G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

. (page 30 of 42)
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attached to a stand. Down the centre runs a wire, C,
terminating above in a ball, A. Several square pieces,
D, F, about one-fourth of an inch thick, are cut out of
the gable, and placed loosely in the holes from which
they are cut. Across each square passes a wire in such
a direction that by inserting the squares one way we
have an uninterrupted line from C to E ; but putting
them in crosswise, we break the continuity of the con-
ductor at D and F. Connect the end of the wire, E,
with the outside of a Leyden jar ; and, having inserted
the square so that the conducting line may be un-
broken, pass a charge through the wire by connecting
the ball A with the inside of the jar. A report will be
heard, but neither of the loose pieces will be displaced.
Now let one of the pieces remain in the same position, and place the other
crosswise ; then, on passing a powerful charge through the wire, the former
will remain undisturbed, while the latter will be thrown out of the gable by
the mechanical action of the fluid in leaping over the break.

798. Among the mechanical effects of an electric ' dis-
charge may be mentioned the perforation of thin non-
conducting substances, such as a card or a piece of paper.
Glass one-twelfth of an inch thick may be pierced by a dis-
charge from a powerful battery.

799. THE ELECTRIC SPARK. The color of the electric
spark varies according to the medium through which it
passes. In ordinary air and oxygen, it is bluish white ;
in rarefied air, violet , in nitrogen, a purplish blue ; in
hydrogen, crimson ; in carbonic acid and chlorine, green.

Flyer. What is the effect of darkening the room ? To what may motion be com-
municated on tho principle of the flyer ? 797. What apparatus further illustrates the
mechanical power of electricity? Describe the Thunder House, and the experiment
performed with. it. 798. What other mechanical effect of an' electric discharge
is mentioned ? 799. What does the color of the electric spark depend on ? What



THE ELECTRIC SPAKK.



307



The length and intensity of the spark depend on the
electrical intensity of the body from which it proceeds.
Sparks may be taken from the prime conductor of a pow-
erful machine at a distance of more than two feet. In a
given machine, the positive conductor yields much more
powerful sparks than the negative.

800. Ignition l>y the Electric Spark. Inflammable sub-
stances may be set on fire by the electric spark, as is shown
by several experiments.

Stand on the insulating stool, touch the prime conductor with one hand,
and from the other transmit a spark to a burner from which a current of gas
is issuing, the gas will be ignited. In houses thoroughly dried by furnace
heat, persons, by simply running over the carpet, have been sufficiently
charged with electricity to light gas with a spark from the finger. Present a
candle just extinguished, with its wick still glowing, to a prime conductor,
so that a spark may pass through the snuff to the candle, and it will be re-
lighted. A person on an insulating stool charged with electricity may set
fire to a cup of ether by presenting to it an icicle, through which the spark
is transmitted. With a suitable apparatus, a fine wire may be melted by
sending through it a charge from a powerful battery.

801. TJie Electrical Fire Home. Fi^ 2S6
Rosin may be ignited with the

apparatus known as the Electrical
Fire House (Fig. 286). Brass wires,
insulated by being enclosed in
glass tubes,enter the opposite sides
of the house, and terminate on the
inside in two knobs, B, C, a short
distance apart. These knobs are
loosely covered with tow and
sprinkled with powdered rosin.
When a charge is passed from A
to D, the rosin is ignited, and the
flame seen through the windows
gives the house the appearance of
being on fire.

802. Apparatus for firing Gunpowder. This apparatus consists of two

is its color in ordinary air and oxygen ? In rarefied air? In nitrogen? In hydro-
gen ? In carbonic acid and chlorine ? "What do the length and intensity of the spark
depend on ? At what distance have sparks been taken from a powerful machine?
How do the sparks from the positive conductor compare with those from the nega-
tive? 800. What is the. effect of the electric spark on inflammable substances?
Prove this with several experiments. What is the effect of sending a powerful
charge through a fine wire ? 801. Describe tha Electrical Fire House, and the ex-




TUK ELECTRICAL FIRE HOUSE.



308



ELECTRICITY.




insulating glass pillars fixed in a stand,
to one of which is attached a wire termi-
nating in a ball, to the other a wooden
cup for holding the powder. The chains
c, d, being connected respectively with
the inner and outer surface of a Leyden
jar, a spark is made to pass from b to A,
which ignites the powder.

