What Makes an Electro Magnet Lift Things?
The working parts of an electric lift magnet are as follows:
_A Shell._--This is a steel casting heavily ribbed on the top for
strength, and also to assist in radiating the heating effect from the
coil.
It is usually made circular in shape, the outside rim forming one pole,
while the lug in the center forms the other. The coil fits in between
these poles, thus making a magnet similar to the ordinary horseshoe
type.
_A Bottom Plate._--The under side of the magnet is closed by a very
tough and hard non-magnetic steel plate, in order to protect the coil.
As well as being non-magnetic, this plate also has sufficient strength
to resist the severe wear to which a magnet is necessarily subjected.
_A Terminal Box._--A one-piece heavily-constructed steel casting bolted
to the top of the shell, containing and protecting the brass sockets
into which the wires from the coil terminate, forms the Terminal Box.
The sockets are made to receive plugs placed on the end of the
conductor wire, by which the magnet is connected with the generator.
_A Coil._--This consists of a round insulated wire which is passed,
while being wound, through a cement-like substance, heavily coating
each individual strand.
A low voltage of current is then passed through the coil, a sufficient
length of time, to thoroughly dry out and bake the coating. This
renders the magnet absolutely fireproof, eliminating all danger of
short circuiting of the coil.
When finished it is well taped to protect the outside wire from
becoming chafed.
The coil is made slightly smaller than the inside dimensions of the
shell and the remaining space is filled with an impregnating compound,
which hardens to the consistency of pitch.
This renders the coil thoroughly waterproof; also forms a cushion to
prevent injury from the severe jars and shocks, received when dropping
a magnet on its load.
_A Controller._--The rapidity with which it is necessary to turn
current on and off while operating a magnet, creates what is called a
“back kick.” Unless this is dissipated quickly it is very destructive
to the coil.
A special controller dissipates this back kick through a set of
resistance coils placed in the controller. By means of an automatic
arrangement, connection with these coils is made instantly upon
breaking the current between the magnet and generator.
A system of control used prevents undue heating of the coil. This
enables the magnet to lift as large a load after a long steady run as
at the start.
What Is a Lodestone?
A lodestone is a variety of the mineral named magnetite which is a
natural magnet. The name magnet comes from the name of the mineral
magnetite and this in turn derived its name from the fact that it was
first discovered in Magnesia. The word magnet really means the “Stone
of Magnesia.”
A lodestone is one of the mysteries of nature. Its properties can
more nearly be understood if we examine an artificial magnet, which
is generally made in the form of either a straight bar or a shoe.
An artificial magnet is made of iron. If you drop a bar magnet into
a box of iron filings, the filings attach themselves to the bar. If
you examine it closely you observe that most of the filings attach
themselves to the ends of the bar. Therefore we call the ends of the
bar the poles of the magnet.
If you suspend a magnetic needle at its center of gravity so that it
is absolutely free to turn, you will soon find one end of the needle
pointing north and the other south of course. The end which is pointed
toward the north is called the north pole and the other the south pole.
If you have a horse-shoe magnet, you can demonstrate this for yourself.
Rub the end of your magnet over a sewing needle and oil the needle so
that when you lay it on the surface of a glass of water it will float.
Then look at it closely. You will see the needle slowly turn until
finally it becomes quite still. If you have a compass at hand so that
you know surely which is north and which is south, you will find one
end of the needle pointing north and the other south. You can then
place the end of your magnet against the outside of the glass and draw
the needle toward your magnet. Your horse-shoe magnet has its north and
south poles close together.
If you have a bar magnet and the end of the needle with the eye in it
is pointing north, you can drive the needle on the surface of the water
away from you by touching the outside of the glass opposite that end of
the needle with the north pole of your magnet. On the other hand, if
you reverse the experiment and place the south pole of your magnet to
the side of the glass, the needle will come toward the magnet. In other
words then the like poles of a magnet repel each other and the unlike
poles attract each other.
