration: Fig. 121]
Suppose we now repeat our experiment in which we tried to light the
gasolene in the watch crystal, using now the spark coil of the boat
instead of our small "home-made" coil. In Fig. 121, B is the battery
of five dry cells. _S_ is a switch. _V_ is the vibrator, which, like
the hammer of an electric bell, makes and breaks the primary circuit.
Of course the coil has a core of iron, although that is not here
represented, and, of course, the coil has many hundred turns instead
of the few here represented, and of course also it is built up of many
layers instead of one as here represented. The secondary has very many
more turns than the primary, but those in which the primary current
passes are common to both circuits. There is also a condenser--not here
represented, and not to be described in this book. The result of all
this is that the secondary circuit has a voltage of between 5000 and
10,000, and a spark jumps across the gap at _c_ between one sixteenth
and one eighth of an inch long. This spark is hot enough to light the
gasolene which I have put in the watch crystal at _c_.
[Illustration: Fig. 122]
Let us return to the bell for a few minutes. I have here a miniature
lamp which requires 10 volts and .1 ampere, that is, 1 watt, which I
will connect at _S_ (Fig. 122). When now I close the primary circuit
with two cells at _P_ you notice that the lamp lights up, but faintly.
It is not receiving .1 ampere. Remember we have only .75 watt at our
disposal and this lamp requires 1 watt. Hence it is getting only three
quarters enough energy. We connect in a third cell and now it lights up
to full brilliancy. The resistance of this lamp must be about 100 ohms.
(10 volts)/(100 ohms) = .1 ampere
The resistance of the four boys might have been 60,000 ohms, and the
voltage of the secondary circuit might in that case have been, say, 150.
(150 volts)/(60,000 ohms) = .0052 ampere
How does it happen that the secondary current had a pressure of 150
volts on the boys but cannot supply even the 10 volts required by the
lamp?
Perhaps we can be brought to appreciate the answer to that question
best by asking ourselves some others quite like it.
Why did not the man who built our mill two generations ago locate it
upon the small stream that flowed near his house? The small stream was
more conveniently located for him and it has quite as much fall as he
got at the foot of this lake. We sometimes express the fact by saying
that the "head of water" or the water pressure was quite as much in one
of these cases as the other.
One boy said that the stream sometimes gives out. Another one said that
it never did have water enough to run that wheel. "Undoubtedly the
trouble is with the quantity," said I, "but I want to show you that we
cannot maintain the pressure unless there is sufficient quantity back
of it."
[Illustration: Fig. 123]
In Fig. 123, suppose _A_ represents a small, slim tank of water three
feet high. The water-wheel _W_, requires one gallon of water a minute
pushed along by a three-foot head of water pressure to run it. The
supply pipe _S_ is bringing into the tank not more than one quart of
water per minute. A gate at _R_ enables us to regulate the flow of
water, as we regulate the flow of electricity, by using more or less
resistance. Now it is evident that if we close the gate, or partially
close it, and allow the tank to fill with water, we may then open the
gate and run the wheel for a short time, but the level of the water in
the tank soon begins to fall and the pressure grows less and the wheel
stops moving. It is just so with all generators of electric current.
If we take from them more than they can supply continuously the
voltage falls. This is notoriously true of dry cells. Like the water
tank represented in Fig. 123, they "run down" if used continuously
to furnish, say, one ampere of current, but they may furnish it for
a short time, the voltage rapidly falling meanwhile. Then if given a
short rest they "pick up" and will again furnish full pressure. The
voltage of a dry cell falls somewhat when it is required to give the
very small amount of current required to actuate a volt meter, say .015
ampere. Hence, our volt meter will not quite correctly show what the
voltage of a single cell would be on open circuit. Notice that, when
I put one cell upon this volt meter the needle shows 1.42 volts; but
when I put four cells in series upon it the needle indicates six volts,
as nearly as we can read it. That is, the voltage of each cell in this
case appears to be 1.5. What has increased the voltage of a cell from
1.42 to 1.50? Simply this: when .015 ampere, the amount required by
the volt meter, was taken from one cell it reduced its pressure, but
when a multiplier with ten times the resistance was added we secured
our reading by using only .006 ampere of current, and this did not
appreciably reduce the true pressure of the cells.
