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 [908] alternative it was suggested that electricity was a form of energy, but this proved untenable.
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.
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 [909] 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.
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.