THE LIBRARY OF WORK AND PLAY
CARPENTRY AND WOODWORK
By Edwin W. Foster
ELECTRICITY AND ITS EVERYDAY USES
By John F. Woodhull, Ph.D.
GARDENING AND FARMING
By Ellen Eddy Shaw
HOME DECORATION
By Charles Franklin Warner, Sc.D.
HOUSEKEEPING
By Elizabeth Hale Gilman
MECHANICS, INDOORS AND OUT
By Fred T. Hodgson
NEEDLECRAFT
By Effie Archer Archer
OUTDOOR SPORTS, AND GAMES
By Claude H. Miller, Ph.B.
OUTDOOR WORK
By Mary Rogers Miller
WORKING IN METALS
By Charles Conrad Sleffel.
[Illustration: Drawing by J. Hodson Redman. Harold Sending the C. Q. D.
Message (_See page 355_).]
_The Library of Work and Play_
ELECTRICITY AND ITS
EVERYDAY USES
BY JOHN F. WOODHULL, PH.D.
[Illustration]
McGOWEN-MAIER & CO.
CHICAGO, ILL.
ALL RIGHTS RESERVED, INCLUDING THAT OF TRANSLATION
INTO FOREIGN LANGUAGES, INCLUDING THE SCANDINAVIAN
COPYRIGHT, 1911, BY DOUBLEDAY, PAGE & COMPANY
PREFACE
Why do we pursue one method when instructing an individual boy out of
school, and a very different method when teaching a class of boys in
school?
The school method of teaching the dynamo is to begin with the bar
magnet and, through a series of thirty or forty lessons on fundamental
principles, lead up to the dynamo, which is then presented, with
considerable attention to detail, as a composite application of
principles. This might be styled the synthetic method. He who teaches
a boy out of school is pretty likely to reverse this order and pursue
the analytic method. The class in school has very little influence in
determining the order of procedure. The lone pupil with his questions
almost wholly determines the order of procedure. Out of school no
one has the courage to deny information to a hungry boy; in school
we profess to put a ban upon information giving, and we do quite
effectually deaden his sense of hunger. The school method rarely yields
fruit which lasts beyond the examination period; on the other hand, a
considerable number of boys have become electrical experts without the
aid of a school. This book is the story of how my boy and I studied
_electricity_ together. We have had no other method than to attack our
problems directly, and _principles_ have come in only when they were
needed.
My boy had learned to read when very young by having stories read to
him while he watched the printed pages. The construction of sentences
out of words and words out of letters had come to him very incidentally
but all in due time, and when he first went to school rather late in
life for a beginner he found himself more proficient than the other
boys of his own age both in reading and in understanding the printed
pages. I could see no good reason why he should not pursue the same
method in studying electricity.
We live in a modern apartment house in a great city. My boy likes to
visit engine rooms and talk with the engineers about their machinery.
His mother and I always encourage him to talk with us about the things
in which he is most interested. If the family is alone at dinner, he is
quite likely to lead the conversation into the field of electricity.
When particularly burdened with my work I have learned to find relief
by giving an afternoon to Harold, who generally takes me to some
electrical store or power station or to ride by electric train out into
the country.
CONTENTS
CHAPTER PAGE
I. The Dynamo and The Power Station 3
II. Dynamo continued--The Magnet 11
III. The Ammeter 25
IV. The Wattmeter 35
V. The Electric Motor 43
VI. Applications of the Electro-magnet 57
VII. Electric Heating 97
VIII. Applications of Electric Heating 107
IX. Lighting a Summer Camp by Electricity 160
X. How Electricity Feels 168
XI. The Electric Sparking Equipment for a Gasolene
Engine 178
XII. Electricity From Central Stations 204
XIII. Electricity From an Old Mill 218
XIV. Doing Chores by Electricity 240
XV. Electric currents from Chemical Action and Chemical
Action from Electric Currents 248
XVI. Electrocution at Millville 271
XVII. The Telephone 274
XVIII. Electric Bell Outfit for the Cottage 296
XIX. Using Electricity to Aid the Memory 300
XX. The Electric Brick Oven 305
XXI. Electric Waves 309
XXII. Ringing Bells and Lighting Lamps by Electric
Waves 324
XXIII. Telegraphing by Electric Waves 329
XXIV. Halley's Comet and Electric Waves 333
XXV. How the Idea of a Universal Ether Developed 339
XXVI. Electric Currents Cannot Be Confined to Wires 349
XXVII. Wireless Telegraphy In Earnest 355
ILLUSTRATIONS
Harold Sending the C. Q. D. Message _Frontispiece_
FACING PAGE
Testing a Generator 8
Wiring 16
Wattmeter 40
Testing the Telegraphy Outfit 62
Electric Bell 72
Feeling Electricity 174
Operating the Switchboard 204
Induction Coil of a Wireless 330
ELECTRICITY AND ITS EVERYDAY USES
I
THE DYNAMO AND THE POWER STATION
One day Harold expressed a desire to see the dynamos, five miles away,
which furnish the electric light in our apartment. So I told him to
invite his best friend to accompany us and we would go.