The induced current from our bell when held back by 60,000 ohms
of resistance in the four boys was able to push with 150 volts of
pressure, and .0025 ampere passed without noticeably reducing this
pressure, but when the same current was held back by only 100 ohms in
the filament of the lamp nearly forty times as much current passed, and
the pressure dropped to something less than ten volts.
"We will try an experiment to show how the voltage will suddenly fall
when we reduce the resistance of your four bodies.
[Illustration: Fig. 124]
"Fill these two empty tin pails in which our lunch was brought with
water from the lake and sprinkle in the salt left over from the lunch.
Now twist a bare copper wire around the bail of each pail and connect
these with the bell so as to get the induced current from its magnet.
(See Fig. 124.) Let the two pails of water be the terminals of the
two wires at _S_. Now you four boys wet your hands in the water and
then join hands, and those at the two ends of the line put your free
hands upon the outside of the pails of water while I close the primary
circuit. You of course feel the current just as you did when you held
the spikes in your hands in a former experiment. But now you two end
boys put your free hands into the salt water, and you instantly get a
very smart shock. The resistance is no longer 60,000. It has dropped
way down to 2000, and if the voltage had remained at 150 you would
have received a terrible shock, but the voltage has dropped down to
five. It is as though you had been pushing very hard against a post and
it suddenly gave way. You cannot push against a thing which offers
no resistance. So the voltage falls when resistance is reduced, and
particularly if the source of supply has very little capacity. Here
is another experiment you must try when you go back to the city. At
a certain water faucet in my laboratory the pressure is disagreeably
high. The water flows with great force and spatters badly. We can
easily reduce the pressure so that the water will flow in a limpid
stream. Fig. 125 shows the situation; _f_ is the faucet, and in the
pipe underneath the sink there is a stop-cock _c_. This may be adjusted
permanently so that the faucet _f_ will act pleasantly. The same
thing is represented again at the gas stove. Let _f_ in the Fig. 125
represent a gas cock at the stove. Suppose the pressure is so high that
the gas flames pass more gas than is readily consumed. It is possible
to adjust a stop-cock like c further back in the pipe so as to produce
hotter flames, get rid of the poisonous fumes of half burned gas, and
cut down the monthly gas bills one half.
[Illustration: Fig. 125]
"My garden hose will usually throw a stream across the street, which
is very desirable when one wishes to sprinkle the street, but this
pressure is disastrous when I wish to sprinkle the flowers. Turning
down the stop-cock at the nozzle makes it shoot a smaller stream but
more spiteful in pressure, knocking the flowers to pieces and washing
the soil away from their roots. But if I partially close the stop-cock
at the side of the house where the hose is attached I may have the
stream of water flow as gently as I choose.
"I should meet precisely the same situation if I tried to ring an
ordinary electric bell by a 110-volt current, and I should use the same
method of overcoming the difficulty.
"The great virtue of the dynamo is that it can furnish a large supply
so that the voltage is kept constant on a great flow of current.
[Illustration: Fig. 126]
"I have not forgotten the question, but have tried to work toward its
answer all this time. The question is, why did Ernest get a shock this
morning when he touched only one binding post, and when the battery
of five cells is not capable of giving shocks to any one who touches
its binding posts directly? We need one more diagram to give the final
answer. In Fig. 126 _e_ represents the binding post from which the
shock was received. _B_ is the battery of five cells, _C_ is the
spark coil, _G_ is the engine cylinder, _f_ is the spark plug. When
one wishes to start the engine he closes the switch _S_. This makes a
continuous conductor from the battery to the metal cylinder itself. As
the engine rolls over it closes the gap in the conductor at _d_ for an
instant. The primary circuit is then completed and the current passes
from _B_ to the cylinder, through the metal of the cylinder to _d_,
then to the coil _C_, where it passes through a portion of the coil and
then back to the battery. The vibrator on the coil causes the magnetic
field to rapidly vary in strength. This induces a secondary current in
the whole coil which, because it passes through a very great number
of turns, has a high voltage. This passes from _C_ through _B_ to the
base of the engine, then up the walls of the cylinder to the plug _f_,
then jumps across the gap at _a_, causing the spark which explodes the
mixture of gasolene and air in the cylinder. The spark plug _f_ is
porcelain--an exceedingly good insulator. Through the centre of this
passes a wire from _a_ to _e_. The current passes up this and back to
_C_. Now the engine rests upon the floor of the boat, and Ernest stood
upon the same floor. The wood of this floor when dry and clean is a
very good insulator, but when wet, and particularly when wet with water
that has ever so slight an amount of any salt in solution, it becomes
a conductor for such high tension currents. When therefore Ernest,
standing upon the floor of the boat, touched the binding post, _e_,
this induced current of high voltage found it about as easy to pass
from the metal of the engine cylinder through the wood to his body and
through his body to _e_ as to jump across the short air gap at _a_.
