If you take a glass rod and rub it with a piece of flannel or silk, it
will be found to have acquired a property which it did not formerly
possess: namely, the power of attracting to itself such light bodies as
dust or bits of thread and paper.
Hold such a rod over some small bits of paper and watch them jump up to
meet it, just as if the glass rod were a magnet attracting small pieces
of iron instead of paper.
The agency at work to produce this mysterious power is called
_electricity_, from the Greek word "Elektron," which means _amber_.
Amber was the first substance found to possess this property.
[Illustration: Fig. 17.—An Electrified Glass Rod will Attract Small Bits
of Paper.]
The use of amber begins with the dawn of civilization. Amber beads have
been found in the royal tombs at Mycenae and at various places
throughout Sardinia, dating from at least two thousand years before our
era.
Amber was used by the ancient world as a jewel and for decoration.
The ancient Syrian woman used distaffs made of amber for spinning. As
the spindle whirled around it often rubbed against the spinner’s
garments and thus became _electrified_, as amber always does when it is
rubbed. Then on nearing the ground it drew to itself the dust or bits of
chaff or leaves lying there, or sometimes perhaps attracted the fringe
of the clothing.
The spinner easily saw this, because the bits of chaff which were thus
attracted would become entangled in her thread unless she were careful.
The amber spindle was, therefore, called the "harpaga" or "clutcher,"
for it seemed to seize such light bodies as if it had invisible talons,
which not only grasped but held on.
This was probably the first intelligent observation of an electrical
effect.
In the eighteenth century, when Benjamin Franklin performed his famous
kite experiment, electricity was believed to be a sort of fiery
atmospheric discharge which could be captured in small quantities and
stored in receptacles such as Leyden jars.
Franklin was the first to prove that the lightning discharges taking
place in the heavens are electrical.
The story of his experiment is very interesting.
He secured two light strips of cedar wood, placed cross-wise and covered
with a silk handkerchief for a kite. To the top of the upright stick of
the kite was fastened a sharp wire about a foot long. The twine was of
the usual kind, but was provided with a short piece of silk ribbon and a
key. The purpose of the ribbon was possible protection against the
lightning running through his body, silk being a "non-conductor," as
will be explained a little farther on. The key was secured to the
junction of the silk ribbon and the twine, to serve as a convenient
conductor from which to draw the sparks—if they came. He did not have to
wait long for a thunderstorm, and as he saw it gathering he went out
with his son, then a young man twenty-two years of age. The great clouds
rolled up from the horizon, and the gusts of wind grew fitful and
strong. The kite felt a swishing blast and began to rise steadily,
swooping this way and that as the breeze caught it. The thunder muttered
nearer and nearer and the rain began to patter on the grass as the kite
flew higher.
The rain soon began to fall heavily, compelling Franklin and his son to
take refuge under a near-by shed. The heavy kite, wet with water, was
sailing sluggishly when suddenly a huge low-lying black cloud traveling
overhead shot forth a forked flame and the flash of thunder shook the
very earth. The kite moved upward, soaring straight into the black mass,
from which the flashes began to come rapidly.
Franklin watched the silk ribbon and the key. There was not a sign. Had
he failed? Suddenly the loose fibers of the twine erected themselves.
The moment had come. Without a tremor he advanced his knuckle to the
key. And between his knuckle and the key passed a spark! then another
and another. They were the same kind of little sparks that he had made
hundreds of times with a _glass tube._
And then as the storm abated and the clouds swept off towards the
mountains and the kite flew lazily in the blue, the face of Franklin
gleamed in the glad sunshine. The great discovery was complete, his name
immortal.
The cause of lightning is the accumulation of the electric charges in
the clouds, the electricity residing on the surface of the particles of
water in the cloud. These charges grow stronger as the particles of
water join together and become larger. As the countless multitude of
drops grows larger and larger the "potential" is increased, and the
cloud soon becomes heavily charged.
Through the effects of a phenomenon called _induction,_ and which we
have already stumbled against in the experiment with the tacks and the
magnetic chain, the force exerted by the charge grows stronger because
of a charge of the opposite kind on a neighboring cloud or some object
on the earth beneath. These charges continually strive to burst across
the intervening air.
As soon as the charge grows strong enough a vivid flash of lightning,
which may be from one to ten miles long, takes place. The heated air in
the path of the lightning expands with great force; but immediately
other air rushes in to fill the partial vacuum, thus producing the
terrifying sounds called _thunder_.
In the eighteenth century, electricity was believed to be a sort of
fiery atmospheric discharge, as has been said. Later it was discovered
that it seemed to flow like water through certain mediums, and so was
thought to be a fluid. Modern scientists believe it to be simply a
vibratory motion, either between adjacent particles or in the ether
surrounding those particles.
