ENERGY.
Energy of motion and of position—Energy can be
transformed, not created or destroyed—Not created by free
will—Conservation of mechanical power—Convertibility of
heat and work—Nature of heat—The steam-engine—Different
forms of energy—Gravity—Molecular energy—Chemical
energy—Dynamite—Chemical affinities—Electricity—Produced
by friction—By the voltaic battery—Electric currents—Arc
light—Induction—Magnetism—The magnetic needle—The electric
telegraph—The telephone—Dynamo-electric engine—Accumulator.
Those ultimate elements, however, atoms and ether, only give us what may
be called the dead half of the universe, which could not exist without
the constant presence of the animating principle of force or energy.
Energy is the term generally adopted in the language of science, for
force is apt to be associated with human effort and with actual motion
produced, while energy is a comprehensive term, embracing whatever
produces or is capable of producing motion. Thus, if we bend a cross-bow,
the force with which it is bent may either reappear at once in the flight
of the arrow, if we let go the string; or it may remain stored up, if we
fix the string in the notch, ready to reappear when we pull the trigger.
In the former case it is called energy of motion, in the latter energy of
position. It is important to realise this distinction clearly, for many
of the ordered and harmonious arrangements of the universe depend on the
polarity, or conflict with alternate victories and defeats, between those
two forms of energy.
[Illustration]
Thus if A B is a pendulum suspended at the point A, if we move it from
its position of rest A C to A B and hold it there, its whole energy
is that of position. If we let it go it swings backwards and forwards
between the positions _A B_ and _A D_, and but for the resistance of the
air and the friction at the point of suspension, it would so swing for
ever. But in thus swinging what happens? From A B to A C energy of motion
keeps gaining on energy of position, until when the pendulum reaches C,
it has annihilated it. Energy of position has entirely disappeared, and
the whole original force expended in raising the pendulum to A B exactly
reappears in the force or momentum of the pendulum at its lowest point.
But is this victory final? By no means; energy of position having touched
bottom, gathers, like Antæus, fresh vigour for the contest, and from the
position A C upwards it gains ground on its adversary until when the
pendulum reaches A D it is in its turn completely victorious.
The same alternation between energy of motion and of position takes place
in all rhythmical movements such as waves, which, whether in water, air,
or ether, are propagated, as in the case of the pendulum, by particles
forced out of their position of rest and oscillating between the two
energies.
[Illustration]
Thus if waves run along an elastic wire A B, the particle P, which has
been forced into the position _p_, oscillates backwards and forwards
between _p_ and _q_, beginning with nothing but energy of position
at _p_, losing it all for energy of motion at P, and regaining it at
_q_. All wave-motions therefore—that is to say, all sound, light, and
heat—depend on this primitive polarity.
If we have got this definition of the two forms of energy clearly
into our heads, we shall be the better prepared for this further
generalisation—the grandest, perhaps, in the whole range of modern
science—that energy, like matter, is indestructible, and can only be
transformed, but never created or annihilated.
This is at first sight a more difficult proposition to establish in
the case of energy than in that of matter. In the latter case we have
nothing in our experience that can lead us to suppose that we have ever
created something out of nothing; but in the former, our first impression
undoubtedly is that we do create force. If I throw a stone at a bird I
have an instinctive impression that the force which projects the stone
is the creation of my own conscious will; that I had the choice either
to throw or not to throw; and that if I had decided not to throw, the
impelling force would never have existed. But, if we look more closely at
the matter, it is not really so. The chain of events is this: the first
impulse proceeds from the visual rays, which, concentrated by the lens of
the eye on the retina, give an image of the bird; this sends vibrations
along the optic nerve to the brain, setting in motion certain molecules
of that organ; these again send vibrations along other nerves to certain
muscles of the arm and hand, which contract, and by doing so give out the
energy of movement which throws the stone. All this process is strictly
mechanical; the eye acts precisely like a camera obscura in forming the
image; the nerve-vibrations, though not identical with those of the wires
of an electric telegraph, are of the same nature, their velocity can be
measured, and their presence detected by the galvanometer; the energy of
the muscle is stored there by the slow combustion of the food we have
eaten, in the oxygen of the air we have breathed. Take any of these
conditions away, and no effort of the will can produce the result. If
the nerve is paralysed, or the muscle, from prolonged starvation, has no
energy left, the stone will not be thrown, however much we may desire to
kill the bird.
Again, precisely the same circle of events takes place in numerous
instances without any intervention of this additional factor of conscious
will. We breathe mechanically, the muscles of the chest causing it
to rise and fall like the waves of the ocean, without any deliberate
intention of taking air into the lungs and exhaling it. Nay more, there
are instances of what was at first accompanied by the sensation of
conscious will, ceasing to be so when the molecular movements had made
channels for themselves, as when a piano-player, who had learned his
notes with difficulty, ends by playing a complicated piece automatically.