Another interesting way to show this is to take two lodestones or two
magnets and let a lot of iron filings attach themselves to the ends
of them. Then when you have done this, point the two north poles of
the magnets or lodestones at each other close together. You will be
intensely interested in seeing how quickly the mysterious something
that is in the magnets makes the filings on the two ends of the magnet
try to get away from each other. On the other hand when you put a north
and south pole together, they form a union of the iron filings.
Another strange thing about a magnet is that if you break it in two,
each half will be a complete magnet in itself with a north and south
pole also, and this is true no matter how many times you break it
into pieces. From this we learn that each tiny particle or molecule
throughout the bar is a magnet by itself.
[Illustration: WHAT A LODESTONE IS
This is a picture of a complete electro magnet. The magnet is attached
to the arm of a crane by the loop in the center and when the magnet
then comes in contact with any kind of iron or steel it lifts it as
soon as the current is turned on. By making the electric current
stronger, greater weight can be lifted. Many tons of material can be
lifted at one time. An electro magnet will do the work of many men at
much less cost.]
[Illustration: In this picture we see the magnet lifting a great weight
of miscellaneous pieces of scrap iron. As many as twenty tons can be
lifted and transferred from one place to another at one time.]
Some things can be magnetized while others cannot. Many substances have
not the property of magnetizing other substances when they have once
been attracted by a magnet. These are called magnetic substances. They
remain magnetized only as long as they are in touch with the magnet;
other substances when once magnetized become permanent magnets. Steel
and lodestone have this faculty. A compass needle is an artificial
magnet which becomes a permanent magnet when rubbed with a magnet.
What Is Electricity?
If you pass a hard rubber comb through your hair, in frosty weather, a
crackling sound is produced, and the individual hairs show a tendency
to stick to the comb. After being drawn through your hair a few times,
you may notice that the comb has become charged with electricity. This
electricity is produced by friction. Not only rubber but many other
substances become electrified by friction, such as a bar of sealing
wax rubbed with flannel, or a glass rod rubbed with silk, will show
the same qualities, and these simple experiments teach us many of the
fundamental facts about electricity.
Some simple experiments will be found instructive and interesting. Rub
with flannel a stick of sealing wax until it is electrified and then
bring it close to a pith ball which should be hung by a silk thread.
The pith ball will at once be attracted to the sealing wax, and, if
brought quite close, the ball will adhere to the wax for a few moments,
and then fly away from it. The ball will now be repelled by the sealing
wax instead of being drawn toward it. Now take a glass rod, rub it with
a silk cloth after drying it thoroughly. When the pith ball is brought
close to the glass rod it also will at first be attracted toward the
glass and, if brought in contact with the glass, the pith ball will
adhere as before. It will also then fly away in the same way it did
from the sealing wax. Repeat these experiments with the sealing wax now
and you will find the ball will be attached, as it was at first, but if
it touches the wax it will again adhere for a moment and then fly away.
By using the sealing wax and glass rod alternately and bringing them
into contact with the pith ball, you discover that when it is attracted
by one, it is repelled by the other, and that, after it has been in
contact with either for a few moments it is no longer attracted by it.
We learn thus that the electricity in the glass and the sealing wax
are not the same. To distinguish the two kinds of attraction, we say
the glass is charged with positive, or vitreous electricity, while the
charge on the sealing wax is called negative, or resinous electricity.
When the pith ball was touched with the sealing wax, it became filled
with negative electricity, and was then no longer attracted by the
wax, but was repelled by it and attracted by the glass rod; but when
the ball had been filled with positive electricity, it was repelled by
the glass and attracted by the wax. We conclude from these facts that
bodies filled with the same kind of electricity repel each other, while
bodies filled with opposite kinds of electricity attract each other.