There are two things upon which he may congratulate himself.
"1. While the coil stepped up the voltage so high it reduced the
available quantity of the current, so that the shock was a safe one.
"2. He received only a portion of the current which passed. The major
part of it passed across the gap at _a_, otherwise we should have
noticed that the engine missed an explosion when he touched the binding
post."
The only part of this electrical outfit from which one may receive
a shock is that line from _e_ to _C_. The greatest difference in
electric pressure is always to be found between the two extremities
of the electric generator; as, for example, between the carbon end
and the zinc end of the battery, the positive and negative poles of
the dynamos; the right-hand and left-hand end of this coil. Since the
right-hand end is connected by good conductors with the metal of the
engine and with the floor of the boat and through it with our bodies,
we are in the same electrical condition as the right end of the coil;
but the left-hand end and the wire connecting it with _e_ are forced
by the varying magnetic field into a very different state of electric
tension, and it is insulated from the engine and from us by the
porcelain spark plug. We say that the "difference in potential" between
the two sides of this system is 5000 to 10,000 volts.
The water in this lake flows through the stream at the other end of the
lake to the ocean. The water of the ocean evaporates to form clouds.
Clouds drift over the land and drop their rain to replenish the lake.
The difference in water level between this lake and the ocean is twenty
feet. A difference in water level is what makes it a water power and
it is what occasioned the building of our mill. This difference of
water level corresponds in our electric generators to the difference
in potential. The difference in potential maintained by our battery
of five cells when not producing current is 7.5 volts. The difference
in potential between the two ends of our coil, when the battery is
agitating its magnetic field, is perhaps a thousand times as much, or
7500 volts.
The boys took their swim in the lake and afterward, while we were all
on shore lying on the grass, they brought up again the question of the
machine-shop. They were anxious to know if I had any plans in regard to
it. I said I had been thinking about it a good deal over night but had
been waiting to hear their plans. Well, they thought it would be good
to have a turning lathe, but could not think of anything else unless it
might be a grindstone run by power. I said I had thought of a Central
Station Electric Plant. At this they all sat up.
"Hydro-electric stations are the proper thing now," I remarked. "On the
Rio Grande River in Colorado they are constructing several plants where
water power will be utilized to generate electricity for use more than
one hundred and fifty miles away. For transmitting electricity to such
a distance they step up the voltage, or electro-motive force as it is
called, to 100,000 volts.
They are harnessing the Au Sable River in Michigan to generate
electricity and transmit it at 135,000 volts e. m. f. to towns nearly
two hundred miles away. Electricians use e. m. f. for electro-motive
force, just as you boys use "exams." as slang for the motive force in
school.
Of course we are aware that since 1896 some of the water power of
Niagara had been converted into electric power to run street cars and
factories and furnish electric light and electric heat as far away as
Buffalo, twenty-six miles distant.
About $18,000,000 are now being invested in hydro-electric enterprises
even in Mexico.
By this time the boys were all standing up and staring at me, while
Harold inquired if I were talking in my sleep. "I have at any rate
succeeded in waking you all up," said I, "and what I have said is not
altogether a joke. Let me explain somewhat at length."
XII
ELECTRICITY FROM CENTRAL STATIONS
Large dynamos generate electricity very much more cheaply than small
machines can, and machines which have a full load continually produce
the current very much more cheaply than those which run upon very light
load part of the time. The largest central stations with load evenly
distributed for the whole day could furnish electricity profitably at
four cents per kilowatt hour. There are many small electric lighting
plants which furnish current from sundown to midnight only at fifteen
cents per kilowatt hour, with little profit. The transformer (Fig.