When we were some distance from the station the boys noticed the very
tall chimneys and inquired why tall chimneys were needed for dynamos. I
explained that the dynamos were run by steam-engines, and steam-engines
required the burning of coal. "Oh!" said Ernest, Harold's friend, "I
read in the paper that electricity is the rival of steam and is going
to drive out the steam-engine." I suggested that we were about to see
some steam-engines driving electricity out of that power station. But
more seriously, I explained that steam-engines were used for many
years as locomotives to draw the trains on the elevated railroads of
New York City, and when at last they were displaced by electric trains
some people thought that it was a case of electricity driving out
steam, whereas what had really happened was that the steam power for
running those trains had been concentrated at a central station, and
its power was merely transmitted to the trains by means of electricity.
The trains were, therefore, run by steam power quite as much as ever.
In like manner, the surface cars of New York a few years ago were run
by a cable, which was merely a very long belt used to transmit to the
cars the power of steam-engines located at a central station. When they
were changed to electric cars, electricity became the successful rival
of nothing else than a twisted wire cable. The cars still run by steam
power as before, but that power is transmitted by electricity instead
of the discarded cable. Steam has driven out the horse as a power for
drawing street cars, and electricity has enabled us to gather all the
steam engines into central stations, where now they are furnishing the
power for moving surface, elevated, and subway cars for street traffic,
as also trains for suburban travel. Central station steam-engines
are producing a vast amount of power, distributed all over the city
by means of electricity, for doing a great variety of work and for
furnishing electric light and heat, all of which we shall presently
study. "Just before we go into this central station, can you tell me
how the elevator is run in our apartment house?" "It is an electric
elevator," said Harold. "And where does the electricity come from?" I
inquired. "Well, I know that it comes from the street mains, but do
they come from this power station?" "Yes," said I, "and we will now go
in and see the steam-engines which lift you up stairs many times each
day by sending electricity to run that elevator. If you choose to do
so, you may claim for purposes of discussion that your elevator is run
by steam."
As we entered the building we came first to the dynamo room and both
boys noticed that the tone which met their ears was that which I had
produced for them in the telephone the night before. "I shall try to
show you before we get through," I said, "that these dynamos are doing
something which makes iron pulsate sixty times a second and that that
is the cause of the pitch of this tone. But let us begin with the coal
which is the source of all this power.
"This particular station at the present time is burning forty tons of
coal an hour. That is as much as Mr. ---- uses to heat his twelve-room
house for a whole year. One pound of coal is capable of liberating
enough energy to supply 5-3/4 horse-power for an hour. (Written for
short 5-3/4 H.P.H.) One ton of coal is capable of furnishing (2,000 ×
5-3/4) 11,500 H.P.H. Forty tons would yield 460,000 H.P.H. But the best
furnaces, boilers, and steam-engines are terribly wasteful of energy.
About nine tenths of all this energy is wasted and only one tenth, or
about 46,000 horse-power per hour, is delivered by the steam-engines to
the dynamos.
"Coal is already scarce in the world and the supply is rapidly being
exhausted. Meanwhile we are growing more dependent upon coal. A century
ago we used scarcely any power except that of men, horses, and oxen,
and what little heat men then used came chiefly from wood. They lived
in cold houses, attended cold churches and schools, did not ride in
steam or electric cars, and did not have power plants. Our wood is
nearly all gone, our coal is going, and we are very rapidly growing
more dependent upon heat and power, our chief source of which is coal.
Wind power is too uncertain to depend upon, and we turned our backs
upon water-power when we began to crowd into cities. What little
water-power there is, however, is nearly all in use.
"There is great need both that we learn how to save the major part
of the energy of the coal which we now waste, and that we find a
substitute for the coal to use when that is gone.
"A part of the heat from the forty tons of coal which is being burned
in this particular power plant goes into the water in the boilers. It
converts this water into steam. The steam, if free to expand into the
air, would occupy about one thousand seven hundred times the volume
of the water. We compel it to expand through the cylinders of the
steam-engine, using its force of expansion to make wheels go around--to
make the dynamo revolve. These dynamos are not _devices for producing
power but merely for transmitting_ the power of these steam-engines
to far away places where it may be used, as, for instance, in our
apartment house, where we are unwilling to walk upstairs and want some
power to carry us.
"Our own apartment is fifty feet above the street. I weigh one hundred
and sixty-five pounds. If I walk up stairs from the street to our
apartment in one minute, which is the rate of a rather slow elevator,
I work at the rate of one quarter of a horse-power. One hundred and
sixty-five pounds raised two hundred feet in one minute requires one
horse-power. You boys each weigh about half as much as I do, and if one
of you walks up the same stairs in one minute you exert half the power
that I do, or if you run up the stairs in half a minute you exert the
same power, that is, one quarter of a horse-power. When we three walk
up together in one minute we exert one half horse-power. If we all
three run up the stairs in half a minute we expend one horse-power.
Now, the speed of elevators for apartment houses is about one hundred
feet a minute. We are unwilling to walk up stairs, not because we are
lazy but because we have the New York haste, and so we employ elevators
which run at the rate of about one hundred feet a minute.
