SOME OF THE RESULTS OF FARADAY'S DISCOVERIES, AND THE PRINCIPLE OF
ENERGY.
In early days, _the spirit of the amber_, when aroused by rubbing,
came forth and took to itself such light objects as it could easily
lift. Later on, and the spirit gave place to the _electric effluvium_,
which proceeded from the excited, or charged, body into the
surrounding space. Still later, and a fluid, or two fluids, acting
directly upon itself, or upon matter, or on one another, through
intervening space without the aid of intermediate mechanism, took the
place of the electric effluvium--a step which in itself was, perhaps,
hardly an advance. Then came the time for accurate measurement. The
simple _observation_ of phenomena and of the results of experiment
must be the first step in science, and its importance cannot be
over-estimated; but before any quantity can be said to be known, we
must have learned how to _measure_ it and to reproduce it in definite
amounts. The great law of electrical action, the same as that of
gravitation--the law of the inverse square--soon followed, as well as
the associated fact that the electrification of a conductor resides
wholly on its surface, and there only in a layer whose thickness is
too small to be discovered. The fundamental laws of electricity having
thus been established, there was no limit to the application of
mathematical methods to the problems of the science, and, in the hands
of the French mathematicians, the theory made rapid advances. George
Green, of Sneinton, Nottingham, introduced the term "potential" in an
essay published by subscription, in Nottingham, in 1828, and to him we
are indebted for some of our most powerful analytical methods of
dealing with the subject; but his work remained unappreciated and
almost unknown until many of his theorems had been rediscovered. But
the idea of a body acting where it is not, and without any conceivable
mechanism to connect it with that upon which it operates, is repulsive
to the minds of most; and, however well such a theory may lend itself
to mathematical treatment and its consequences be borne out by
experiment, we still feel that we have not solved the problem until we
have traced out the hidden mechanism. The pull of the bell-rope is
followed by the tinkling of the distant bell, but the young
philosopher is not satisfied with such knowledge, but must learn "what
is the particular go of that." This universal desire found its
exponent in Faraday, whose imagination beheld "lines" or "tubes of
force" connecting every body with every other body on which it acted.
To his mind these lines or tubes had just as real an existence as the
bell-wire, and were far better adapted to their special purposes.
Maxwell, as we have seen, not only showed that Faraday's system
admitted of the same rigorous mathematical treatment as the older
theory, and stood the test as well, but he gave reality to Faraday's
views by picturing a mechanism capable of doing all that Faraday
required of it, and of transmitting light as well. Thus the problem of
electric, magnetic, and electro-magnetic actions was reduced to that
of strains and stresses in a medium the constitution of which was
pictured to the imagination. Were this theory verified, we might say
that we know at least as much about these actions as we know about the
transmission of pressure or tension through a solid.
With regard to the _nature_ of electricity, it must be admitted that
our knowledge is chiefly negative; but, before deploring this, it is
worth while to inquire what we mean by saying that we know what a
thing is. A definition describes a thing in terms of other things
simpler, or more familiar to us, than itself. If, for instance, we say
that heat is a form of energy, we know at once its relationship to
matter and to motion, and are content; we have described the
constitution of heat in terms of simpler things, which are more
familiar to us, and of which we _think_ we know the nature. But if we
ask what _matter_ is, we are unable to define it in terms of anything
simpler than itself, and can only trust to daily experience to teach
us more and more of its properties; unless, indeed, we accept the
theory of the vortex atoms of Thomson and Helmholtz. This theory,
which has recently been considerably extended by Professor J. J.
Thomson, the present occupier of Clerk Maxwell's chair in the
University of Cambridge, supposes the existence of a perfect fluid,
filling all space, in which minute whirlpools, or vortices, which in a
perfect fluid can be created or destroyed only by superhuman agency,
form material atoms. These are _atoms_, that is to say, they defy any
attempts to sever them, not because they are infinitely hard, but
because they have an infinite capacity for _wriggling_, and thus avoid
direct contact with any other atoms that come in their way. Perhaps a
theory of electricity consistent with this theory of matter may be
developed in the future; but, setting aside these theories, we may
possibly say that we know as much about electricity as we know about
matter; for while we are conversant with many of the properties of
each, we _know_ nothing of the ultimate nature of either.