The case of animals also raises another difficulty. Suppose a retriever
dog sees his master shoot at and miss a hare: shall he obey the
promptings of his animal instinct and give chase, or those of his higher
moral nature which tell him that it is wrong to do so without the word
of command? It is hard to see how this differs from the case of a man
resisting or yielding to temptation; and how, if we assign conscious will
to the man, we can deny it to the dog.
Reasoning from these premises, some philosophers have come to the
conclusion that man and all animals are but mechanical automata, cleverly
constructed to work in a certain way fitting in with the equally
preordained course of outward phenomena; and that the sensation of will
is merely an illusion arising as a last refinement in the adjustment
of the machinery. But here comes in that principle of duality or
polarity, by which a proposition may be at once true and untrue, and
two contradictory opposites exist together. No amount of philosophical
reasoning can make us believe that we are altogether machines and not
free agents; it runs off us like water from a duck’s back, and leaves us
in presence of the intuitive conviction that to a great extent
Man is man and master of his fate.
If this be an illusion, why not everything—evidence of the senses,
experiment, natural law, science, as well as morality and religion?
To pursue this farther would lead us far astray into the misty realm of
metaphysics, and I refer to it only as showing that the principle of the
conservation of energy, standing as it does in apparent contradiction to
our natural impressions, requires a fuller demonstration than the kindred
principle of the indestructibility of matter.
[Illustration]
In the case of ordinary mechanical power it had been long known that the
intervention of machinery did not create force, but only transformed it.
If a weight of 1 lb., A, just balances a weight of 2 lb., B, by aid of a
pulley, and by the addition of a minute fraction, such as a grain, raises
it 1 foot, it will be invariably found that A has descended 2 feet. In
other words, 1 lb. working through 2 feet does exactly the same work
as 2 lbs. working through 1 foot. And whatever may be the intervening
machinery the same thing holds good, and the work put in at one end comes
out, neither more nor less, at the other, except for a minute loss due
to friction and resistance of air. If a force equal to 1 lb. is made, by
multiplying the intermediate machinery, to raise a ton a foot from the
ground, exactly as much force must have been exerted as if the ton had
been divided into 2,240 parts of 1 lb. each, and each part separately
lifted.
But although energy cannot be created, at first sight it seems as if it
might be destroyed, as when the ton falls to the ground and seems to have
lost all its energy, whether of motion or of position. But here science
steps in and shows us that it is not destroyed, but simply transformed
into another sort of motion, which we call heat.
Some connection between mechanical work and heat had long been known, as
in the familiar experiment of rubbing our hands together to warm them;
and the practice known to most primitive races of obtaining fire by
twirling a stick rapidly in a hole drilled in a block of wood; a practice
described by the old Sanskrit word ‘pramantha,’ which means an instrument
for obtaining fire by pressure or friction, and which, translated into
Greek, has been immortalised by the legend of Prometheus. But it was
reserved for recent years, and for an English philosopher, Dr. Joule,
to give scientific precision and generality to this idea, by actually
measuring the amount of heat produced by a given amount of work, and
showing that they were in all cases convertible terms, so much heat for
so much work, and so much work for so much heat. He did this by measuring
accurately by a thermometer the heat added to a given amount of water by
the work done by a set of paddles revolving in it, set in rapid motion by
a known weight descending through a known space. The unit of work being
taken as that sufficient to raise 1 kilogramme through 1 metre, and that
of heat as that required to raise the temperature of one kilogramme of
water by 1° Centigrade, the relation between them, as found by a vast
number of careful experiments, is that of 424 to 1. That is, one unit of
heat is equal to 424 units of work.
In this, and all cases requiring scientific precision, it is better to
use the units of the metrical system than our clumsy English standards;
but it may be sufficient for the ordinary reader to take the metre,
which is about 39·37 inches, as practically a yard, and the kilogramme,
which is 15,432 English grains, as practically equal to 2 lbs. This is
sufficient to show the much greater energy of the invisible forces which
act at minute distances, than that of gravity and other forces which do
appreciable mechanical work, the energy of a weight falling from a height
of more than 1,300 feet being only sufficient to heat its own weight by
1°.
This proof of the convertibility of work into heat gives much greater
precision to our ideas respecting the real nature of heat and its kindred
molecular and atomic energies. Heat is clearly not a material substance,
for a body does not gain weight by becoming hotter. In the case of all
ponderable matter down to the atoms, which are only of the size of
cricket-balls compared to that of the earth, any combination which adds
matter adds weight, and the weight of the product exactly equals the
sum of the weights of the separate factors which have united to form
it. Thus, if iron is burnt in oxygen gas, the product, oxide of iron or
rust, weighs more than the original iron by just as much as the weight
of the oxygen which has been consumed. But heat, light, and electricity
add nothing to the weight of a body when they are added to it, and take
nothing away when they are subtracted. The inference is unavoidable that
heat, like light, is not ponderable matter, but an energy transmitted
by waves of the imponderable medium known as ether. This is confirmed
by finding that when a ray from the sun is analysed by passing through
a refracting prism, one part of the spectrum shows light of various
colours, while another gives heat. The hottest part of the spectrum lies
in the red and beyond it, showing that the heat-waves are longer, and
their oscillations slower, than those of light. Heat-waves also may be
made to interfere, and to become polarised, in a manner analogous to the
phenomena exhibited by those of light.