803. THE ELECTKOPHOKTJS.
Small quantities of electricity
maybe accumulated with a sim-
ple apparatus known as the Electrophorus, which to a cer-
tain extent answers as a substitute for the electrical
machine.

The electrophorus consists of a cake of a resinous mixture 8 or 10 inches
in diameter, and a somewhat smaller plate of metal with a rounded edge and
a glass handle, by which it may be raised without drawing off the electricity.
Excite the resinous mixture with fur, and placing on it the metallic plate,
touch the upper surface of the latter for an instant to let its negative elec-
tricity escape. Then raise the metallic plate by the insulated handle, and
on presenting a conductor a spark will be given. Place the metallic plate
again upon the rosin, and on raising it another spark may be withdrawn. A
Leyden jar may thus be slowly charged. Left on the rosin, the metallic
plate will remain charged for a long time, and may be conveniently used as
occasion requires in experimenting.

804. ELECTROSCOPES. Electroscopes are instruments
for detecting the presence of electricity, and determining
whether it is positive or negative. They appear in various
forms, the simplest being the pith ball suspended by a
silk thread, represented in Fig. 266. The attraction of the
pith ball in its natural state by any substance presented to
it, indicates the presence of electricity in the latter. When
the pith ball is charged with positive electricity, its attrac-
tion by any substance indicates negative electricity in the
latter, and its repulsion positive. When the pith ball is



periment performed with it. 802. Of what does the apparatus for firing gunpowder
consist ? 803. With what may small quantities of electricity be accumulated ? Of
what does the Electrophorus consist ? How is it worked ? 804. What are Electro-
scopes ? What is the simplest form of the electroscope ? How is the presence of elec-
tricity in any substance indicated? When the pith ball is positively charged, what
iocs its attraction by any substance indicate ? What, its repulsion ? When the pith



THE ELECTROMETER.



309




QUADRANT ELEC-
TROMETER.



charged with negative electricity, its attraction by any sub-
stance indicates positive electricity in the latter, its repul-
sion negative.

805. ELECTROMETERS. Electrometers are
instruments for measuring approximately the
quantity of electricity in a given conductor
or other body. Electrometers, more or less
sensitive, are made in different forms ; one of
the simplest is the Quadrant Electrometer,
shown in Fig. 288.

A slender ivory rod, with a pith ball attached to its
lower end, is suspended from a wooden pillar so as to
swing freely like a pendulum. The pivot on which it
turns is the centre of a semicircular scale attached to the
pillar ; and the whole apparatus terminates in a brass pin
which may be inserted in the top of a prime conductor.
The greater the quantity of electricity in the latter, the
farther from the pillar the pith ball will swing, and this
distance is indicated by the scale.

806. ELECTRICAL INDUCTION. An electrical atmosphere
surrounds every excited body. An insulated conductor
situated within this atmosphere becomes excited, and when
thus affected is said to be electri-
fied by induction. The phenom-
ena of electrical induction are con-
stantly exhibited.

807. Electrical induction is illustrated with
the apparatus represented in Fig. 289. c a d is
a brass cylinder with rounded ends, insulated
on a glass support and furnished at one ex-
tremity with a pith ball electroscope, f. On
bringing the end d within a few inches of a
prime conductor, the pith balls, which be-
fore hung close together, instantly separate,
indicating the presence of electricity. Since
the cylinder is not in contact with the prime INDUCTION APPARATUS.

ball is negatively charged, what does its attraction indicate ? What, its repulsion ?
805. What are Electrometers ? What is. one of the simplest forms called ? Describe
the Quadrant Electrometer, and its mode of operation. 806. By what is every ex-
cited body surrounded? When is a body said to bo electrified ly induction?
607. Describe the apparatus for illustrating electrical induction, and the experiments



Fig. 289.




310 ELECTRICITY.

conductor and receives no sparks from it, it is obviously electrified by in'duc
tion. Its neutral and latent electricity is decomposed by the electrical at-
mosphere which surrounds the prime conductor : the negative portion is
attracted towards d, and the positive repelled to c, where it charges the two
balls, and thus causes them to separate. If the cylinder is removed from
the neighborhood of the prime conductor, the pith balls immediately fall to-
gether; it is only when within the atmosphere of the prime conductor that
they indicate any electrical excitement.