[Illustration: Photograph by Helen W. Cooke. Testing a Generator]
"These dynamos enable us to employ the power of this central station
to run the elevator in our apartment house. Here is a dynamo rolling
over now in the act of sending out power, some of which goes to that
elevator; and standing beside it is another waiting to be used when
necessary. Examining these dynamos, we find that they are composed
of nothing else than iron and copper. About all that we can say of
these mysterious machines is that the moving iron generates the
electricity and the copper leads it away.
[Illustration: Fig. 1]
"Each one of these dynamos has many hundred tons of iron in it. A
huge wheel of iron, thirty-two feet in diameter, one hundred feet in
circumference, portions of which are surrounded by insulated copper
conductors, forms the centre-piece of the machine. This movable part
weighs four hundred tons. Around about this is a fixed ring of iron,
portions of which are surrounded by insulated copper conductors.
Ordinarily the ring which is stationary is called 'the field,' and the
wheel, which rotates, is called 'the armature,' although these terms
are sometimes reversed for certain reasons. The movable part in these
machines rotates about once a second, that is, its circumference moves
a little faster than a mile a minute. The iron moving at this high rate
of speed creates ether streams or electric currents, which are led off
by the copper conductors. The generation of electricity on a large
scale requires large masses of iron and high velocity."
I noticed that the boys stood before this machine in a state of
utter bewilderment, bewildered as a man who is told that what he had
considered north is really south, bewildered as a man who, having
wandered through a maze of city streets, looks up at length and
unexpectedly finds the building he has been seeking towering before
him. The questions they asked were entirely without thought. "What is
inside of it?" "Simply more iron and copper, such as you see on the
surface," I replied. "But what makes it go?" "The steam engines, of
course, four of which you see, are coupled directly to each dynamo."
"But where does it get its electricity?" "Don't forget that you are
looking at _a generator_ of electricity. Big mass of iron--rapid
motion! That is the whole truth. But it cannot satisfy you as an answer
until you have become used to it. We have seen all that we ought to
see here to-day. Let us drop the whole matter now, but return to my
laboratory to-morrow, and I will give you the next step which will help
you."
The boys did no talking upon their return journey. Whether one may say
they were thinking or not I cannot tell, but certainly their ideas were
incubating.
II
THE DYNAMO, CONTINUED--THE MAGNET
When we had gathered at my laboratory the next day I took down a spool
of one pound No. 24 cotton-covered copper wire (Fig. 2 _A_), which had
its centre filled with wire nails. The boys had seen it before and
remembered it. With flexible wires I connected the two ends of the wire
on this spool to a sensitive ammeter, _B_, which had its zero in the
middle of the scale, and I laid down upon the table a bar magnet, _C_.
[Illustration: Fig. 2]
"Here," I said, "is a dynamo complete." The bar magnet furnishes the
'field' and this spool of copper wire, _A_, which I will move back and
forth immediately over the magnet from end to end, is 'the armature.'
_D_ and _e_ are the line wires and the circuit is completed through
the ammeter to show whether we are generating electricity. And now as
I move this armature along the field you see the needle of the ammeter
move to the right from zero to ten. When the armature is moved in the
opposite direction along the field the needle moves in the opposite
direction past zero and on to ten at the left. The moving of the needle
in the ammeter shows that we are generating electricity. The swinging
to and fro of the needle shows that we are generating an alternating
current of electricity. It is a mere matter of detail whether we move
the armature or the field, as I will show you by letting the spool A
rest quietly upon the table and moving the magnet to and fro lengthwise
across the end of the spool. Or I may accomplish the same results by
moving them both in opposite directions. It is simply necessary that
they move _with reference to each other_. Some dynamos are made with
stationary fields and rotating armatures, some with stationary armature
and rotating fields, and some with both parts designed to rotate in
opposite directions.
"Magnetism is not confined to the magnet. It extends more or less
widely into the region about it. It is this region affected by the
magnet that we designate its magnetic field. By bringing this sensitive
compass needle into the region of this bar magnet from all directions,
I show you that it has a slight power to change the direction of the
needle when about a foot away. This power grows rapidly greater as the
distance grows less. Of course its field extends rather indefinitely,
but we may say that this particular magnet has an appreciable field
extending about one foot in all directions from it. We find upon
examination that some magnets have bigger and stronger fields than
others, that all have their strongest fields when first magnetized
and lose their strength gradually, _but never entirely_. We find that
hardened iron and steel hold magnetism longer than soft iron, _but
all iron is magnetized somewhat at all times_. Iron that is feebly
magnetized can be made into a strong magnet by bringing it into a
strong magnetic field. The earth is a feeble magnet, and that is why
it gives direction to the compass needle. That is also probably the
reason why every piece of iron upon the earth is a magnet, or, to put
the cause back another step, we may say that whatever causes the earth
to be a magnet also causes every piece of iron upon the earth to be
likewise a magnet.