There can be no doubt, therefore, that heat, like light, is an energy
or mode of motion, transmitted by waves of an imponderable ether, and
that it acts on the molecules and atoms of matter by the accumulated
successive impulses of those waves on the molecules and atoms which are
floating in it, or rather which are revolving in it, in definite groups
and fixed orbits, like miniature solar systems or starry universes. We
can now see how heat performs work, and why work can be transformed into
it.
Heat performs work in two ways. First, it expands bodies—that is, it
draws their molecules farther apart against the force of cohesion which
binds them together or keeps them moving in definite orbits at definite
distances. It is as if it increased the velocity, and therefore the
centrifugal force of a system of planets, and so caused them to revolve
in wider orbits. The expansion of mercury in a thermometer affords a
familiar instance of this effect of heat and the readiest measure of its
amount. Secondly, it increases the energy of the molecular motions, so
that they dart about, collide, and vibrate with greater force. Thus, as
heat increases, evaporation increases, for molecules on the surface are
projected with so much force as to get beyond the sphere of the cohesive
attraction which binds them to the system, and they dart off like comets
into space. Finally, as heat increases, and more and more work is done,
against the centripetal force of cohesion, most substances, and doubtless
all if we could get heat enough, are converted from solids into fluids,
and ultimately into gases, in which latter state the molecules have got
altogether beyond the sphere of their mutual attraction, and tend to
dart off indefinitely in the direction of their own proper centrifugal
motions, unless confined, in which case they dart about, collide,
rebound, and exercise pressure on the containing surface.
Conversely, if heat expands bodies, it is given out when they contract.
Thus the enormous quantity of heat poured out for millions of years by
the sun, is probably owing mainly to the mechanical force of contraction
of the original cosmic matter condensing about the solar nucleus.
Again, when gases suddenly expand, their temperature falls, which is
the principle by which artificial ice is procured, and frozen beef and
mutton are brought from America and Australia, producing, such are the
complicated relations of modern society, agricultural depression, fall of
rents, and a serious aggravation of the Irish question.
As an example of the converse proposition of the transformation of heat
into mechanical work, the steam-engine affords the aptest illustration.
The original power came from the sun millions of years ago, and did work
by enabling the leaves of plants to overcome the strong mutual affinity
of carbon and oxygen in the carbonic dioxide in the air, and store up the
carbon in the plant, where it remained since the coal era in the form of
energy of position. By lighting the coal, or in other words separating
its molecules more widely by heat, we enable them to exert once more
their natural affinity for oxygen, and burn, that is recombine into
carbonic dioxide. The heat thus produced turns water into steam, which
passes through a cylinder, either into a condenser if the steam is at low
pressure, or into the outer air if it has been superheated and brought
to a higher pressure than that of the atmosphere. The difference of the
pressure or elasticity of the steam in the boiler, and of the same steam
when it is condensed or liberated, is available for doing work, and,
being admitted and released alternately at the two ends of the cylinder,
drives a piston up and down, which, by means of cranks and shafts,
turns a wheel or does whatever work is required of it. In doing this,
heat disappears, being converted into work, and the amount of heat would
exactly equal that into which the work would be converted according to
Joule’s law, if it could all be utilised without the loss necessarily
incurred by friction, radiation, and the still more important absorption
of latent heat required to convert water at boiling-point into vapour
of the same temperature. This latter is not really an annihilation of
the heat, but its conversion into work done in separating the molecules
against the force of cohesion. The whole heat, therefore, is transformed
into work, mainly molecular work in tearing molecules asunder, and the
residue into mechanical work turning spindles and driving locomotives and
steamboats.
The intermediate machinery here, including the water in the boiler, is
merely the means of applying the original energy in the particular way
we desire. The essential thing is the transformation of a certain amount
of heat into work by passing, in accordance with the laws of heat, from
a hotter to a colder body. The last condition is indispensable, for the
nature of heat is to seek an equilibrium by passing from hot to cold, and
no work can be got out of it in the reverse way. On the contrary, work
must be expended and turned into heat to restore the temperature which
has run down. The case is analogous to that of water, which, if raised by
evaporation or stored up in reservoirs at a level above the sea, can be
made to turn a wheel while it is running down; but when it has all run
down to the sea level, can do no more work, and can only be pumped up
again to a higher level by the expenditure of fresh work. Owing to this
tendency of heat we can see that, although matter and energy are to all
appearance indestructible, the present constitution of the universe is
not eternal. The animating energy of heat is always tending to obliterate
differences of temperature, and bring all energy down to one uniform dead
level of a common average, in which no further life, work, or motion are
possible. Fortunately this consummation is far off, and for many tens
or hundreds of millions of years the inhabitants of this tiny planet
may feel fairly secure, and need not, like the late Dr. Cumming, of
millenarian celebrity, introduce breaks in the leases of their houses to
provide against the contingency of the world coming to an end at an early
date.