If the cylinder cad, instead of being insulated, is connected with the
earth, its positive electricity is driven off to the latter, while the negative
portion is retained. If the cylinder is then removed, its communication with
the earth being first cut off, it will remain excited with negative electricity.

808. ELECTRICITY FROM STEAM. Electricity is devel-
oped during the escape of steam from an orifice. This fact
was discovered in 1840 by a workman attending a steam-
engine ; who, happening to take hold of the safety-valve
with one hand while the other was in a jet of steam escap-
ing from a fissure, received an electric shock. The experi-
ment was repeated, and it was found that a person with
one hand in a jet of escaping steam could give a shock witli
the other to any one in contact with the boiler or the brick
work supporting it. The electricity in question is produced
by the friction of minute particles of water against the sides
of the orifice.

As soon as this fact came to the knowledge of scientific men, an appara-
tus known as the Hydro-electric Machine was invented for the purpose of
experiment. It consists of a steam boiler from three to six feet long, mount-
ed on insulating pillars, with an arrangement for letting the steam escape in
jets against a plate covered with metallic points, which acts like a prime con-
ductor. This machine develops electricity in prodigious quantities, its power
being equal to that of four large plate machines combined. It yields sparks
twenty-two inches long, in such quick succession that they resemble a sheet
of flame.

809. ATMOSPHERIC ELECTRICITY. The atmosphere, be-
sides the neutral and latent electricity which resides in it
as in all other substances, contains more or less free elec-



performcd with it? How may the cylinder be charged with negative electricity?
898. Under what circumstances is electricity produced by steam ? State the circum-
stances attending this discovery. What was found when the experiment was repeat-
ed? How is the electricity in question produced? What instrument was invented
for the sake of further experiment? Describe the Hydro-electric machine. To what
(a its power equal? What is said of its sparks? 809. What does the atmosphere






ATMOSPHERIC ELECTRICITY. 311



tricity, the quantity increasing with the distance from the
earth's surface. This is proved by sending up arrows con-
nected by a conducting metallic wire with a delicate eleo
trometer. The higher the arrows rise, the more the elec-
trometer is affected. An experimenter in England, by
connecting a number of pointed conductors with an insu-
lated wire a mile long and raised a hundred feet above the
earth's surface, has collected enough electricity to charge
a battery of fifty jars every three seconds.

810. Origin. The free electricity in the atmosphere is
due 1. To the friction of large masses of air of different
densities on each other. 2. To the condensation of atmos-
pheric vapors into a liquid forrcb a process which develops
electricity in great abundance. 3. To the chemical changes
involved in the growth of trees and plants. 4. To evapo-
ration, particularly in the case of water filled with vegetable
matter undergoing decomposition.

As these processes are not always going on with the
same activity, it follows that the quantity of free electricity
present in the atmosphere differs at different times and
places.

811. St.-Elmo^s Fire. "When the atmosphere is very
abundantly charged with electricity, its presence is indi-
cated by various luminous phenomena. Hence the brilliant
light called St. Elmo's Fire, which frequently appears at
night on the tops of masts, the points of bayonets, and the
tips of the ears of horses. It is simply the superabundant
electricity of the atmosphere, attracted by a pointed con-
ductor, into which it silently passes. Such phenomena are
most common during thunder-storms, when as many as
thirty have been seen in different parts of the same vessel.
Sometimes they resemble sheets of flame, and extend three
feet in length ; at others they take the form of globes



contain? To what is the free electricity in the atmosphere proportioned? II ow is
this proved ? What has been done in this connection in England ? 810. To what
four processes is the free electricity in the atmosphere chiefly due ? Why is the quan-
tity of free electricity in the atmosphere different at different times ? 811. When aro
luminous phenomena observed in the atmosphere ? Describe the phenomenn known



312 ELECTRICITY.

of fire, attaching themselves to the yard-arms and mast-
heads.

