ELECTRICITY

_How may electricity be called into activity?_

By _mechanical power_, by _chemical action_, by _heat_, and by _magnetic

influence_.

_What is the most ordinary way of exciting electricity?_

By _friction_.

_Do we know any reason why the means above enumerated should develop

electricity from its latent condition?_

We are _entirely ignorant_ upon this subject.

_When you rub a piece of paper with India-rubber, why does it adhere

to the table?_

Because the _friction_ of the India-rubber against the surface of the

paper develops _electricity_, to which this adhesiveness is mainly to be

attributed.

_Does electricity present any appearance by which it can be known?_

No; electricity, like heat, is in itself _invisible_, though often

accompanied by both _light_ and _heat_.

_When a substance, by friction or by any other means, acquires the

property of attracting other bodies, in what state is it said to

be?_

It is said to be _electrified_, or _electrically excited_; and its

motion towards other bodies, or of other bodies towards it, is ascribed

to a force called electric attraction.

_Does an electrified body exercise any other influence than an

attractive one?_

It _does_; for it will be found that light substances, after _touching_

the electrified body, will _recede from it_ just as actively as they

approached it before contact. This is termed _electric repulsion_.

Thus, if we take a dry glass rod, rub it well with silk, and present it

to a light pith ball, or feather, suspended from a support by a silk

thread, the ball or feather will be attracted towards the glass. After

it has adhered to it a moment, it will fly off, or be repelled. The same

will happen if sealing-wax be rubbed with dry flannel, and a like

experiment made; but with this remarkable difference, that when the

glass repels the ball, the sealing-wax attracts it, and when the wax

repels, the glass will attract. These phenomena are examples of

_electrical attraction_ and _repulsion_.

_What is a non-electrified body?_

One that holds its own natural quantity of electricity _undisturbed_.

_What happens when an electrified body touches one that is

non-electrified?_

The electricity contained in the former is _transferred_ in part to the

latter.

Thus, on touching the end of a suspended silk-thread with a piece of

excited wax, the silk will be excited, as will be shown by its moving

towards a book, piece of metal, or any other object placed near it.

_Do all bodies conduct or allow electricity to pass through them

equally well?_

Although there is no substance that can _entirely prevent_ the passage

of electricity, nor any that does not oppose _some resistance_ to its

passage, yet it moves with a much greater facility through a certain

class of substances than through others. Those substances which

facilitate its passage are called conductors; those that retard or

almost prevent it, are called non-conductors.

_What substances are good conductors of electricity?_

The _metals_, _charcoal_, the _earth_, _water_, and _most fluids_,

except oils, the _human body_, etc., are good conductors.

_What substances obstruct the passage of electricity, or are

“non-conductors”?_

_Glass_, _resin_, _oil_, _silk_, _sulphur_, _dry air_, etc., etc., are

non-conductors.

_What is an electrical machine?_

An electrical machine is an arrangement by which quantities of

electricity can be collected and discharged.

One type of the electrical machine most usually employed consists of a

large circular plate of glass, mounted upon a metallic axis, and

supported upon pillars fixed to a secure base, so that the plate can, by

means of a handle, be turned with ease. Upon the supports of the glass,

and fixed so as to press easily but uniformly on the plate, are four

rubbers; and flaps of silk, oiled on one side, are attached to these,

and secured to fixed supports by several silk cords. When the machine is

put in motion, these flaps of silk are drawn tightly against the glass,

and thus the friction is increased, and electricity excited.

_Do we know what electricity is?_

No; a complete and final answer to this question is no more possible

than the answer to the question--what is _life_? The _theory_ of

electricity, however, opens up possibilities of the most fascinating

nature; it gives us a wonderfully clear conception of which might be

called the inner mechanism of electricity; and it even introduces us to

the very atoms of electricity.

_Give a short outline of the theory of electricity._

EARLY THEORIES.--Early writers on the nature of electricity supposed it

to be either a fluid of peculiar properties or else two fluids whose

properties were complementary to each other or of opposite kinds;

Franklin held the _one fluid_ theory. Later physicists arrived at the

conclusion that whatever electricity might be, it was not a material

substance. As an alternative it was suggested that electricity was a

form of energy, but this proved untenable.