Dismissing, then, to the remote future any speculations as to the failure
of this essential element of active energy, let us rather consider the
various protean forms in which it shows itself.
1. The energy of visible motion, which, as we have seen, may be
transformed into an equivalent amount of energy of position.
2. Molecular energy, which causes the cohesive attraction, repulsion, and
other proper motions of these minute and invisible particles of matter.
3. Energy of heat and light, which are transmitted by waves of the
assumed imponderable medium called ether.
4. Energy of chemical action, by which the small ultimate particles of
ponderable matter, called atoms, separate and combine into the various
combinations of molecules constituting visible matter, in obedience to
certain affinities, or inherent attractions and repulsions.
5. Electrical energy, which includes magnetism as a special instance.
All these forms of energy may exist, as in the case of visible energy,
either as energies of motion or of position, and the actual constitution
of the universe is due in a great measure to the alternation of these
two energies. Thus all wave-motion, whether it be of the waves of the
sea grinding down a rocky coast, of the air transmitting sound, or of
ether transmitting light and heat, are instances of energies of motion
and of position, conflicting with one another and alternately gaining
the victory. So also a pound of gunpowder or dynamite has an immense
energy of position, which, when its atoms are let loose from their
mutual unstable connection by heat or percussion, manifests itself in an
enormous energy of motion, which is more or less destructive according to
the rapidity with which the atoms rush into new combinations.
Let us consider these different energies a little more in detail. The
energy of visible motion is manifested principally by the law of gravity,
under which all matter attracts other matter directly as the mass and
inversely as the square of the distance. It is a universal and uniform
law of matter, and can be traced without change or variation from the
minutest atom up to the remotest double star. The energy of living force
might, at first sight, be considered as another of the commonest causes
of visible motion; but, when closely analysed, it will be found that what
appears as such is only the result of molecular energy of position stored
up in the living body by chemical changes during the slow combustion of
food, and that nothing has been added by any hypothetical vital force.
The conscious will seems to act in those cases simply as the signalman
who shows a white flag may act on a train which has been standing on the
line waiting for it. The energy which moves the train is due entirely
to the difference of heat, which has been developed by the combustion
of coal, between the steam in the boiler and the steam when allowed to
escape into the air; and this energy came originally from the sun, whose
rays enabled the leaves of growing plants to decompose carbonic dioxide
and store up the carbon in the coal. Of this force of gravity causing
visible motion we may say that it is comparatively a very weak force,
which acts uniformly over all distances great or small.
Molecular energies, on the other hand, act with vastly greater force, but
at very small distances, and appear sometimes as attractive and sometimes
as repulsive forces. Thus solid bodies are held together by a force of
cohesion which is very powerful, but acts only at very small distances,
as we may see if we break a piece of glass and try to mend it by pressing
the broken edges together. We cannot bring them near enough to bring the
molecular attraction again into play and make the broken glass solid.
But the same glass acts with repellent energy if another solid tries
to penetrate it, so that we can walk on a glass floor without sinking
into it. Heat also, by increasing the distance between the molecules,
first weakens the cohesive force so that the solid becomes fluid, and
finally overcomes it altogether, so that it passes into the state of gas
in which the centripetal attraction of the molecules is extinguished,
and they tend to recede further and further from each other under the
centrifugal force of their own proper velocities. The great energy of
molecular forces will be apparent from the fact that a bar of iron, in
cooling 10° Centigrade, contracts with a force equal to a ton for each
square inch of section, as exemplified in the tubular bridge across the
Menai Straits, where space has to be allowed for the free contraction and
expansion of the iron under changes of temperature.
Chemical energy, or the mutual attractions and repulsions of atoms, is
even more powerful than that of molecules. It displays itself in their
elective affinities, or what may be called the likes and dislikes,
or loves and hatreds, of these ultimate particles. Perhaps the best
illustration will be afforded by that ‘latest resource of civilisation,’
dynamite. This substance, or to give it its scientific name,
nitro-glycerine, is composed of molecules each of which is a complex
combination of nine atoms of oxygen, five of hydrogen, three of nitrogen,
and three of carbon. Of these, oxygen and hydrogen have a strong affinity
for one another, as is seen by their rushing together whenever they get
the chance, and by their union forming the very stable compound, water.