"But thanks to Oersted in Denmark in 1819 and Faraday in England in
1821 and Joseph Henry in Albany, N. Y., in 1827, we have learned to
make exceedingly powerful magnets by sending a current of electricity
in a whirl around the iron. This is the meaning of the coils of copper
wire around iron cores in the dynamo, in electric bells, in telegraph
sounders, in motors, etc., etc. To prevent the electric current from
taking the shortest route, through the iron core or through the
successive layers of copper wire, the iron core and the wire must
be covered with something like wood or paper or cotton or silk or
rubber--such things as electricity does not readily pass through--that
is, insulating material.
"Joseph Henry, while teaching in the Albany Academy, was the first to
make electro-magnets. There was no such thing as wire covered with an
insulating material then in the market, and he wound all his wire
with silk ribbon. But in the year 1834 he made magnets which lifted
thirty-five hundred pounds, to the astonishment of every one. A pair of
such electro-magnets as I have here (Fig. 3), each consisting of one
pound of No. 24 cotton covered copper wire, eight hundred feet long,
wound in one thousand turns about an iron core two inches in diameter,
will lift several hundred pounds: much more than we three can lift, as
I shall now show you."
[Illustration: Fig. 3]
The cores of the two magnets were bolted fast to an iron beam, and a
large bar of iron with a ring in it was laid across the other free ends
of the magnet cores. I made connections with the electric lighting
circuit (that in my laboratory is what is called a direct current),
and sent a current of electricity around the coils. The two boys and
I tugged at the ring in the iron bar to no avail. We were unable to
pull the iron bar away from the magnet. But when I opened the switch
and cut off the electric current, one boy with one finger in the ring
lifted the bar with perfect ease.
"Electro-magnets are now made with a magnetic intensity 90,700
times that of the earth's magnetism. Electro-magnets are used for
hoisting iron castings weighing many tons. Here is a picture of an
electro-magnet lifting a whole wagon load of kegs of nails from the
wagon to the hold of a ship.
"Electro-magnets are our only means of utilizing electricity for
power. It is the pull of electro-magnets that moves the electric car.
Electro-magnets are now used for pulling all the trains out of the
Grand Central Depot in New York City.
"Let us now compare the strength of our electro-magnet with that of the
bar magnet used in our former experiment."
I opened and closed the switch, which sent the electric current through
my magnet coils at frequent intervals, and the two boys, each with a
compass needle, searched the field for magnetic effects. They found
that the magnetic field extended six or eight feet, but this piece of
research was broken up by a new idea which appeared to strike them both
at the same instant, for they shouted both together, "Let's use this
electro-magnet in place of the bar magnet for our dynamo experiment!"
[Illustration: Photograph by Helen W. Cooke. Wiring]
"That is surely the next step in our programme," said I, "but you will
need a steam-engine to move an armature in this magnetic field, will
you not, judging from the struggle we had with that iron bar a few
minutes ago?" The boys looked quite hopeless until I said, "The best
thing about the electro-magnet remains yet to be told. You have perfect
control of its strength by changing the amount of electricity which you
send around the coil.
"By means of an instrument which works like the motorman's controller
on the electric car, I may control the amount of electricity which
flows, just as well as you may control the flow of water by a faucet or
stop-cock. By this means I will control the strength of the magnet so
that you may move the armature in your dynamo experiment.
"In 1821, Faraday, at the Royal Institution, London, learned that he
could produce magnetism by means of the electric current, and, in
1831, he learned that the reverse was also true, namely, that he could
produce electricity from magnetism. This idea coming as the result of
ten years of incessant search made him shout and dance like a child.
You are feeling a little of the pleasure of his discovery."
[Illustration: Fig. 4]
I then fastened one of the coils upon the table underneath a small
bench (Fig. 4) and sent an electric current around it. The other coil,
_B_, connected with the ammeter was pushed back and forth along the
surface of the bench over this coil. The boys found that the more
electric current I sent around the coil _A_, that is, the stronger I
made the magnetic field, the harder it was to move the coil _B_. They
found that the nearer _B_ was to _A_ the harder it was to move it. They
found that the faster they moved _B_ the more electricity was produced.
They tried laying _B_ upon its side upon the bench and thus moving
it. They tried taking _B_ off the bench and moving it on all sides of
_A_. They found it much harder to move in some ways than in others,
but in all cases they found that the harder they had to work the more
electricity was developed, as was shown by the ammeter.
"The dynamo is any machine which will convert mechanical work into
electricity. The magneto is one form of a dynamo which you have used
much at the summer cottage, but have never seen the inside of. Here are
several (see Figs. 5, 6, and 8) which I will let you examine inside and
out, and with these I must leave you to yourselves for a time."
When I returned I asked the boys why these dynamos were called
_magnetos_. "Because they have steel magnets for their fields," they
replied. "There are several magnets bent in the shape of a horseshoe."
"Yes," I said, "in this case the field is made stronger by taking
several magnets. Have you noticed any armature?" "Yes, it is made of
iron with insulated copper wire wound around it."
"Please recall that the amount of energy you expend in going upstairs
depends on two things: (1) your weight and (2) the speed with which you
move. Also recall that the amount of electricity you could generate
with a dynamo depended upon the amount of energy you expended.