812. Fire-balls. To electricity are also attributable the
Fire-balls which are from time to time observed darting
through the atmosphere, at heights of thirty miles and up-
wards, and with velocities of from five to thirty-three miles
in a second. These balls sometimes vanish suddenly, leaving
behind them a luminous track ; at other times they explode
into smaller balls or sparks ; and at others again they arc
'accompanied with showers of meteoric stones. Falling or

shooting stars are the same phenomena on a smaller scale,
and in lower regions of the atmosphere.

813. Lightning and Tliunder. The grandest of all the
phenomena produced in the atmosphere by electricity is
Lightning. Lightning is nothing more than the spark
which accompanies the passage of the electric fluid from
one cloud to another, or between a cloud and the earth.
Thunder is the crackling sound produced at the same time
by the sudden rush of air into the vacuum which the elec-
tric fluid, as it darts with inconceivable rapidity, leaves
behind it. Flashes of lightning are sometimes several
miles in extent ; and, as the crackling sound is produced
at every point of their course, it does not reach our ear all
at the same instant. Hence the rolling or rumbling of
thunder, which is in some cases prolonged by successive
echoes from neighboring mountains or clouds.

814. That lightning and thunder are produced by an
electric discharge, though previously suspected, was first
experimentally proved in 1752, by Benjamin Franklin,
whom the world recognizes alike great as a philosopher
and a patriot.

Impressed with the conviction that lightning and the electric spark were
identical, Franklin determined to test its truth by trying to collect electricity

as St. Elmo's Fire. At what time is it most common ? What different forms does it
assume ? 812. What other phenomena are attributable to electricity ? What be-
comes of these fire-balls ? What are -shooting stars? 813. What is the grandest of
nil the electrical phenomena of the atmosphere? What is Lightning? What is
Thunder ? How is the rolling of thunder accounted for ? 814. By whom and when
was it proved that lightning and thunder are produced by an electric discharge ?



j

FKANKLLN'S EXPERIMENT. 313

from the clouds during a thunder-storm. With this view he made arrange-
ments for extending a wire to a great height from a steeple then in course of
erection in Philadelphia. The work advanced but slowly ; and while anx-
iously watching its progress one day, he observed a boy's kite far up in tho
air, and higher than he could hope to get his wire even when the steeple
should be finished. It struck him at once that with this simple toy he could
make the desired experiment, letting the string perform the part of the con-
ducting wire. Accordingly, he made a cross of two strips of cedar, to the
extremities of which he fastened the four corners of a silk handkerchief,
using this as a covering that his kite might be able to withstand the rain and
wind accompanying a thunder-shower. A sharp-pointed wire extended a
foot from the top of the cross, to draw off the electricity from the clouds.

The kite thus constructed was raised by Franklin and his son in the first
thunder-storm that occurred in June, 1752. Hempen twine was used, at the
lower end of which a key was fastened for a prime conductor, while the whole
was insulated by a silk ribbon fastened to a non-conductor sheltered from the
wet. With intense anxiety the philosopher awaited the result. A cloud
passed without any electrical indications, and he began to despair of success.
Another came, and now to his indescribable joy he saw the loose fibres of
the twine stand out every way and follow his finger as it passed to and fro.
Presenting his knuckle to the key, he received a spark ; and as soon as the
twine was wet with rain, and its conducting power thus increased, the elec-
tricity was abundant. A Ley den jar was charged from the key, with which
spirits were set on fire, and other experiments performed. This discovery
raised its author to the first rank among the philosophers of his day. His
own feelings at the triumphant result of his experiment may be imagiriecl.
" Convinced of an immortal name, he felt he could have been content if that
moment had been his last."

Franklin's experiment was repeated with success in various parts of Eu-
rope. There was no room left for doubting the identity of lightning with
the electric spark. In later times this identity has been further confirmed by
phenomena connected with the electric telegraph. Reports as loud as that
of a pistol are often heard in telegraph offices during a storm, and to ensure
the safety of the operators the wires have to be connected by conductors
with the earth. Even in clear weather it is sometimes found difficult to fix
the wires on the poles, in consequence of numbness produced in the hands
by electricity conducted to them by the wires.

815. Effects of Lightning. Lightning produces both
mechanical and chemical effects. Its mechanical effects are
very powerful. It crushes huge trees, rends off their
branches, and sometimes tears their trunks into fragments.