But while the theory of electricity has scarcely advanced beyond the
point at which it was left by Clerk Maxwell, the practical
applications of the science have experienced great developments of
late years. Less than a century ago the lightning-rod was the only
practical outcome of electrical investigations which could be said to
have any real value. [OE]rsted's discovery, in 1820, of the action of
a current on a magnet, led, in the hands of Wheatstone, Cooke, and
others, to the development of the electric telegraph. Sir William
Thomson's employment of a beam of light reflected from a tiny mirror
attached to the magnet of the galvanometer enabled signals to be read
when only extremely feeble currents were available, and thus rendered
submarine telegraphy possible through very great distances. The
discovery by Arago and Davy, that a current of electricity flowing in
a coil surrounding an iron bar would convert the bar into a magnet, at
once rendered possible a variety of contrivances whereby a current of
electricity could be employed to produce small reciprocating
movements, or even continuous rotation, where not much power was
required, at a distance from the battery. An illustration of the
former is found in the common electric bell; it is only necessary that
the vibrating armature should form part of the circuit of the
electro-magnet, and be so arranged that, while it is held away from
the magnet by a spring, it completes the battery circuit, but breaks
the connection as soon as it moves towards the magnet under the
magnetic attraction. To produce continuous rotation, a number of iron
bars may be attached to a fly-wheel, and pass very close to the poles
of the magnet without touching them; when a bar is near the magnet,
and approaching it, contact should be made in the circuit, but should
be broken, so that the magnet may lose its power, as soon as the bar
has passed the poles; or the continuous rotation may be produced from
an oscillating armature by any of the mechanical contrivances usually
adopted for the conversion of reciprocating into continuous circular
motion. But all such motors are extremely wasteful in their employment
of energy. Faraday's discovery of the rotation of a wire around a
magnetic pole laid the foundation for a great variety of
electro-motors, in some of which the efficiency has attained a very
high standard. About ten years ago, Clerk Maxwell said that the
greatest discovery of recent times was the "reversibility" of the
Gramme machine, that is, the possibility of causing the armature to
rotate between the field-magnets by sending a current through the
coils. The electro-motors of to-day differ but little from dynamos in
the principles of their construction. The copper disc spinning between
the poles of a magnet while an electric current was sent from the
centre to the circumference, or _vice versa_, formed the simplest
electro-motor. All the later motors are simply modifications of this,
designed to increase the efficiency or power of the machine.
Similarly, the earliest machine for the production of an electric
current at the expense of mechanical power only, but through the
intervention of a permanent magnet, was the rotating disc of Faraday,
described on page 262. This contrivance, however, caused a waste of
nearly all the energy employed, for while there was an electro-motive
force from the centre to the circumference, or in the reverse
direction, in that part of the disc which was passing between the
poles of the magnet, the current so generated found its readiest
return path through the other portions of the disc, and very little
traversed the galvanometer or other external circuit. This source of
waste could be, for the most part, got rid of by cutting the disc into
a number of separate rays, or spokes, and filling up the spaces
between them with insulating material. The current then generated in
the disc would be obliged to complete its circuit through the external
conductor. If we can so arrange matters as to employ at once several
turns of a continuous wire in place of one arm, or ray, of the copper
disc, we may multiply in a corresponding manner the electro-motive
force induced by a given speed of rotation. All magneto-electric
generators are simply contrivances with this object. The iron cores
frequently employed within the coils of the armature tend to
concentrate the lines of force of the magnet, causing a greater number
to pass through the coils in certain positions than would pass through
them were no iron present. The electro-motive force of such a
generator depends on the strength of the magnetic field, the length of
wire employed in cutting the lines of force, and the speed with which
the wire moves across these lines. The point to aim at in constructing
an armature is to make the resistance as small as possible consistent
with the electro-motive force required. As there is a limit to the
strength of the magnetic field, it follows that for strong currents,
where thick wire must be employed, the generator must be made of large
dimensions, or the armature must be driven at very high speed to
enable a shorter length of wire to be used.
The so-called "compound-interest principle," by which a very small
charge of electricity might be employed to develop a very large one by
the help of mechanical power, was first applied about a century ago in
the revolving doubler. Long afterwards, Sir William Thomson availed
himself of the same principle in the construction of the "mouse-mill,"
or replenisher. The Holtz machine, the Voss and Wimshurst machines,
and the other induction-machines of the same class, all work on this
principle. It may be illustrated as follows: Take two canisters, call
them A and B, and place them on glass supports. Let a very small
positive charge be given to A, B remaining uncharged. Now take a brass
ball, supported by a silk string. Place it inside A, and let it touch
its interior surface. The ball will, as shown by Franklin, Cavendish,
and Faraday, remain uncharged. Now raise it near the top of the
canister, and, while there, touch it. The ball will become negatively
electrified, because the small positive charge in A will attract
negative electricity from the earth into the ball. Take the ball, with
its negative charge, still hanging by the silk thread, and lower it
into B till it touches the bottom. It will give all its charge to B,
which will thus acquire a slight negative charge. Raise the ball till
it is near the top of B, and then touch it with the finger or a metal
rod. It will receive a positive charge from the earth because of the
attraction of the negative charge on B. Now remove the ball and let it
again touch the interior of A. It will give up all its charge to A;
and then, repeating the whole cycle of operations, the charge carried
on the ball will be greater than before, and increase in each
successive operation, the electrification increasing in geometrical
progression like compound interest. A Leyden jar having one coating
connected to A and the other to B, may thus be highly charged in
course of time. A pair of carrier balls or plates, or a number of
pairs, may be used instead of one. The carriers, just before leaving A
and B, may be put in contact with one another instead of being put to
earth; they may be mounted on a revolving shaft, and the forms of A
and B modified to admit of the revolution of the carriers, and all the
necessary contacts may be made automatically. We thus get various
forms of the continuous electrophorus, and if the carriers are mounted
on glass plates, and rows of points placed alongside the springs or
brushes used for making the contacts, when the charges on the carriers
become very strong, electricity will be radiated from the points on to
the revolving glass plates, which will thus themselves take the place
of the metal carriers. Such is the action in the Voss and other
similar machines.