Therefore, the strength of the electric current which this machine
may produce depends upon two things: (1) the strength of the magnetic
field against which you must pull and (2) the speed of the motion of
the armature. Evidently this field is made as strong as it is possible
to make it with steel magnets. Now is there any device for giving high
speed to the armature?"
"Yes, indeed," said the boys, "one has a pulley so that it may be
connected by a belt with a gas engine, and the others have each a large
cog-wheel working into a smaller one. We found in one of them that a
single revolution of the crank gave six revolutions to the armature."
I found that the boys had made large-sized drawings of the parts, and
were preparing to report on the magneto as a form of dynamo at the next
meeting of the Science Club, which we had started among the boys in
school.
[Illustration: Fig. 5]
"I will loan you some apparatus so that you may give a very interesting
demonstration on that subject," said I, "only let me show you how to
use it first. Connect the binding posts _D_ and _E_ of this magneto
(Fig. 5) with my ammeter. Turn the crank _very_ slowly and notice that
the needle of the ammeter swings to and fro with each revolution of
the armature. That shows that you have not only a _dynamo_, but an
_alternating current_ dynamo.
[Illustration: Fig. 6]
"Now connect the binding posts _d_ and _e_ of this magneto (Fig. 6)
with a short piece of copper wire. Turn the crank and you notice that
this dynamo rings two electric bells. Turn slowly and you notice
that the alternations of the current are numbered by the strokes on
the bells. The hammer swings to and fro just as the needle of the
ammeter did. Each bell therefore receives one stroke of the hammer for
each revolution of the armature. Now try to turn the crank steadily
at the rate of one revolution per second. The armature is making
six revolutions, or cycles, per second and you now have not only
an alternating current dynamo but a _six-cycle alternating current
dynamo_. The lighting circuit used in our apartment is a _sixty_-cycle
alternating current. To be sure the armature of the dynamo which
generates that current revolves only once a second, but it carries
coils enough upon its rim to make that number of alternations.
"Now connect this telephone receiver with the binding posts _D_ and
_E_ of this magneto (Fig. 7). Unscrew the cap of the receiver. Move to
one side the iron diaphragm and turn slowly the crank of the magneto.
Notice that the diaphragm vibrates in time with the alternations of
the dynamo. Replace the diaphragm, screw on the cap, hold the receiver
to your ear and turn the crank as fast as you can. You will probably
be able to make about sixteen cycles per second. The receiver in that
case is giving forth a sound of the same pitch as a sixteen-foot closed
organ-pipe.
[Illustration: Fig. 7]
"Connect the telephone receiver to the binding posts _D_ and _E_ of
this magneto (Fig. 8), and by means of a belt connect the pulley to
this series of cog-wheels. Now you may turn the crank and readily make
the armature revolve at the rate of sixty cycles per second, and you
notice that you get the same tone that we heard in the dynamo room of
the power station and the same tone the telephone receiver gave when I
connected it to a coil in our apartment. The tone which is produced by
sixty vibrations per second is very nearly that of the _C_ two octaves
below middle _C_ on the piano. Try it along with the piano and you will
find it a little flat. This string on the piano is making sixty-four
_vibrations per second_.
[Illustration: Fig. 8]
[Illustration: Fig. 9]
"Now connect this miniature telephone switchboard lamp with the magneto
(Fig. 9) and turn the crank fast. The lamp lights up to full brilliancy
and you notice that the light is steady, although it is made by an
alternating current passing through the filament in one direction,
stopping entirely, and then passing in the opposite direction. The
filament has no time to cool off, provided you turn fast enough, but
try turning a little slower and you will notice the flickering of the
lamp."
III
THE AMMETER
[Illustration: Fig. 10]
At the last meeting of the Science Club so many questions were asked,
which the demonstrators could not answer, that a programme committee,
to whom such questions might be referred thereafter, was appointed.
It was made the duty of this committee to assign to various members
the task of searching for satisfactory answers, and when the material
was ready to be reported to the club, the programme committee
determined the time and order of presentation. I found that I had been
made an honorary member of this committee and that it was expected
that I should steer the committee. I told them that I accepted this
appointment with the understanding that the fellow who steers is always
the smallest man in the crew, and if they would do all the work I would
enjoy the honorary title of cockswain. Secretly, however, I appreciated
that this was in effect adding several courses to my already rather
heavy programme. I must, under the régime, direct a large number of
inexperienced students in library research, in laboratory research, and
in the art of giving demonstrations with apparatus and experiments to
audiences.
The most urgent questions, as also those which were next in the natural
order, concerned the _ammeter_. I told the committee to make that the
subject of the next meeting and to send to my laboratory on a certain
day the person or persons whom they might appoint to report upon it.
[Illustration: Fig. 11]
I find that the boys never come singly, but generally in pairs. When
the boys came they found lying upon the table an ammeter (Fig. 11).
I told one of them to take out the three screws in the front and remove
the face of the instrument. I had told the boys that the instrument
cost sixty dollars and that letting them open it was like letting
them open my watch. As soon as the face came off one of the boys
exclaimed that from my reference to the watch he had expected to see
very complicated machinery with many wheels, but from the exceeding
simplicity of the mechanism he could not see why it should cost sixty
dollars. I told him that although it was a fine piece of workmanship
it was fortunately very easy to understand, and I asked them if it
reminded them of anything else that they had ever seen. After a few
moments of reflection they agreed that it was very much like one of the
magnetos. "Well," said I, "where is the field?"