When two substances are charged, as we say, with electricity of
opposite kinds and are brought into contact, and left so for some time,
the two charges disappear, one appearing to neutralize the other. From
this, we conclude, and rightly, that any substance not electrified,
contains equal amounts both positive and negative electricity. When,
therefore, we rub a piece of glass with silk, we are not creating
electricity, but only separating the different kinds. The positive
electricity adheres to the glass, and the negative remains behind,
on the silk. In the same manner, when we electrify sealing wax with
flannel the negative kind remains in the sealing wax and the flannel
becomes charged with the positive. Whenever a body is electrified by
friction, both kinds of electricity are produced; it is impossible to
produce one kind without the other.
[Illustration: WHAT ELECTRICITY IS
Magnets are particularly valuable in lifting raw material in a steel
mill. The red-hot pig-iron, from which steel is made, can be handled
easily in this way, whereas it would be impossible to handle same by
hand. Sometimes great quantities of iron are broken up by the magnet. A
weight of many tons is lifted by the magnet and allowed to fall on the
material to be broken up. The weight falls as soon as the current is
turned off.]
[Illustration:
Weight of wheel, 8160 lbs.
Pieces of machinery which cannot be lifted by men on account of their
great weight and shape are handled easily.]
You must rub the entire glass rod or bar of sealing wax to electrify
the whole of it. If only a part of the glass rod or sealing wax is
rubbed, only that part becomes electrified, as may be shown by trying
to attract a pith ball with the part that has not been rubbed.
~WHAT GOOD AND BAD CONDUCTORS OF ELECTRICITY ARE~
If, however, the charged part of the sealing wax is brought into
contact with a metal rod resting on, say, a drinking glass, the rod
becomes charged, not only where it is brought into contact, but all
over its surface. Substances over which electricity flows readily
are called conductors of electricity. All metals are of this kind.
Things like glass and sealing wax over which electricity does not flow
readily, are called non-conductors, or insulators. Water, the human
body, and the earth are good conductors and rubber, porcelain, most
resins, and dry air are non-conductors.
You have already learned that substances charged with opposite kinds of
electricity attract each other, and substances charged with the same
kind repel each other. We will try to discover why substances charged
with either kind of electricity attract small light objects, such as
pith balls, when these latter are not charged with electricity. As we
have discovered, all substances which have remained undisturbed have
both kinds of electricity present in them, in equal amounts. Now, when
an uncharged body is brought near a charged body, the two kinds of
electricity in the uncharged body have a tendency to separate. The kind
opposite in character, to that on the charged body, is attracted toward
the charged body, and the other kind is repelled. Thus, if our bar of
sealing wax, charged with, let us say, negative electricity, is brought
near a pith ball, the positive electricity in the ball is attracted
to the side nearest the scaling wax, and the negative electricity
is repelled to the farther side. As the positive electricity on the
pith is nearer to the scaling wax than the negative, its attraction
for the negative charge, on the sealing wax, is stronger than the
repulsion between the negative electricities of the two objects, and
consequently, the ball is attracted to the sealing wax. If the charged
sealing wax is brought near a good conductor, which is supported on
some non-conducting substance, such as glass, silk, or rubber, over
which electricity will not flow, a much more complete separation of the
two kinds of electricity occurs on the conductor than on the pith ball.
If the charged sealing wax is brought near one end of a metal rod so
placed, the charge of negative electricity upon the sealing wax will
attract the positive electricity on the metal, to that end, and will
repel the negative electricity to the other end. When a pith ball, hung
by the silk thread, is brought close to either end of the metal rod,
when the charged sealing wax is near the other end, the pith ball will
be attracted toward the rod; but will not be attracted if placed close
to the middle of the rod. This proves that the metal rod is electrified
only in the parts nearest to and farthest away from the charged body.
The two kinds of electricity neutralize each other at the parts in
between.
If now we take two conductors and place them end to end, we have
for all practical purposes, a single conductor. It has the decided
advantage, however, of being easily separated into two parts. When an
electrified substance is brought close to one end of such a conductor,
a charge of one kind is attracted to the near portion of the conductor,
and a charge of the opposite kind is repelled to the farther part. By
separating the two parts of the conductor, we learn that one of the
ends, which have been in contact, is charged with positive and the
other with negative electricity.