But after Faraday had shown how to construct a magneto-electric
machine, the idea of applying the "compound-interest principle," and
thus converting the magneto-electric machine into the "dynamo,"
occurred apparently simultaneously and independently to Siemens,
Varley, and Wheatstone. The first dynamo constructed by Wheatstone is
still in the museum of King's College, London. Wilde employed a
magneto-electric machine to generate a current which was used to
excite the electro-magnet of a similar but larger machine, having an
electro-magnet instead of a permanent steel magnet. The electro-magnet
could be made much larger and stronger than the steel magnet, and from
its armature, when made to revolve by steam power, a correspondingly
stronger current could be maintained. The idea which occurred to
Siemens, Varley, and Wheatstone was to use the whole, or a part, of
the current produced by the armature to excite its own electro-magnet,
and thus to dispense with the magneto-electric machine which served as
the separate exciter. When a part only of the current is thus
employed, and is set apart entirely for this duty, the machine is a
"shunt dynamo;" when the whole of the current traverses the
field-magnet coils as well as the external circuit, it is a "series
dynamo." The apparent difficulty lies in starting the current, but a
mass of iron once magnetized always retains a certain amount of
"residual magnetism," unless special means are taken to get rid of
it, and even then the earth's magnetism would generally induce
sufficient in the iron to start the action. Commencing, then, with a
slight trace of residual magnetism, the revolution of the armature
generates a feeble current, which passing round the magnet coils,
strengthens the magnetism, whereupon a stronger current is generated,
which in turn makes the magnet still stronger, and so on until the
magnet becomes saturated or the limit of power of the engine is
reached, and the speed begins to diminish, or a condition of affairs
is reached at which an increased current in the armature injures the
magnetic field as much as the corresponding increase in the
field-magnet coils strengthens it, and then no further increase of
current will take place without increasing the speed of rotation. In a
true dynamo the whole of the energy, both of the current and of the
electro-magnets, is obtained from the source of power employed in
driving the machine.
But Faraday's discovery of electro-magnetic induction led to practical
developments in other directions. Graham Bell placed a thin iron disc
in front of the pole of a bar magnet, and wound a coil of fine wire
round the bar very near the pole. The ends of the coils of two such
instruments he connected together. When the iron disc of one
instrument approached the pole of the magnet, the lines of force were
disturbed, fewer escaped radially from the bar, and more left it at
the end, so as to go straight to the iron disc; thus the number of
lines of force passing through the coil was altered, and a current was
induced which, passing round the coil of the other instrument,
strengthened or weakened its magnet, and caused the iron disc to
approach it or recede from it, according to the way in which the coils
were coupled. Thus the movements of the first disc were faithfully
repeated by the second, and the minute vibrations set up in the disc
by sound-waves were all faithfully repeated by the second instrument.
This was Graham Bell's telephone, in which the transmitter and
receiver were convertible.