It was early discovered that electricity would travel through some
mediums but not through others. These were termed respectively
"conductors" and "non-conductors" or insulators. Metals such as silver,
copper, gold, and other substances like charcoal, water, etc., are good
conductors. Glass, silk, wool, oils, wax, etc., are non-conductors or
insulators, while many other substances, like wood, marble, paper,
cotton, etc., are partial conductors.
There seems to be two kinds of electricity, one called "static" and the
other "current" electricity. The former is usually produced by friction
while the latter is generated by batteries or dynamos.
A very simple and well-known method of generating static electricity is
by shuffling or sliding the feet over the carpet. The body will then
become _charged_, and if the knuckles are presented to some metallic
object, such as a gas-jet or radiator, a stinging little spark will jump
out to meet it.
[Illustration: _From the author’s "Wireless Telegraphy and Telephony" by
permission._ *A Double Lightning Discharge from a Cloud to the Earth.*]
The electricity is produced by the friction of the feet sliding over the
carpet and causes the body to become electrified.
Warm a piece of writing-paper, then lay it on a wooden table and rub it
briskly with the hand. It soon will become stuck to the table and will
not slide along as it did at first. If one corner is raised slightly it
will tend to jump right back. If the paper is lifted off the table it
will tend to cling to the hands and the clothing. If held near the face
it will produce a tickling sensation. All these things happen because
the paper is electrified. It is drawn to the other objects because they
are _neutral_, that is, do not possess an electrical charge.
[Illustration: Fig. 19.—A Piece of Dry Writing-Paper may be Electrified
by Rubbing.]
All experiments with static electricity perform better in the winter
time, when it is cool and clear, than in the summer. The reason is that
the air in winter is drier than in summer. Summer air contains
considerable moisture and water vapor. Water vapor is a _partial_
conductor of electricity, and the surrounding air will therefore conduct
the static electricity away from your apparatus almost as fast as it can
be produced in the summer time.
[Illustration: Fig. 20.—A Surprise for the Cat.]
Some day during the winter time, when it is cool and clear, and the cat
is near a fire or a stove, stroke the cat rapidly with the hand. The fur
will stand up towards the hand and a faint crackling noise will be
heard. The crackling is caused by small sparks passing between the cat
and the hand. If the experiment is performed in a dark room, the sparks
may be plainly seen. If you present your knuckle to the cat’s nose a
spark will jump to your knuckle and somewhat surprise the cat.
If the day is brisk and cool, so that everything outside is frozen and
dry, try combing the hair with a rubber comb. Your hair will stand up
all over your head instead of lying down flat, and the faint crackling
noise, showing that sparking is taking place as the comb passes through
the hair, will be plainly heard. The electricity is produced by the
friction between the hair and the comb.
Electricity may be produced by friction between a number of substances.
A hard rubber rod, a glass rod, a rubber comb or a stick of sealing-wax
may be very easily electrified by rubbing them briskly with a piece of
dry, warm flannel.
*Electroscopes* are devices for detecting the presence of static
electricity.
[Illustration: Fig. 21.—A Paper Electroscope.]
A very simple form of electroscope may be made in much the same manner
as the paper compass described in the last chapter. It may be cut out of
writing-paper and mounted on a pin stuck through a cork. If an
electrified rod is held near the electroscope it may be made to whirl
around in the same manner as a compass needle when a bar magnet is
brought to it.
*The Pith-Ball Electroscope* is a very simple device, in which a ball of
cork or elder pith is hung by a fine silk thread from an insulated
support. A suitable electroscope may be made from a glass bottle having
a piece of wire thrust into the cork to support the pith ball. When the
electrified rod is presented to the pith ball, it will fly out towards
the rod.
[Illustration: Fig. 22.—A Pith-Ball Electroscope.]
If the pith ball is permitted to touch the glass rod, the latter will
transfer some of its electricity and charge the ball. Almost immediately
the pith ball will fly away from the glass rod, and no matter how near
the rod is brought, it will refuse to be touched again.
This action is much the same as that of the magnetized needle suspended
from a thread when the similar pole of the magnet is presented to it.
When the rod is first presented to the pith ball, the latter is neutral
and does not possess an electrical charge. When the rod has touched the
ball, however, some of the electricity from the rod passes to the ball,
and after this they will repel each other.
The reason is that the rod and the ball are _similarly_ charged and
_similarly charged bodies will repel each other_.
[Illustration: Fig. 23.—A Double Pith-Ball Electroscope.]
If you are a good observer you might have noticed when experimenting
with an electrified rod and the small bits of paper, that some of the
little papers were first attracted and flew upwards to the rod, but
having once touched it, were quickly repelled.