This act of separating the two kinds of electricity upon a conductor by
means of a charge upon another body which is not permitted to come into
contact with the conductor, is called induction, and two charges of
electricity produced in this way are known as induced charges.
There are other ways in which a charge of electricity may be induced
upon a conductor. One end of the conductor may be connected with the
earth by means of some good conducting material, and the charged
substance brought close to the other end. A charge, opposite in
character to the initial charge, is attracted to the end of the
conductor that is near the charged body, and the electricity of the
opposite kind is repelled, through the conductor to the earth. By
securing the connection with the earth, while the charged body is
near the conductor, a charge is obtained upon the conductor, that is
opposite in character to the initial charge. This method of charging
conductors, by induction, is practically the same as the one first
described, for the earth is a conductor of electricity, and corresponds
to the more distant part of the two-piece conductor.
An instrument, known as the electrophorus, is especially designed for
the production of electric charges by induction in the manner just
described. This instrument consists of a brass plate, on an insulating
handle of glass, and a disk of sealing wax, fitted into a brass dish,
whose edges rise somewhat higher than the surface of the wax. In using
the electrophorus the brass dish, or sole, is placed upon some support
that will conduct electricity, and the sealing wax disk is then rubbed
vigorously with a piece of flannel, or catskin, which electrifies the
sealing wax, with negative electricity. The brass plate is then taken
by the glass handle and brought close to the charged sealing wax. The
charge of negative electricity on the wax attracts a charge of positive
electricity to the under surface of the plate and repels a negative
charge to its upper surface. If the charged plate is now brought into
contact with the edge of the brass dish the negative charge, on the
back of the plate, flows away, through the legs of the dish, to the
earth, but the positive charge remains on the under surface, where
it is bound, by the attraction of the negative charge on the disk of
sealing wax. If the brass plate is now removed, it will be found to be
charged with positive electricity.
The negative charge upon the sealing wax is not reduced or diminished
by its action in charging the brass plate, and it is possible to charge
the plate an indefinite number of times by means of one charge on the
sealing wax.
The charges of electricity, produced in any of the ways that have been
described, are necessarily small, and the disturbance produced, when
they are destroyed by bringing oppositely charged conductors together,
is very slight, merely a little snapping noise and, perhaps, a small
spark, that seems to leap from the positively charged conductor to
the negatively charged one, when they come very close together. By
the use of electrical machines of various kinds, in some of which
the electricity is produced by friction, and in others by induction,
conductors may be charged with much larger quantities of electricity,
and the disturbance produced by their discharge is greatly increased.
The noise produced is louder and the spark much brighter, and leaps
from one conductor to the other, while they are much farther apart.
It is possible to produce still larger charges of electricity upon
conductors if they are arranged so as to form what are called
condensers.
What Is a Leyden Jar?
One of the commonest forms of condenser is the Leyden jar, which is so
named because it was invented at Leyden, in Holland. This is a glass
jar, upon the outside of which is fastened a coating of tinfoil, that
covers the bottom of the jar and extends two-thirds of the way up
the sides. Inside the jar there is a similar coating of tinfoil, and
through the top of the jar, which is usually made of wood, extends a
metal rod. On the upper end of the rod, there is a metal ball, and, at
the lower end, is attached a chain which runs down to the bottom of the
jar and rests upon the inner tinfoil coating.
In using the Leyden jar, the ball on the metal rod that runs through
the top of the jar is connected with an electrical machine, and the jar
is supported upon some conducting material, through which electricity
may be conveyed from the outer coating of tinfoil to the earth. If the
inner coating of tinfoil is now charged with positive electricity, by
means of the electrical machine, it induces, upon the outer coating
of foil, a charge of negative electricity, which is bound by the
attraction of the positive charge on the inside of the jar. At the
same time, the positive electricity, on the outer coating of foil, is
repelled, through the conducting support, to the earth.
The charge that can be communicated to the coating of the foil, inside
the Leyden jar, is greatly increased by the presence of a charge of the
opposite kind of electricity, on the coating on the outside of the jar.