[Illustration: Fig. 12]
"Is this horseshoe arrangement a magnet?" they inquired.
"There is a compass needle right at your hand waiting to answer that
question," I replied. They immediately found that it was a magnet.
"Well," I said, "to be really sure that it is a magnet you must find
a portion of it that will _repel_ a portion of your compass needle as
well as other portions in both horseshoe and needles which attract each
other." Whereupon, they found that the portion marked _N_ (Fig. 13)
repelled the blue end of the compass needle and attracted strongly the
bright end of the needle, while the portion marked _S_ did the reverse.
"We will call _N_ and _S_ the poles of the magnet. This is simply a
steel bar magnet bent into the shape of a horseshoe."
[Illustration: Fig. 13]
"You told us," remarked one of the boys, "that steel magnets gradually
lose their strength. How then can this be correct as a measuring
instrument?"
"It is the purpose of the iron case to enable this magnet to retain its
magnetism, and if you will examine its field, as we did that of another
magnet upon a former occasion, you will find that although this is a
strong steel magnet its field does not extend outside of the iron case.
It is as though we could box up magnetism and keep it from escaping.
"Now if this is like the magneto, where is the armature? The spool-like
thing between the poles of the magnet looks just like the armature in
one of the magnetos.
"Yes, it has an iron core with a coil of insulated wire around it, and
you remember that when an electric current is sent around a piece of
iron, that iron is made into a magnet, and if it is a magnet it must
have poles. It is very delicately poised upon a pivot and will act
exactly like your compass needle, which is also a little magnet with
poles. I will send an electric current through the wire which surrounds
this armature, and you notice that the needle which it carries moves
to the right. Notice that the lower end of this armature acts like the
blue end of your compass needle in that it is repelled from the pole
_N_ of the field and is attracted toward _S_ of the field. In like
manner, the upper end or pole of the armature is repelled from _S_
and attracted to _N_ of the field. The blue end of the compass needle
is called its north pole because it points north under the magnetic
influence of the earth, and so we may call the lower end of the
armature its north pole.
"The electric current which I am sending through the armature comes
first through one ordinary 16-candle-power electric lamp which you
see lighted on this 'resistance board,' as it is called, and you
notice that the needle points to .5. This means that half an ampere of
electricity is passing through this lamp. I will now send the current
through a 32-candle-power lamp, and you notice that the needle points
to one, indicating that one ampere is required to light that lamp. But
what prevents the needle from going farther, and what brings it back to
zero each time?" The boys discovered a very small spring, like the hair
spring of a watch, coiled around the pivot of the armature. "So, then,
one ampere of electricity gives magnetism to this armature so that it
may pull against its coiled spring hard enough to carry the needle to
the point one. Twice as much electricity will give it magnetism enough
to carry it to two, and so on across the scale.
"The full name of this instrument is Ampere meter, which by usage
has been shortened to ammeter. It was named in honour of André Marie
Ampère, who was born at Lyons, in France, in 1775, the year our
Revolutionary War broke out. He died in 1836. When Oersted made his
famous discovery of the action of an electric current upon a magnetic
needle, in 1819, Ampère was in middle life (forty-four), and took up
the same line of research with great vigour. The next year, 1820, he
discovered what you will doubtless enjoy rediscovering now.
"You will notice that the binding posts on the bottom of this ammeter
are marked, one positive, +, and the other, negative -. The electric
current now enters the instrument by the post marked + and after
passing around the armature leaves by the post marked -. I will reverse
the connections and thus send the current around the armature in the
other direction, and you notice that its poles are now reversed. The
lower end which was formerly the north pole of the armature has now
become the south pole, as proven by the fact that it is repelled
from the south pole of the field and attracted to its north pole.
This carried the needle to the left, and inasmuch as the zero is in
the middle of the scale we may with this instrument both measure the
amount of current and tell its direction. You will recall that when
we connected the magneto with this instrument, it indicated that
the magneto sent the current first in one direction and then in the
other, which we call an 'alternating current.' But you notice that
the current which I am using in this laboratory flows continuously
in one direction. This is called the 'direct current.' We shall find
out how a dynamo may produce a direct current at another time. Let
us not forget, however, that we have repeated Ampère's discovery, and
found out that the direction in which we send the current around an
electro-magnet determines which end shall be its north and which its
south pole. If you will note carefully which way the wire is wound
around the armature you will see that when I send the current in at
the positive post it is passing around the north pole of the armature
opposite to the direction in which the hands of a clock move. If I
reverse the current it passes around the lower end of the armature
_in the same direction as the hands of a clock move_ and then this
end becomes a south pole. This is 'Ampère's rule,' and it is what
candidates for admission to college are very careful to learn.
"Before we replace the face of this ammeter I must call your attention
to a wire running by a short cut from one binding post to the other,
_s_ (Fig. 14). Suppose _a_ represents the wire around the armature.