[Illustration: =HOW ELECTRICITY IS PRODUCED BY THE ACTIVITY OF

ELECTRONS=

_Diagram of the Atoms composing the finest point of a piece of Amber_

_ELECTRONS flying off from the SILK Atoms, across on to the AMBER Atoms,

and so charging the Amber with what is called NEGATIVE Electricity_

_Diagram of the Atoms composing the thinnest possible thread of SILK;

each Atom charged with innumerable NEGATIVE ELECTRONS whirling around

within or on the Atoms._]

ELECTRON THEORY.--This, with certain reservations, is held by the

scientific world of today. All matter is believed to be constituted of

minute particles called “atoms,” whose diameter has been estimated at

about one millionth of a millimeter. Up to a few years ago the atom was

believed to be quite indivisible, but it has been proved beyond doubt

that this is not the case. An atom may be said to consist of two parts,

one much larger than the other. The smaller part is negatively

electrified, and is the same in all atoms; while the larger part is

positively electrified, and varies according to the nature of the atom.

The small negatively electrified portion of the atom consists of

particles called “electrons,” and these electrons are believed to be

indivisible units or atoms of negative electricity.

The electrons in an atom are not fixed, but move with great velocity, in

definite orbits. They repel one another, and are constantly endeavoring

to fly away from the atom, but they are held in by the attraction of

the positive core. So long as nothing occurs to upset the constitution

of the atom, a state of equilibrium is maintained and the atom is

electrically neutral; but immediately the atom is broken up by the

action of an external force of some kind, one or more electrons break

their bonds and fly away to join some other atom. An atom which has lost

some of its electrons is no longer neutral, but is electro-positive; and

similarly, an atom which has gained additional electrons is

electro-negative.

THE ELECTRIC CURRENT.--A current of electricity is believed to be

nothing more or less than a stream of electrons, set in motion by the

application of an electro-motive force. Some substances are good

conductors of electricity, while others are bad conductors or

non-conductors. In order to produce an electric current, that is a

current of electrons, it is evidently necessary that the electrons

should be free to move. In good conductors, which are mostly metals, it

is believed that the electrons are able to move from atom to atom

without much hindrance, while in a non-conductor their movements are

hampered to such an extent that interatomic exchange of electrons is

almost impossible.

_Does electricity seem to exist in two different states or

conditions?_

It does; and to designate these two conditions, the terms positive and

negative have been employed. Thus a body which has an overplus of

electricity is called positive, and one that has less than its natural

quantity is called negative.

_Do light, heat, and electricity appear to have some properties in

common?_

They _do_; each may be made, under certain circumstances, to _produce_

or _excite_ the other. All are so light, subtle, and diffusive, that it

has been found impossible to recognize in them the ordinary

characteristics of matter. Some suppose that light, heat, and

electricity are all _modifications_ of some common principle.

_Why does the fur of a cat sparkle and crackle when rubbed with the

hand in cold weather?_

Because the friction between the hand and fur produces an excitation of

_positive electricity_ in the _hand_ and _negative_ in the _fur_, and an

interchange of the two causes a spark, with a slight noise.

_Why does this experiment work best in very cold weather?_

Because the air is then _very dry_, and does not _convey away_ the

electricity as fast as it is excited; if the air, on the contrary, were

_moist_, the electricity would be _conducted off_ nearly as fast as it

was excited by friction, and its effects would not therefore be so

manifest.

_With what velocity is electricity transmitted through good

conductors?_

With a velocity so great that the most rapid motion produced by art

appears to be actual rest when compared to it. Some authorities have

estimated that electricity will pass through copper wire at the rate of

_two hundred and eighty-eight thousand miles in a second_ of time--a

velocity greater than that of light.

_What agents are undoubtedly the most active in producing and

exciting electricity in the operations of nature?_

The _light_ and the _sun’s rays_.

_Do some animals have the power of exciting electricity within

themselves?_

There _are_ certain animals which are _gifted_ with the extraordinary

power of _producing electrical phenomena_ by an effort of muscular or

nervous energy. Among these the electrical eel and the torpedo are most

remarkable.

_How powerful a charge of electricity can the electrical eel send

forth when in full vigor?_

Sufficient to _knock down a man or stun a horse_.