Each of these charges attracts the other, through the glass of the jar,
and serves to bind or hold it. If either coating of foil is removed,
the charge on the other coating tends to fly off the tinfoil, and will
immediately do so, if a conductor is brought near. It is because the
negative effects of the initial charge, inside the jar, and of the
induced charge outside the jar, make it possible to communicate, to
each coating of foil, a larger charge than it could otherwise be made
to receive, that a Leyden jar is called a condenser.
When a Leyden jar is disconnected from the electrical machine, two
opposite charges of electricity are present on it, one inside and the
other on the outside. If the two coats of tinfoil are now connected, by
means of a condenser, they will at once neutralize each other, and the
jar will be discharged. A jar may be discharged, by simply taking hold
of the tinfoil on the outside of the jar, with one hand, and touching
the metal rod, running through the top of the jar, with the other.
If you do this, there will be a sudden flow of electricity through
your body, your muscles will give a sudden jerk, and you will feel a
peculiar tingling sensation. In other words, you will have received a
shock.
It is not necessary, for the hand that does not grasp the jar, actually
to touch the rod that runs through the top. If the hand is brought
toward the rod, rather slowly, you will see a spark leap across the
space between the rod and your hand, while your hand is still some
distance from the rod. The greater the distance, across which the spark
leaps, the brighter will be the spark, and the stronger the shock
produced. This distance is sometimes spoken of as the length of the
spark, and it indicates the size of the charges on the tinfoil coatings
of the jar.
Who Discovered Electricity?
It may seem difficult to believe, that the tiny spark and weak snapping
noise that are produced when a Leyden jar is discharged, are, in many
respects, the same as lightning and thunder, but it is nevertheless
true. This was proved by Benjamin Franklin, about the middle of the
18th century, in the following way. One afternoon, when a thunder
shower was approaching, he sent up a kite, to the string of which he
fastened a large metal key; and to the key, a ribbon of non-conducting
silk, which he held in his hand. When the rain had been falling long
enough to wet the string thoroughly, it become a good conductor of
electricity, and Franklin found that the key had become charged with
electricity transmitted from the clouds, along the wet kite string.
The non-conducting silk ribbon, that formed the continuation of the
kite string, from the key to his hand, was employed to prevent him from
receiving shocks from the passage of the electricity, through his body,
to the earth.
Up to this point, your attention has been directed in charges of
electricity. You have been told how they may be produced, what some of
their leading properties are, and what effects they produce, when they
are discharged. The subject that will now be explained to you is that
of electric currents.
What Is an Electric Current?
By an electric current, is meant a flow of electricity along a
conductor. The flow of electricity, through your body, when you receive
an electric shock, is a current, but it lasts only for an instant, and
it is difficult to learn much about its nature. By the use of various
devices, it is possible to produce currents, that will continue as long
as we want them, so that we are enabled to study their properties quite
thoroughly.
One of the oldest and simplest forms of apparatus, for producing
electric currents, is that which is known as the voltaic cell. This
form of apparatus may very easily be constructed. Pour some water into
a glass jar, and add a little sulphuric acid. Now place in the water a
strip of clean zinc and one of clean copper. Do not let the strips of
metal touch in the water, but connect them outside the water by means
of a piece of wire. When this has been done, a current of electricity
will be sent up along the wire and through the water between the two
strips of zinc and copper. This current is said to flow along the wire
from the copper, which is called the positive pole of the cell, to the
zinc, which is called the negative pole. In the liquid in the cell
(i.e., the jar), the current travels from the zinc to the copper, thus
completing what is called the electric circuit. Whenever the circuit it
broken, that is, whenever there is a gap made in the wire connecting
the poles, or anything else is done to destroy the completeness
of the path, along which the current travels, the current ceases;
consequently, when it is desirable to stop the current, all that is
necessary is to cut the wire connecting the two strips of copper and
zinc.