Oxygen and carbon have also a very strong affinity, and readily form
the stable product carbonic dioxide gas. Nitrogen, on the other hand,
is a very inert substance; its molecule consists of two atoms of itself
which are bound together by a strong affinity, and can only be coaxed
with difficulty into combinations with other elements, forming compounds
which are, as it were, artificial structures, and very unstable. We
see this in the air, which consists mainly of oxygen and nitrogen, but
not in chemical combination, the oxygen being simply diluted by the
nitrogen, as whisky is with water, with the same object of diluting the
too powerful oxygen or too potent alcohol, and enabling the air-breather
or whisky-drinker to take them into the system without burning up the
tissues too rapidly. If nitrogen had more affinity for oxygen it would
combine chemically with it, and we should live in an atmosphere of
nitrous oxide, or laughing gas.
The molecule, therefore, of nitro-glycerine resembles a house of cards,
so nicely balanced that it will just stand, but will fall to pieces at
the slightest touch. When this is supplied by a slight percussion the
molecule falls to pieces and is resolved into its constituent atoms,
which rush together in accordance with their natural affinities, forming
an immense volume of gas, partly of water in the form of steam where
oxygen has combined with hydrogen, and partly of carbonic dioxide where
it has combined with carbon, leaving the nitrogen atoms to pair off, and
revert to their original form of two-atom molecules of nitrogen gas.
It is as if ill-assorted couples, who had been united by matrimonial
bonds tied by the manœuvres of Belgravian mothers, found themselves
suddenly freed by a decree of divorce _a vinculo matrimonii_, and rushed
impetuously into each other’s arms, according to the laws of their
respective affinities. So striking is the similitude that one of Goethe’s
best-known novels, the ‘Wahlverwandschaften,’ takes its title from the
human play of these chemical reactions. The enormous energy developed
when these atomic forces are let loose and a vast volume of gas almost
instantaneously created, is attested by the destructive force by which
the hardest rocks are shattered to pieces and the strongest buildings
overthrown.
These loves and hatreds, or, as they are termed, chemical affinities and
repulsions of the atoms, are the principal means by which the material
structure of the universe is built up from the original elements.
The earth, or solid crust of the planet we inhabit, consists mainly
of oxidised bases, and is due to the affinity of oxygen for silicon,
calcium, aluminium, iron, and other primary elements of what are called
metals. This affinity enables them to make stable compounds, which, under
the existing conditions of temperature and otherwise, hold together and
are not readily decomposed. Water in like manner, in all its forms of
waves, seas, lakes, rivers, clouds, and invisible vapour, is due to the
affinity between oxygen and hydrogen forming a stable compound. Salt
again is owing to the affinity of chlorine for sodium, and so for nearly
all the various products with which we are familiar, oxygen and nitrogen
in the air we breathe being almost the only elements which exist in their
primary and uncombined state in any considerable quantities, and form
an essential part of the conditions which render our planet a habitable
abode for man and other forms of life.
We shall see presently something more of the nature of these affinities,
and the laws by which they act; but before entering on this branch
of the subject we must consider the remaining form in which the one
indestructible energy of the universe manifests itself, viz. that of
electricity.
Electricity is the most subtle and the least understood of these forms.
In its simplest form it appears as the result of friction between
dissimilar substances. Thus if we rub a glass rod with a piece of silk,
taking care that both are warm and dry, we find that the glass has
acquired the property of attracting light bodies, such as little bits
of paper, or balls of elder-pith. Other substances, such as sealing-wax
and amber, have the same property. Pursuing our research further we find
that this influence is not, like that of gravity, uniform and always
acting in the same direction, but of two kinds, equal and opposite. If we
touch the pith-ball by the excited glass rod, it will after contact be
repelled; but if we bring the ball which has been excited by contact with
the glass within the influence of a stick of sealing-wax which has been
excited by rubbing it with warm dry flannel, the ball instead of being
repelled is attracted.
Conversely, if the pith-ball has been first touched by excited
sealing-wax, it will afterwards be repelled by excited sealing-wax
and attracted by excited glass. It is clear, therefore, that there
are two opposite electricities, and that bodies charged with similar
electricities repel, and with unlike electricities attract, one another.
For convenience, one of these electricities, that developed in glass,
is called positive, and the other negative; and it has been clearly
proved that one cannot exist without the other, and that whenever
one electricity is produced, just as much is produced of an opposite
description. If positive electricity is produced in glass by rubbing it
with silk, just as much negative electricity is produced upon the silk.
Another primary fact is that some substances are able to carry away
and diffuse or neutralise this peculiar influence called electricity,
while others are unable to do so and retain it. The former are called
conductors, the latter non-conductors. Thus, glass is an insulator or
non-conductor, while metal is a conductor of electricity; and the reason
why the substances rubbed together, as glass and silk, must be dry is
that water, in all its forms, is a conductor which carries away the
electricity as fast as it is produced.