_Is the electricity generated by these animals the same as that

occasioned by the ordinary electrical machine?_

It _is the same_, and produces the _same effects_.

_Do vital action and muscular movements in man and animals give rise

to electricity?_

They _do_; and it can be shown by direct experiment that a person cannot

even _contract the muscles of the arm_ without exciting an electrical

action.

_Does change of form or state in bodies generally produce electrical

excitation?_

Change of form or state is one of the _most powerful methods_ of

exciting electricity.

Water, in passing into steam by artificial heat, or in evaporating by

the action of the sun or wind, generates large quantities of

electricity. The crystallization of solids from liquids, all changes of

temperature, the growth and decay of vegetables, are also instrumental

in producing electrical phenomena.

_What is lightning?_

Lightning is _accumulated electricity_, generally discharged _from the

clouds_ to the earth, but sometimes from the earth to the clouds.

_What causes the discharge of an electric cloud?_

When a cloud _overcharged_ with electric fluid approaches another which

is _undercharged_, the fluid rushes from the former _into the latter_,

till both contain the _same quantity_.

_Is there any other cause of lightning besides the one just

mentioned?_

Yes; sometimes mountains, trees, and steeples will discharge the

lightning from a cloud floating near, and sometimes the electricity

passes from the earth into the clouds.

_How high are the lightning clouds from the earth?_

Sometimes they are elevated _four or five miles high_, and sometimes

actually touch the earth with one of their edges; but they are rarely

discharged in a thunder storm when they are more than seven hundred

yards above the surface of the earth.

_What is a thunder storm?_

The _disturbance_ caused in the _air_ when successive discharges of

accumulated electricity take place.

_Into how many kinds has lightning been divided?_

_Three._

_What are they?_

The _zig-zag lightning_, _sheet lightning_, and _ball lightning_.

_Why is lightning sometimes forked?_

Because the lightning cloud is at a _great distance_; and the

_resistance of the air_ is so great that the electrical current is

diverted into a zig-zag course.

_How does the resistance of the air make the lightning zig-zag?_

As the lightning condenses the air in the immediate advance of its path,

it flies from side to side, in order to pass where there is the _least

resistance_.

_Why is the flash sometimes quite straight?_

Because the lightning cloud is near the earth, and as the flash meets

with very little resistance, it is not diverted; in other words, the

flash is straight.

_What is sheet lightning?_

Either the reflection of distant flashes not distinctly visible or

beneath the horizon, or else _several flashes intermingled_.

[Illustration: =FRANKLIN AND HIS KITE=]

_What other form does lightning occasionally assume?_

Sometimes the flash is _globular_, which is the most dangerous form of

lightning.

_Does a discharge produce a flash when it passes through good

conductors?_

It _does not_, but passes quietly and invisibly.

_What is heat lightning?_

Sometimes it is the _reflection_ in the atmosphere of the lightnings of

storms _very remote_, the storms themselves being so far distant that

their thunders cannot be heard. This phenomenon is also occasioned by

the play of silent flashes of electricity between the earth and the

clouds, the amount of electricity developed not being sufficient to

produce any other effects than the mere flash of light.

_Why is lightning more common in summer and in autumn than in spring

and winter?_

Because the heat of summer and autumn produces _great evaporation_, and

the conversion of _water into vapor_ always develops _electricity_.

_How long is the duration of a flash of lightning?_

Arago has demonstrated that it does not exceed the _millionth part of a

second_.

_With what velocity is lightning, or the electric fluid which gives

rise to its appearance, supposed to move?_

Not less than _two hundred and fifty thousand miles per second_.

_By whom was the identity of lightning and electricity first

established?_

By _Dr. Franklin_, at Philadelphia, in 1752.

The manner in which this fact was demonstrated, was as follows:

Having made a kite of a large silk handkerchief stretched upon a frame,

and placed upon it a pointed iron wire connected with the string, he

raised it upon the approach of a thunder storm. A key was attached to

the lower end of the hempen string holding the kite, and to this one end

of a silk ribbon was tied, the other end being fastened to a post. The

kite was now insulated, and the experimenter for a considerable time

awaited the result with great solicitude. Finally, indications of

electricity began to appear on the string; and on Franklin presenting

his knuckles to the key, he raised an electric spark. The rain beginning

to descend, wet the string, increased its conducting power, and vivid

sparks in great abundance flashed from the key.