Electricity, like water, goes more readily through a big conductor than
a small one and more readily through a short than a long conductor. If
_s_ and _a_ were water pipes, each having a stop-cock, we might easily
adjust the cocks so that one tenth of the water would go through _a_
and nine tenths through _s_. Or, indeed, without stop-cocks, the size
and length of _s_ and _a_ might be so apportioned that one tenth of
the water would flow through _a_ and nine tenths through _s_. This is
precisely the adjustment which has been made with reference to the flow
of electricity through this instrument. _s_ is called a 'shunt.' When
the shunt is out all the current goes through _a_ and when the shunt
is in only one tenth of the current goes through _a_. I have two other
shunts, each of which may be put in the place of _s_. With the second
only one hundredth of the current goes through _a_ and with the third
only one thousandth of the current goes through _a_. Thus I have an
instrument which will measure anything from one thousandth of an ampere
up to ten amperes.
[Illustration: Fig. 14]
"In this laboratory we pay about one cent for an ampere of electricity
for one hour. Twice as much coal must be consumed to furnish two
amperes as one, and twice as much coal must be consumed to furnish
an ampere for two hours as for one hour. Hence we need an instrument
which will keep account of time as well as amount of current. Such an
instrument we must look into next.
"Just before we pass to that, however, let me ask if you have ever
heard of a 'shunt-wound' dynamo. Can you guess from the way we have
just used the word 'shunt' what the expression could mean with
reference to a dynamo?" Without hesitation the boys told me that it
meant that the field and armature were wound parallel to one another,
as shown by diagram in Fig. 15. In which case the electric current
which the machine generates divides, part of it going around the
field and part around the armature. Another type, called series-wound
dynamos, is indicated by diagram in Fig. 16, in which case the electric
current goes through field and armature in succession. Under either of
these circumstances, how can the armature move with reference to the
field? The answer will appear in the next chapter.
[Illustration: Fig. 15]
[Illustration: Fig. 16]
IV
THE WATTMETER
We were able to maintain connections between the binding posts of the
ammeter and the movable armature of flexible wires because the armature
never moves more than one third of a revolution, but we now wish to
examine an instrument in which the armature must not only make a
complete revolution but must continue to revolve in the same direction
indefinitely. How are connections made so that an electric current may
pass from the fixed binding posts to the wire of the moving coil? I
will lift the cover off this instrument, which is called a wattmeter,
and let you find the answer to that question.
I sent through the instrument the current from a 32-candle-power lamp.
According to the ammeter, which was also in circuit, the amount was one
ampere.
The armature of the wattmeter revolved slowly and it was not long
before the boys reported that connections for the current were made
by strips of metal sliding on metal plates. The ends of the armature
wire were fastened one to one plate and the other to the other plate,
and the metal strips brush along over the surfaces of the plates. (That
is why they are called "brushes," I said.) And the brushes slide from
one plate to the other each time the armature makes half a revolution.
(That is, the brushes change the connection and thus change the poles
of the armature at the proper instant so that they are always attracted
to the poles of the field toward which they are moving.) This is called
a commutator.
Notice that while the ammeter was like the magneto in having a steel
magnet for its field, the wattmeter is like the dynamo in having
electro-magnets for both armature and field. Notice in the second place
that this instrument is an _electric motor_ since it is made to revolve
by an electric current. If it were made to revolve by some other power
it would generate electricity and would then be called a dynamo.
Indeed, let me tell you something which must at present be nothing
more than a puzzle to you. _Every machine, while it is being driven
by an electric current as an electric motor, is, at the same time,
acting as a dynamo to generate a current in the opposite direction._
Notice in the third place that this is a shunt-wound instrument. The
current which is sent into the instrument divides, and part of it goes
through the field, while part goes through the armature. Motors, as
well as dynamos, are either shunt-wound or series-wound. But notice
finally that the axle on which the armature is carried has a cyclometer
arrangement which keeps account of the number of revolutions. The
armature is going slowly enough for us to count the revolutions. With
watch in hand we found that it made one hundred and twenty revolutions
per minute. I next brought the current to the wattmeter through a
16-candle-power lamp and the ammeter, connected in series, showed that
half an ampere was passing. We counted the revolutions of the wattmeter
and found them to be sixty per minute.
Here, then, is a simple electric motor which will register the amount
of electricity we use. It will register the same amount whether we use
one ampere for one hour or half an ampere for two hours or two amperes
for half an hour. In any case this product is called _one ampere hour_.
But the words printed upon the dials of this instrument are not _ampere
hours_, but _watt hours_ and the name of the instrument is _wattmeter_.
This next requires explanation. Follow me in a little roundabout
journey and the matter will be readily understood when viewed from
another approach.