These facts have given rise to a theory—which is after all not so
much an explanation as a convenient mode of expressing the facts—of
the existence of two opposite electric fluids, which, in the ordinary
or unexcited body, are combined and neutralise one another, but are
separated by friction, and flow in opposite directions, accumulating at
opposite poles, or, it may be, one being accumulated at one pole, while
the other is diffused through some conducting medium and lost sight of.
The active electricity, be it positive or negative, thus accumulated at
one pole, and retained there by the substance in contact with it being a
non-conductor, disturbs by its influence the electrical equilibrium of
any body brought near to it, separates its two fluids, and attracts the
one opposite to itself. This attraction draws the light body towards it
until contact ensues, when the electric fluid of the excited body flows
into the smaller one, so that its opposite electricity is expelled, and
it is in the same condition as its exciter, and therefore liable to be
repelled by a similar exciter, or attracted by an opposite one which
formerly repelled it.
It is evident, without going further, that there is a great analogy
between electrical energy and those of heat and of chemical affinity.
The same mechanical work—viz. friction—which generates heat, generates
electricity. The chief difference seems to be that friction may be
transformed into heat when the same substances are rubbed together, as
in the case of obtaining fire by the friction of wood; but electricity
can only be obtained by friction between dissimilar substances. Thus
no electricity is obtained by rubbing glass upon glass, or silk upon
silk, or upon glass covered with silk, though a slight difference of
texture is sometimes sufficient to separate the electric fluids. Thus
if two pieces of the same silk ribbon are rubbed together, lengthways,
no electricity is produced, but if crossways, one is positively, and the
other negatively, electrified. In this respect the analogy is evident to
chemical affinity, which, in like manner, only acts between dissimilar
bodies.
In order, however, to carry the proof of the identity of these forms of
energy beyond the sphere of vague analogy, we must follow up electricity
far beyond the simple manifestations of the glass rod and sealing-wax,
and pursue it to its origin, in the transformations of chemical action
and mechanical work, in the voltaic battery, the electric telegraph, the
telephone, and the dynamo.
The voltaic battery, in its simplest form, is a trough containing an acid
liquid in which pairs of plates of different metals are immersed. It is
evident that if the action of the acid on each metal were precisely the
same, equal quantities of each would be dissolved in the acid, and the
equilibrium of chemical energies would not be affected. But, the action
being different, this equilibrium is disturbed, and if the sum of these
disturbances for a number of separate pairs of plates can be accumulated,
it will become considerable. This is done by connecting the plates of the
same metal in each cell by a metallic wire covered by some non-conducting
substance. There are, therefore, two wires, one to the right hand, the
other to the left, the loose extremities of which are called the poles
of the battery. If we test these poles as we did the glass rod and stick
of sealing-wax, we find that one pole is charged with positive and the
other with negative electricity. In other words, the chemical energy,
whose equilibrium was disturbed by the unequal action of the acid on the
plates of different metals, has been transformed into electrical energy
manifesting itself, as it always does, under the condition of two equal
and opposite polarities. If we connect these two poles with one another
the two electricities rush together and unite, and there is established
what is called an electrical current circulating round the battery.
As the chemical action of the acid on the metals is not momentary but
continuous, the acid taking up molecule after molecule of the metal,
so also the current is continuous. When we call it a current, the term
is used for the sake of convenience, for as the current, as we shall
presently see, will flow along the wire or other conducting substance
for immense distances, as across the Atlantic, with a velocity of many
thousands of miles per second, we can, no more than in the case of light,
figure it to ourselves as an actual transfer of material particles swept
along as by a river running with this enormous velocity, but necessarily
as a transmission of some form of motion travelling by waves or tremors
through the all-pervading ether in which the atoms of the conducting
wire are floating. Be this as it may, the effect of these electric
currents is very varied and very energetic. It can produce intense heat,
for if, instead of uniting the two poles, we connect them by a thin
platinum wire, it will, in a few seconds, become heated to redness. If
the connecting wire is thicker, heat will equally be generated but less
intense, thus maintaining the analogy to the current which rushes with
more impetuosity through a narrow than through a wide channel. If the
poles are tipped with a solid substance like carbon, whose particles
remain solid under great heat, when they are brought nearly together
intense light is produced and the carbon slowly burns away. This produces
what is called the arc light, which gives such a strong illuminating
power and is coming into general use for lighting up large spaces.
Another transformation is back again into chemical energy, which is shown
by the power of the electric current to decompose compound substances.
If, for instance, the poles of a battery are plunged into a vessel
containing water, the molecules of the water will be decomposed and
bubbles of oxygen gas will rise from the positive, and of hydrogen from
the negative, pole.