_Why was the kite insulated when Franklin fastened the key to the

post with a silk ribbon?_

Because the silk was a _non-conductor_, and would not allow the

electricity received upon the kite to pass off by means of the string to

the ground.

_Was this experiment one of great danger and risk?_

It was; because the whole amount of electricity contained in the thunder

cloud was _liable to pass from_ it, by means of the string, to the

earth, notwithstanding the use of the silk insulator.

_Have we any proof of the utility of lightning rods?_

The experience of a hundred years has shown that when all the _necessary

rules_ have been _observed_, the protection is perfect, as far as human

effort can avail.

_Is a building more or less liable to be struck when furnished with

a good lightning conductor?_

Lightning conductors do _not_, as many suppose, _attract the lightning

toward the building_ on which they are situated; they simply _direct its

course_, and _facilitate_ the _passage_ of the _fluid_ in the most

direct way to the earth, only when a discharge must inevitably occur.

There is no attraction, but the lightning takes the road which offers

the least resistance.

_What is thunder?_

It is a certain _noise_ proceeding apparently from the clouds, which

usually follows, after a greater or less interval, the appearance of a

flash of lightning.

_How is it supposed to be occasioned?_

The usual explanation offered is a _sudden displacement of the air_

produced by the electrical discharges in which the lightning is evolved.

Others have supposed that the passage of the electric current creates a

vacuum, and that the air rushing in to fill it produces the sound. Any

explanation that has yet been offered is not altogether satisfactory.

_What occasions the rolling of the thunder?_

It has been ascribed to the _effect of echo_; but the true cause

probably is, that the sound is developed by the lightning in passing

through the air, and consequently separate sounds are produced at every

point through which the lightning passes.

_Why is thunder sometimes one vast crash?_

Because the lightning cloud is _near the earth_; and as all the

vibrations of the air (on which sound depends) reach the ear at _the

same moment_, they seem like one vast sound.

_Why is the thunder generally heard several moments after the

flash?_

Because it has a _long distance_ to travel. Lightning travels nearly a

_million_ times faster than thunder; if, therefore, the thunder has _a

great distance to come_, it will not reach the earth till a considerable

time _after the flash_.

_Can we not tell the distance of a thunder cloud by observing the

interval which elapses between the flash and the peal?_

Yes; the flash is instantaneous, but the thunder will take a whole

_second of time_ to travel three hundred and eighty yards; hence, if the

flash be five seconds before thunder, the cloud is nineteen hundred

yards off.

_i.e._ 380 × 5 = 1900 yards.

_What is the aurora borealis or northern lights?_

_Luminous appearances_ seen in the _sky_ at night-time. Sometimes

streaks of blue, purple, green, red, etc., and sometimes flashes of

light, are seen.

_What is the cause of the aurora borealis or northern lights?_

_Electricity_ in the higher regions of the atmosphere is undoubtedly an

active agent in producing this phenomenon.

_Is the aurora ever seen in other parts of the heavens than towards

the north?_

In the northern hemisphere it always appears in the _north_, but in the

southern hemisphere it appears in the _south_: it seems to originate at

or near the _poles of the earth_, and is consequently seen in its

greatest perfection within the arctic and antarctic circles.

_What is known concerning the extent of the aurora?_

It is not _local_, but it is seen simultaneously at places widely remote

from each other, as in Europe and America.

_What calculations have been made respecting the height of the

aurora?_

The height of the appearances varies from _one to two hundred miles_;

they sometimes appear within the region of the clouds, and very near to

the earth.

_Do the auroras appear at any particular seasons and times?_

They appear more frequently in the _winter_ than in the summer, and are

only seen at _night_.

_Do they also occur in the day-time?_

The aurora is known to _affect the magnetic needle_ and the telegraph;

and as the effects upon these instruments are noticed by day as well as

by night, there can be no doubt of the occurrence of the aurora at all

hours. The intense light of the sun renders the auroral light invisible

during the day.

_Of what utility are the auroral appearances in the polar regions?_

During the long polar night, when the sun is absent, the aurora appears

with a magnificence unknown in other regions, and affords _light

sufficient_ for many of the _ordinary outdoor employments_.