Eelate the incidents connected with Franklin's great discovery. "What was the re-
sult of this experiment as regards the reputation of its author ? As regards his own
feelings? Where was the experiment repeated ? How has tho identity of lightning
with the electric spark been since confirmed? 815. Mention some of tho mechanical

14



314 ELECTRICITY.

When buildings are struck, large masses of masonry are
displaced; a brick wall more than 12 feet long has been
carried in one piece to a distance of 15 feet. These effects
are analogous to the throwing out of the blocks of wood
from the gable of the Thunder House, as described in
797. It is only (as shown in that experiment) in the case
of imperfect conductors, that is, when obstructions are
presented to the free passage of the electric fluid, that
these effects are produced.

Lightning is also a powerful chemical agent. It decom-
poses water and other substances into their elements. It
sets fire to trees and houses, and melts metallic bodies.
On the tops of mountains it is not unusual to see the sur-
face of the hardest rocks perforated with deep cavities
covered with a vitreous crust, owing to their having been
struck with lightning.

816. Lightning Hods. When a cloud becomes heavily
charged with electricity, if another cloud in a different
electrical state is near it, a discharge takes place between
the two ; in which case there is no danger. But some-
times there is no such adjacent cloud, and a flash of light-
ning darts from the charged cloud to the earth or sea : it
is then said to strike. In such a case, the air being a bad
conductor, the electric fluid in its descent follows any bet-
ter conductor it can find, such as a house, a tree, the mast
of a ship, a living animal, or a human being. Now, if the
objects just mentioned were perfect conductors, the light-
ning would follow them to the earth without doing any
injury ; but they all offer some obstruction to its passage,
and therefore all suffer more or less when struck.

The tallest objects, reaching nearest to the clouds, are the most likely to
be struck. It is therefore imprudent to stand on the top of a hill or near a
tree during a thunder-storm. In the house it is best at such a time not to
sit near a damp wall, a bell wire, a gilded picture frame, or any metallic sub-

ffects of lightning. Only in what case are these effects produced ? State some of
the chemical effects of lightning. 816. When does an electric discharge take place
between two clouds ? When, between a cloud and the earth ? Why are houses,
trees, &c., struck? Why do they suffer damage when struck? What objects are
most likely to be struck ? What positions is it imprudent to take during a thunder-



LIGHTNING EODS.



315



stance, as the electric fluid is sure to select the best conductor iu its path to
the earth if the house should be struck.

817. Having proved lightning to be an electric dis-
charge, Franklin proceeded to devise means for preserving
buildings from its effects. He thus became the inventor
of the Lightning Rod, a simple contrivance which has been
instrumental in saving life and property to an extent that
can not be estimated.

The best material for a lightning rod is copper, but iron is cheaper and
generally preferred. It must extend at least four feet above the building to
be protected, and terminate above in one or more sharp points, which should
be tipped with silver or platinum to keep them from rusting, and thus
losing part of their conducting power. The rod should be
continuous, and of such size that the fluid may follow it freely
without danger of melting it, say three-fourths of an inch
across. It should be placed as close as possible to the wall
and fixed securely to it. The lower end should be divided
into two or more pointed branches, as shown at a, a, a,
in Fig. 290. These branches should slant away from the
building, and at least one of them should sink far enough into
the ground to reach water or soil that is moist. If the build-



Fig. 290.



Fig. 291.




ing is large, and particu-
larly if it has more than
one point projecting up-
ward, it should have sev-
eral rods, either descend-
ingdirectly to the ground,
like c, d, in Fig. 291, or
connected together by a
good conductor, and
timately carried down
like e, f, g, 7i.



E



;



rv3J

ui. n

w r n



818. The security afforded by lightning rods is twofold. In the first place,
terminating in points, they generally draw off the electric fluid silently ; and
secondly, if a discharge takes place, the lightning in its descent will follow
them rather than the inferior conductors to which they aro attached, and
finding a free passage through them will do no injury. Lightning rods Lave
not been found efficacious to a greater distance than forty feet. Within this
limit, they protect a space around themselves equal to twice the height that



storm ? 817. Who invented the Lightning Eod ? Of what materials is the lightning
rod made? What should be its form and size, to ensure the safety of a building?
In what case should a building have several rods? How may they in that case bo
arranged? 818. Iu what two ways do lightning rods conduce to the safety of a build-



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 30 of 42)