The repulsion between two similarly electrified bodies may be shown by a
double electroscope.
A double electroscope is made by hanging two pith balls on two silk
threads from the same support.
Electrify a glass rod and touch it to the pith balls. They will
immediately fly apart because they are electrified with the same kind of
electricity.
*The Gold-leaf Electroscope* is one of the most sensitive means which
can be employed to detect small amounts of static electricity.
[Illustration: Fig. 24.—A Gold-Leaf Electroscope.]
It is a very simple instrument and is easily made in a short time. A
couple of narrow strips of the thinnest tissue paper, or, better still,
two strips of gold leaf, are hung from a support in a wide-mouthed glass
bottle which serves at once to insulate and protect the strips from
draughts of air.
The mouth of the jar is closed by a plug of paraffin wax, through the
center of which passes a small glass tube. A stiff copper wire passes
through the tube. The lower end of the wire is bent at right angles to
furnish support for the strips of gold leaf. A round sheet metal disk
about the size of a quarter is soldered to the upper end of the rod.
If an electrified stick of sealing-wax or a glass rod is presented to
the disk of the electroscope, the strips will repel each other very
strongly. If the instrument is sensitive, the strips should begin to
diverge some time before the rod reaches the disk. It is possible to
make an electroscope so sensitive that chips formed by sharpening a
pencil will cause the strips to diverge.
*There are two kinds of static electricity.* Rub a glass rod with a
piece of silk and then suspend it in a wire stirrup as shown in Figure
25. Excite a second rod also with a piece of silk and bring it near one
end of the suspended one. The suspended rod is _repelled_ and will swing
away from the one held in the hand.
[Illustration: Fig. 25.—Method of Suspending an Electrified Rod in a
Wire Stirrup.]
Now rub a stick of _sealing-wax_ with a piece of _flannel_ until the
sealing-wax is electrified. Then bring the stick of sealing-wax near the
end of the suspended rod. The rod will be _attracted_ to the
sealing-wax.
If you experiment further you will find that two sticks of sealing-wax
will repel each other.
[Illustration: Fig. 26.—Similarly Electrified Bodies Repel Each Other.
Dissimilarly Electrified Ones Attract Each Other.]
This experiment indicates that there are two kinds of electrification:
one developed by rubbing glass with silk and the other developed by
rubbing sealing-wax with flannel.
In the first instance, the glass rod is said to be _positively_
electrified, and in the latter case the sealing-wax is _negatively_
electrified.
The same law that applies to magnetism also holds true in the case of
static electricity, and similarly electrified bodies will repel each
other and dissimilar ones attract.
*The Electrophorus* is an instrument devised by Volta in 1775 for the
purpose of obtaining static electricity.
[Illustration: Fig. 27.—The Electrophorous]
It is easily constructed and will furnish a source of electricity for
quite a number of interesting experiments. An electrophorus consists of
two parts, a round cake of resinous material cast in a metal dish or
pan, and a round metal disk which is provided with an insulating handle.
To make an electrophorus, first procure an old cake or pie tin, and fill
it with bits of resin or sealing-wax. Place the pan in a warm spot upon
the stove where the resin will melt, taking care not to overheat or it
will spatter and possibly take fire. As the resin melts, add more until
the pan is nearly full. When all is melted, remove from the fire and set
it away where it may cool and harden in the pan without being disturbed.
Cut a circular disk out of sheet tin, zinc, or copper, making the
diameter about two inches less than that of the pie pan. Solder a small
cylinder of tin or sheet brass to the center of the disk to aid in
supporting the handle. The latter is a piece of glass tubing about
three-quarters of an inch in diameter and four or five inches long,
placed in the center of the cylinder and secured with molten
sealing-wax.
In order to use the electrophorus the resinous cake must first be beaten
or briskly rubbed with a piece of warm woolen cloth or flannel. Then
place the disk on the cake holding the insulating handle with the right
hand. Touch the cover or the disk momentarily with the forefinger of the
left hand. After the finger is removed, raise the disk from the cake by
picking it up with the glass insulating handle. The disk will now be
found heavily charged with positive electricity, and if the knuckles are
presented to the edge, a spark will jump out to meet them.
[Illustration: Fig. 28.—An Electric Frog-Pond.]
The cover may then be replaced, touched, and once more removed. It will
yield any number of sparks, the resinous cake only needing to be
recharged by rubbing once in a long while.
*An Electric Frog-Pond* may be experimented with by cutting out some
small tissue-paper frogs. Moisten them a little and lay them on the
cover of the electrophorus. Touch the electrophorus with the finger and
then raise it with the insulating handle. If the "frogs" are not too wet
they will jump from the cover upon the table as soon as the cover is
raised.