The production of a current of electricity, by means of an apparatus of
this sort, depends upon the chemical action of the acid in the water
upon the strip of zinc. As long as the acid continues to act upon
the zinc, the current is produced, and when the acid ceases to act
upon the zinc, the current ceases to flow. If the zinc is clean, the
chemical action of the acid ceases, whenever the circuit is broken, and
consequently, when the cell is not being used to produce a current,
the zinc is not destroyed by the acid. But if the zinc is not clean,
small electric currents are set up, within the liquid, between the
zinc and the impurities on its surface, and around the points where
these impurities lie the acid acts upon the zinc and dissolves it. This
action of the acid upon the zinc, when the circuit is broken, is known
as local action, and it is very desirable to prevent it, as far as
possible. For this purpose the zinc is often rubbed with mercury, which
soaks into the zinc and forms a film on its surface, upon which the
impurities float. This treatment of the zinc is known as amalgamation,
and it serves to prevent almost all the local action, due to impurities
of the zinc.
Many other substances, besides zinc and copper, have been found capable
of yielding an electric current, when placed in a suitable liquid, and
many other fluids, besides water that contains a little sulphuric acid,
have been employed to act upon the zinc and copper, or the substances
used in their stead. Numerous cells of different kinds have, therefore,
been devised, but, in all of them, the current is produced by chemical
action. Most of them contain a liquid of some sort, which is called the
exciting fluid, and two solid substances, which are called the elements
of the cell. One of these elements is always much more susceptible to
the chemical action of the exciting fluid, than the other, and this one
is known as the positive element. The other element, upon which the
exciting fluid may have no action, is called the negative element. In
cells in which the elements are zinc and copper, the zinc is always the
positive element. This may seem strange to you, for you have already
learned that the zinc is the negative pole of the cell, but, to avoid
confusion, you must fix well in your mind the fact that the zinc is
not the positive element of a voltaic cell, but its negative pole,
and that the copper, which forms the negative element is the positive
pole of the cell. The currents produced by the various forms of voltaic
cells, vary considerably in strength, but none of them are very strong.
In order to obtain a stronger current, a number of cells must be used
together. Such a collection of cells forms a voltaic battery, and in
some instances, as many as fifty thousand cells have been used in a
single battery.
We have already learned in our study of water that it may be separated
into its elementary gases by sending an electric current through it.
The effect is a chemical one. Water, however, is not the only substance
that is decomposed by electricity; almost all chemical compounds may be
decomposed by the passage of a current through them, provided a current
of sufficient strength is used.
Another effect of the current is its heating effect. It has been found
that the passage of an electric current, through any body, is always
productive of a certain amount of heat. The amount of heat produced
depends upon the strength of the current of electricity, and the
resistance to its passage that is offered by the body through which it
travels. This amount is increased by increasing either the strength of
the current or the resistance of the conductor along which it travels.
We have already learned, that some substances allow electricity to
pass over them very readily, and are therefore called conductors,
while substances through which electricity does not flow readily are
known as non-conductors. No substance is a perfect non-conductor, for
electricity can be made to pass through any substance, if the current
is sufficiently powerful. Neither is any substance a perfect conductor,
for all substances offer some resistance to the passage of an electric
current. Those substances that are ordinarily considered good
conductors offer varying degrees of resistance to electric currents.
For example, a copper wire offers less resistance than an iron wire of
the same length and diameter.
The resistance of a body depends not only upon its material, but also
upon its length and size. In conductors of the same material, the
resistance is directly proportional to the length of the conductor,
and inversely proportional to the square of its diameter. This is not
surprising, for an electric current bears a strong resemblance to a
current of water, in many of its properties, and you know that it is
harder to force water through long, narrow pipes, than through short,
wide ones.
From what has been stated about resistance, you may see, that a current
will produce more heat, in passing through a long fine wire, than
through a shorter and thicker one, and that, of two conductors of the
same length and size, but of different material, one may be heated much
more by a current than will another.
~HOW MAGNETS ARE MADE~
A third effect of the electric current, which has not previously been
mentioned is its magnetizing effect. It is upon this, that some of the
most important effects of electricity depend.