[Illustration: Fig. 17]
When we were estimating the energy required to climb the stairs of an
apartment house, we needed to take into account two factors, (1) our
weight and (2) the time which we took in climbing them. The amount of
coal burned, steam generated, electricity produced, to run our elevator
depends upon two factors, (1) its weight and (2) its speed. That idea
is fundamental. Let us get at it in still another way. Suppose we have
a mill pond, (Fig. 17, _A_). We construct a penstock _p_ and install
a water-wheel, _S_, to operate a mill. Our business increases and we
install more machinery in our mill and must have more power to run
it. We have two ways of getting it, (1) we may lengthen our wheel and
enlarge our penstock so that a greater weight of water will fall upon
the wheel, or (2) we may lengthen our penstock and move the wheel
farther down so that the water will fall upon the wheel with greater
velocity. It is just so with the electric current. Like water it is
driven on in its course by pressure. The unit for electric pressure is
called a volt. If we wish to drive the wattmeter or any other electric
motor twice as fast as now, we may choose whether we shall do so by
doubling the volts of pressure or by doubling the amperes of quantity.
The electric pressure on our mains is about one-hundred and ten volts.
We three together weigh 330 pounds. Our elevator brought us up stairs
at the speed of 100 feet per minute. It requires one horse-power to
raise 330 pounds 100 feet in a minute. The ammeter in the engine room
showed that 7 amperes of electricity were sent through the motor of the
elevator to bring us up. That is, seven amperes at 110-volt pressure
give one horse-power. In the office building across the street where
they use a 220-volt current 3-1/2 amperes are required to take us up
stairs at the same speed. It is necessary that the same amount of coal
be consumed to furnish the horse-power of energy whether we supply it
by means of seven amperes at 110 volts or 3-1/2 amperes at 220 volts.
You notice that the product is 770 in each case. The name given to
this product is _watts_. More accurately 746 watts of electrical power
are equivalent to one horse-power. The name of this unit commemorates
the famous inventor of the steam engine, James Watt (1736-1819). His
monument now overlooks the Clyde at his native town, Greenock, Scotland.
To light a certain lamp, to heat a certain laundry iron, to furnish a
certain amount of power for an electric motor, we must have a definite
number of watts. We may choose whether we will have it at high or low
voltage with correspondingly low or high number of amperes.
[Illustration: Fig. 18]
We will now connect with our laboratory current a 32-candle-power
lamp, an ammeter, and a wattmeter, all in series, Fig. 18, and in
parallel with these a volt meter. This last instrument indicates
the electric pressure. Its mechanism will be examined later. The
volt meter indicates 110 volts and the ammeter shows that one ampere
is passing. The filament in the lamp resists the passage of the
current. It gets quite hot and gives forth as much light as thirty-two
candles. Its resistance is just such that 110 volts of pressure send
one ampere through it. We will now take the reading of the wattmeter,
note the time and read it again later. One hour later its index showed
that 110 watt hours of electrical energy had been converted into light
and heat. This at the usual rate, costs 1.1 cents, one cent per hundred
watt hours or ten cents per thousand watt hours, called a kilowatt
hour. The more common 16-candle-power lamp costs about half a cent an
hour to operate. It requires one horse-power to keep fourteen of them
burning.
[Illustration: Photograph by Helen W. Cooke. Wattmeter]
I will now take you to see the wattmeter which measures all the
electric energy used in this building. You note down its reading and
the date and the next time you come we will read it again and thus
find out how much electricity has been used for electric lights, for
electric ventilating fans, for electric elevators, for electric ovens,
and electric irons in the school of household arts, for electric motors
to run lathes and other machines in the school of technical arts, for
electric experiments in my laboratories and lecture room, for electric
vacuum cleaners and, lastly, for pumping the pipe organ in chapel.
I saw by the boys' faces as they departed what would be the next
question that they would bring to me. Knowing, however, that the hour
was up, they were too polite to press it then.
V
THE ELECTRIC MOTOR
In a few days I received a telephone message, asking if I could appoint
an hour to meet the programme committee in my laboratory. I must
confess that my pleasure in these meetings had increased so much that I
was quite ready to slight other duties, if need be, to engage in them.
Moreover, since my business was education it was not difficult for me
to regard these meetings in the light of a duty quite as important as
my regular class instruction--perhaps more effective. At any rate the
boys and I managed to get together. May God forgive the man who essays
to teach boys, but does not love to be with them.
Of course at the last meeting of the Science Club every one wanted
to know how we ran a pipe organ by electricity. Moreover the
Electrical Show was coming on in the city, and cows were to be
milked by electricity, dishes were to be washed by electricity, rugs
and furniture were to be cleaned by electricity, and innumerable
distracting and distressing things were to take place. I told the boys
that really only two kinds of things were to be done by electricity at
the show, and if they would give me two one-hour appointments I would
furnish them with the key to the whole show. We might as well begin
to-day with the pipe organ question.
A pipe organ is operated by air. It has bellows which are simply one
form of an air pump. A boy is often employed to turn a crank which
works the bellows. Down in the basement underneath our pipe organ I
will show you how a half-horse-power electric motor takes the place of
a boy. We found a dark and dirty corner where a boy used to stand and
turn a crank every time æsthetically inclined people enjoyed an organ
recital in the room above. Science, which has not been given credit for
being _humanitarian_, put an electric motor into that dark corner and
sent the boy up stairs to hear the music. The motor _grumbled_ at the
dirt in the corner and compelled the janitor to keep it clean.