127) makes it possible to gather all this generation of electricity
for sparsely settled districts into large central stations, located
sometimes far away from the consumer perhaps, where there is abundant
power in some water-fall, thus saving the expense of coal for running
the dynamos.
[Illustration: Photograph by Helen W. Cooke. Operating the Switchboard]
A few years ago there were no central stations for this purpose. Now
according to the latest census reports there are in the United States
about 30,000 plants, including those which belong to certain cities,
that generate electricity for sale, and there are twice as many more
isolated plants to furnish light and power in factories, hotels, etc.
[Illustration: Fig. 127]
The money invested in central station business now exceeds six billion
dollars, and the annual output of electric current is sufficient to
keep eight billion 16-candle-power carbon filament electric lights
burning continuously night and day. All this has more than doubled in
the last five years. Central stations are now furnishing about five
times as much current for heating, cooking, and charging automobiles as
they did five years ago. About one third of all the central stations
depend on water power.
[Illustration: Fig. 128]
[Illustration: Fig. 129]
We might take as the type of hydro-electric central station, that
is, one which generates electricity by water-power, the Glenwood
Station of the Central Colorado Power Company. This station has
two 9000 horse-power water turbines. Each water-wheel drives an
alternating-current generator which develops 4000 volts of e. m. f.
These water wheels and generators are shown in Fig. 129. The penstocks
are to be seen coming through the back wall of the building. They bring
water at 170 foot head, or about seventy-five pounds per square inch
static (standing) pressure. Three huge transformers, each weighing
twenty-six tons, step up the e. m. f. from 4000 to 100,000 volts.
These are the cylinders shown in Fig. 130. They simply contain a great
many coils of copper wire with a vast amount of iron at the centre.
They accomplish in a large way what our spark coil does in a lesser
degree. But why go to all this expense to produce such a dangerous
and troublesome voltage? The answer briefly is, that while it is
dangerous and troublesome the expense is not so great as it would be
to supply by any other method the electric current required. Denver
and numerous other places, large and small, require electric current.
From one to two hundred miles away on the Grande River, there is
vast power running to waste. We have to choose on the one hand between
buying power in the shape of coal and distributing power plants to
those various localities where electricity is needed, and on the other
using this water-power, which is now running to waste, to generate
electricity which we may transmit and distribute throughout the one
hundred and eighty-five miles to Denver, Leadville, Boulder, Dillon,
Idaho Springs, etc. But electric energy transmitted a long distance
suffers great loss.
[Illustration: Fig. 130]
Suppose, for instance, I needed to supply fifty amperes at one
hundred-volt pressure ten miles distant from the generator, and had
a conductor the size of a trolley wire to bring the current. The
resistance of the trolley wire is one ohm for every two miles, or
five ohms. The drop in voltage is found by multiplying the amperes
of current by the ohms of resistance. Ten miles from the central
station, therefore, the drop on fifty amperes would be 50 × 5 = 250
volts. It would, therefore, be necessary to maintain a pressure of 350
volts at the generator to deliver the fifty amperes at 100 volts. The
energy supplied by the generator is 350 volts × 50 amperes = 17,500
watts = 17.5 K. W. The energy delivered to the consumer is 100 volts
× 50 amperes = 5000 watts = 5 K. W. In order to deliver fifty cents'
worth of electricity per hour to the consumer it would, in this case,
be necessary to generate $1.75 worth of electricity at the central
station. That is, about seventy per cent. of the energy generated would
be wasted in transmission. If now we decide to generate this electrical
energy at ten times as high voltage it will be necessary to transmit
only one tenth as many amperes, or five. In this case the drop in
voltage would be 5 amperes × 5 ohms = 25 volts. It would be necessary
to maintain 1025 volts of pressure at the generator to deliver to
the consumer the five amperes at 1000 volts = 5000 watts. That is, to
deliver 5000 watts in this case we must generate 1025 volts × 5 amperes
= 5125 watts, and less than 2-1/2 per cent. of the energy generated
would be lost in transmission.
If now the consumer must have his energy delivered at 100 volts, we
must introduce a step-down transformer at his end of the line which may
give him 50 amperes at 100 volts = 5000 watts. This transformer, being
small, will cause a loss of 15 or 20 per cent., but if there were a
very large amount to transform it might be done with a loss of only 4
per cent.
[Illustration: Fig. 131]
[Illustration: Fig. 132]