By coiling a wire around a bar of iron or steel, and then sending an
electric current through it, the piece of iron, or steel, is made to
show magnetic properties. By this is meant, as you doubtless know, that
the iron will now attract other pieces of iron, or steel, to it. The
strength of this attraction depends upon the strength of the current,
and upon the number of turns of wire around the bar. By increasing
either the strength of the current, or the number of turns in the
coil of wire, around the bar of iron, the strength of its magnetic
attraction is increased. When the current is stopped, the magnetic
properties of the iron disappear almost completely. A magnet, that
depends upon a current of electricity for its magnetic power, is called
an electro-magnet.
Besides electro-magnets there are others, which are called permanent
magnets. Electro-magnets are composed of soft iron, the softer the
better, and, as soon as the current of electricity ceases to flow
around them, their magnetic properties disappear. Permanent magnets,
on the contrary, are made of steel, and their magnetism is independent
of the action of a current of electricity. No coil of wire is wound
around them, and no current is employed to maintain their magnetic
properties. A piece of steel may be made to become a permanent magnet,
by passing a current of electricity, for a considerable time, through
a coil of wire wound around it, or by allowing a piece of steel to
remain for some time in contact with a strong magnet. When a current of
electricity passes through a coil of wire, wound around a bar of steel,
it takes longer to magnetize the steel than it would to magnetize iron,
but, when the current ceases, the magnetism does not all disappear from
the steel. A portion of it remains, and the steel becomes permanently
magnetic.
If a thin bar of steel is magnetized, and is then suspended by its
middle, so that it can spring freely, it will be found that one end
tends to point toward the north, and the other toward the south.
Whenever the bar is swung out of this position, it swings back to it,
and if the north end is turned entirely around to the south, it does
not remain, but swings back to its former position. This shows that
there is a difference in the magnetism at the two ends of the magnet.
To indicate this difference, the north-seeking end of a magnet is
called the positive pole of the magnet, and the south-seeking end is
known as the negative pole.
By suspending two bar magnets, in the manner described, it can be shown
that the positive and negative poles of the magnets act like positive
and negative charges of electricity. Poles of the same kind repel, and
poles of opposite kinds attract, each other.
Permanent magnets are usually made in two forms, either straight
or horseshoe shaped. A compass needle, as has been shown, is an
example of a straight magnet. The horseshoe variety, which has a
little bar of iron, called the keeper, laid across the poles is a
common toy. Electro-magnets are seldom seen, except in electrical
instruments or machinery. The pictures shown on the following pages
give us a bird’s-eye view of some of the wonders performed by these
electro-magnets. Tons and tons of material are picked up and held
securely by one of these magnets as easily as you can hold on to an
apple.
Why Does a Bee Have a Sting?
The bee’s sting is given him as a weapon of defence. Primarily it is
for the sole purpose of enabling him to help defend the hive from his
enemies. Sometimes when he is attacked away from the hive he uses his
sting to defend himself. When he does so, he injects a little quantity
of poison through the sting and that is what causes the inflammation.
How Does a Honey Bee Live?
The bee lives in swarms of from 10,000 to 50,000 in one house. In the
wild state the house or hive is located in a hollow tree generally.
These swarms contain three classes of bees, the perfect females or
queen bees, the males or drones, and the imperfectly developed females,
or working bees. In each hive or swarm there is only one perfect female
or queen whose sole mission is to propagate the species. The queen is
much larger than the other bees. When she dies a young working bee
three days old is selected as the new queen. Her cell is enlarged by
breaking down the partitions, her food is changed to “royal jelly
or paste” and she grows into a queen bee. The queen lays 2,000 eggs
per day. The drones do not work and after performing their duty as
males are killed by the working bees. The female bees do the work of
gathering the honey. They collect the honey from the flowers, they
build the wax cells, and feed the young bees. When a colony becomes
overstocked, a new colony is sent out to establish a new hive under the
direction of a queen bee.