Another effect of electrical currents is that of attraction and repulsion
on one another. If two parallel wires, free to move, carry currents
flowing in the same direction as from positive to negative, or _vice
versâ_, they will attract one another; if in opposite directions, they
will repel. Electrical currents also work by way of induction, that is,
they disturb the electrical equilibrium of bodies brought within their
influence and induce currents in them. Thus, if we have two circular
coils of insulated wire placed near each other, one on the right hand,
the other on the left, and connect the extremities of the right-hand coil
with the poles of a battery, when the connection is first made and the
current begins to flow, a momentary current in the opposite direction
will pass through the left-hand coil. This will cease, and as long as
the current continues to flow through the right-hand coil there will be
no current through the other; but if we break the contact between the
right-hand coil and the battery, there will be again a momentary current
through the left-hand coil, but this time in the same direction as the
other. The same effect will be produced if, instead of making and
breaking contact in the right-hand coil, we keep the current constantly
flowing through it, and make the right-hand coil alternately approach
and recede from the other coil. In this case, when the right-hand coil
approaches, it induces an opposite current in the left-hand one; and when
it recedes, one in the same direction as that of the primary.
These phenomena of induction prepare us to understand the nature of
magnets, and the magnetic effects produced by electrical currents. If
an insulated wire is wrapped round a cylinder of soft or unmagnetic
iron, and a current passed through the wire, the cylinder is converted
into a magnet and becomes able to sustain weights. If the current
ceases, the cylinder is no longer a magnet, and drops the weight. A
magnet is therefore evidently a substance in which electric currents are
circulating at right angles to its axis, and a permanent magnet is one in
which such currents permanently circulate from the constitution of the
body without being supplied from without. The earth is such a magnet, and
also iron and other substances, under certain conditions.
This being established, it is easy to see why an electrical current
deflects the magnetic needle. If such a needle is suspended freely near
a wire parallel with it, on a current being passed through the wire it
must attract if similar, or repel if dissimilar, the currents which are
circulating at right angles to the axis of the needle, and thus tend to
make the needle swing into a position at right angles with the wire so
that its currents may be parallel to that of the needle. This is the
reason why the needle in its ordinary condition points to the north
and south, or rather to the magnetic poles of the earth, because its
currents are influenced by the earth currents which circulate parallel to
the magnetic equator. The deviation of the needle from this direction,
caused by any other current, like that passed along the wire, will depend
on the strength of the current, which may be measured by the amount of
deflection of the needle. The direction in which the needle deflects,
viz. whether the north pole swings to the right or to the left, will
depend on the direction of the current through the wire. The direction
of the circular currents which form a magnet is such that if you look
towards the north pole of a freely suspended cylindrical magnet—i.e. if
you stand on the north of it and look southwards—the positive current
will ascend on your right hand, or on the west side, and descend on the
east. It follows that unlike poles must necessarily attract, and like
poles repel one another, for in the former case the circular currents
which face each other are going in the same, and in the latter in
opposite directions.
The reader is now in a position to understand the principle of the
electric telegraph, that wonderful invention which has revolutionised
human intercourse and, to a great extent, annihilated space and time. It
originated in the discovery made by Oersted, a Danish _savant_, that the
effect of an electric current was to make a magnet swing round, in the
endeavour to place itself at right angles to it. The conducting power of
insulated copper wire is such that it practically makes no difference
whether one of the wires connected with the pole of a battery is two
feet or 2,000 miles in length, and the earth, being a conducting medium,
supplies an equal extension from the other pole, so that a closed
electric circuit may be established across the Atlantic as easily as
within the walls of a laboratory.
If, therefore, a magnetic needle is suspended at the American end, it
will respond to every electrical current, and to any interruption,
renewal, or reversal of that current established in England. The needle
may thus be made to swing to the right or left, by forming or reversing a
current through the wire; and it will return to its position whenever the
current is interrupted, and repeat its movement whenever the current is
renewed. In fact it may be made to move like the arm of the old-fashioned
telegraph, or of a railway signal. It only remains to have a machine by
which the operator can form and interrupt currents rapidly, and a code by
which certain movements of the needle stand for certain letters of the
alphabet, and you have the electric telegraph.
There are many ingenious applications of the machinery, but in principle
they all resolve themselves into transformations of energy. Chemical
energy is transformed into electric energy, and that again into
mechanical work in moving the needle.
The telephone is another instance of similar transformations. Here spoken
words create vibrations of the air, which cause corresponding vibrations
in a thin plate or disc of metal at one end, which are conveyed by
intermediate machinery to a similar disc at the other end, whose
vibrations cause similar vibrations in the air, reproducing the spoken
words at a distance which may be a great many miles from the speaker.
The great inventions of modern science which have so revolutionised
society are all instances of the laws of the conservation of energy. Man
makes the powers of nature available for his purposes by transforming
them backwards and forwards, now into one, now into another form of
energy, as required for the result he wishes to attain. He wants
mechanical power to pump water or drive a locomotive or steamboat: he
gets it from the steam-engine, by transforming the energy of heat in
coal, which came ages ago from the energy of chemical action produced
by the sun’s rays in the green leaves of growing plants. He wants to
send messages in a few seconds across the Atlantic: he does it by
transforming chemical energy into electricity in a voltaic battery,
sending its vibrations along a conducting wire, and converting it at
the far end into mechanical power, making a magnetic needle turn on its
axis and give signals. If, instead of sending a message, he wants to
hold a conversation at a distance, he invents the telephone, by which
sound-vibrations of air are transformed into vibrations of a disc, then
into electric currents, then into vibrations of a distant disc, and
finally back again to spoken words. Or, if he wants light, he turns
electricity into it by tipping the poles of his battery with carbon and
bringing them close together.
The latest inventions of electrical science—the dynamo and the
accumulator—afford remarkable instances of this convertibility of one
primitive energy into different forms. In the instance just quoted of
obtaining light from electricity by the voltaic battery, the cost has
hitherto proved an obstacle to its adoption. The electrical energy is
all obtained from the transformation of the heat produced in the cells
by the chemical action on the metal used, which is commonly zinc. Now,
the heat of combination of zinc with oxygen is only about one-sixth of
that of coal, while the cost of zinc is about twenty times as great.
Theoretically, therefore, energy got by burning zinc costs 120 times
as much as that got by burning coal. Practically the difference is not
nearly so great, for there is very little loss of energy in the battery
by the process of conversion, while the best steam-engine cannot convert
into work as much as twenty per cent, of the heat energy in the coal
consumed. Still, after making every allowance, the cost of energy from
zinc remains some twenty times as great as from coal, so that unless some
process is found for obtaining back the zinc as a residual product, there
is no prospect of this form of electricity being generally available for
light or for mechanical power.
The dynamo is an instrument invented for the mechanical generation of
electricity by taking advantage of the principle that electrical energy
is produced by moving magnets near coils of wire, or coils of wire near
magnets. A current is thus started by induction, and, once started,
the mechanical power exerted in making the magnet or coils revolve is
continually converted into electricity until the accumulated electrical
energy becomes very powerful. The original energy comes of course from
the coal burned in the steam-engine which makes the magnet or coils
revolve.
The principle of the conservation of energy is well illustrated by the
fact that as the dynamo generates an electric current if made to revolve,
conversely it may be made to revolve itself if an electric current is
sent through it from an exterior source. It is, therefore, available not
only as a source of light in the former case, but as a direct source of
mechanical power in the latter. It is on this principle that electric
engines are constructed and electric railways are worked. Here also it
is a question of cost and convenience, for you can only get electricity
enough either to light a street or to drive an engine, by an original
steam-engine or other motive power to work the dynamo, and a system of
conducting wires to convey the electricity to the place where the light
or power is wanted. Where the motive power is supplied by nature, as in
the case of tidal or river currents or waterfalls, it is quite possible
that power may be obtained in this way to compete with that obtained
directly from the steam-engine; but there are as yet considerable
practical difficulties to be overcome in the transmission of any large
amount of energy for long distances.
To overcome some of these difficulties the accumulator has been invented,
which affords yet another remarkable instance of the transformation of
energy. It consists of two lead plates immersed in acidulated water. When
a strong electrical current is sent through the water, it decomposes it,
the oxygen going to one lead plate and the hydrogen to the other. The
oxygen attacks the lead plate to which it goes, forming peroxide of lead;
while the hydrogen reduces any oxide in the other plate, producing pure
lead, and leaving a film of surplus hydrogen on the surface. The charging
current is then reversed, so that the latter plate is now attacked and
the former one reduced, when the current is again reversed. By continuing
this process the surfaces of both lead plates become porous, so that they
present a large surface, and can therefore hold a great deal of peroxide
of lead. The charging current being now broken, the oxygen which has been
forcibly separated from the liquid seeks to recombine with hydrogen;
and if the two lead plates are joined by a wire, this effort of the
oxygen generates an electrical current in the opposite direction to the
original one, which is the current utilised. Electricity is thus stored
up in a portable box, where it can be kept till wanted, when it is drawn
out by connecting the plates, and as a large amount of energy has been
accumulated the current which is produced lasts for a considerable time.
Unfortunately accumulators are bulky, heavy, and expensive, and nearly
half the energy of the original charging current is lost in obtaining the
reversed or working current. They are therefore not as yet adapted for
general use, though perfectly capable of supplying either light or motive
power, for both which purposes they have been successfully applied in
special cases. The future both of electric power and electric lighting
is now reduced entirely to a question of cost; and though it is hard to
beat gas and the steam-engine, with cheap coal, and air and water for
nothing, it is possible that by using natural sources of power to move
dynamos, and by obtaining zinc back as a residual product in batteries,
electricity may in certain cases carry the day.