But another and an earlier application of Faraday's discoveries is
found in the induction coil. A short length of thick wire and a very
great length of thin wire are wound upon an iron bar. The ends of the
long thin wire, or secondary coil, form the terminals of the machine;
the short thick wire, or primary coil, is connected with a battery,
but in the circuit is placed an "interrupter." This is generally a
small piece of iron, or hammer, mounted on a steel spring opposite one
end of the iron core, the spring pressing the hammer back against a
screw the end of which, like the back of the hammer, is tipped with
platinum; and this contact completes the battery circuit. When the
current starts, the iron core becomes a magnet, attracts the hammer,
breaks the contact, stops the current, the magnetism dies away, the
hammer is forced back by the spring, and then the cycle of events is
repeated. But the starting of the current in the primary causes a
great many lines of magnetic force to pass through each of the many
thousand turns of wire in the secondary, especially as the iron core
conducts most of the lines of force of each turn of the primary almost
from end to end of the coil, and thus through nearly all the turns of
the secondary. This action might be further increased by connecting
the ends of the iron core with an iron tube or series of longitudinal
bars placed outside the whole coil. When the primary current ceases,
all these lines of force vanish. Thus during the starting of the
primary current, which, on account of self-induction, occupies a
considerable time, there will be an inverse current in the secondary
proportional to the rate of increase of the primary; and while the
primary is dying away, there will be a direct current in the secondary
proportional to its rate of decrease. The primary current cannot be
increased at a faster rate than corresponds to the power of the
battery, but by making a very sharp break it may be stopped very
rapidly. Still, however rapidly the circuit is broken, self-induction
causes a spark to fly across the gap until the energy of the current
is used up. The introduction of the condenser, consisting of a number
of sheets of tinfoil insulated by paper steeped in paraffin wax, and
connected alternately with one end or the other of the primary coil,
serves to increase the rapidity with which the primary current died
away, by rapidly using up its energy in charging the condenser, and
produces a corresponding diminution in the spark at the
contact-breaker. This rapid destruction of the primary current causes
a correspondingly great electro-motive force in the secondary coil,
and thus very long sparks are produced between the terminals of the
secondary coil when the primary current is broken, though no such
sparks are produced when the primary current starts. If the secondary
coil be connected up with a galvanometer, so that there is a metallic
circuit throughout, it will be found that just as much electricity
flows in one direction through the circuit at the break of the primary
as flows in the other direction at the make, the difference being that
the first is a very strong current of great electro-motive force but
lasting a very short time, the second a feebler current lasting a
correspondingly longer time.
* * * * *
But though the recent advances in electrical science have been very
great, the grandest triumph of this century is the establishment of
the principle of the conservation of energy, which has settled for
ever the problem of "the perpetual motion," by showing that it has no
solution. This problem was not simply to find a mechanism which should
for ever move, but one from which energy might be continuously derived
for the performance of external work--in fact, an engine which should
require no fuel. But in spite of all that has been proved, numbers of
patents are annually taken out for contrivances to effect this
object.
We have seen how Rumford showed that heat was motion, and how he
approximately determined its mechanical equivalent. Seguin, a nephew
of Montgolfier, endeavoured to show that, when a steam-engine was
working, less heat entered the condenser than when the same amount of
steam was blown idly through the engine. This Hirn succeeded in
showing, thus proving that heat was actually used up in doing work.
Mayer, of Heilbronn, measured the work done in compressing air, and
the heat generated by the compression, and assumed that the whole of
the work done in the compression, and no more, was converted into the
heat developed, which was the same thing as assuming that no work was
done in altering the positions of the particles of gas. From these
measurements he deduced a value of the mechanical equivalent of heat.
The assumption which Mayer made was shown experimentally by Joule to
be nearly correct. Joule proved that, when air expands from a high
pressure into a vacuum, no heat is generated or absorbed on the whole.
This he did by compressing air in an iron bottle, which was connected
with another bottle from which the air had been exhausted, the
connecting tube being closed by a stop-cock. The whole apparatus was
immersed in a bath of water, and on allowing the air to rush from one
vessel into the other, and then stirring the water, the temperature
was found to be the same as before. When the iron bottles were in
separate baths of water, that from which the air rushed was cooled,
and that into which it rushed was heated to the same extent. Joule and
Thomson afterwards showed that a very small amount of heat is absorbed
in this experiment. Joule also showed that the heat generated in a
battery circuit is proportional to the product of the electro-motive
force and the current, or to the product of the resistance and the
square of the current, which, in virtue of Ohm's law, is the same
thing. This relation is often known as Joule's law. He also proved
that, for the same amount of chemical action in the battery, the heat
generated was the same, whether it were all generated within the
battery or part in the battery and part in an external wire; and that
in the latter case, if the wire became so hot as to emit light, the
heat measured was less than before, on account of the energy radiated
as light. With a magneto-electric machine he employed mechanical power
to produce a current, and the energy of the current he converted into
heat. In all cases he found that, _whatever transformations the energy
might undergo in its course, a definite amount of mechanical energy,
if entirely converted into heat, always produced the same amount of
heat_; and he thereby proved, not only that heat is essentially
_motion_, but that it corresponds precisely with that particular
dynamical quantity which is called _energy_; and thus justified the
attempt to find a relation between heat and energy, or to express the
mechanical equivalent of heat as so many foot-pounds.
Joule then set to work to determine, in the most accurate manner
possible, the number of foot-pounds of work which, if entirely
converted into heat, would raise one pound of water through 1 deg. Fahr.
The best known of his experiments is that in which he caused a paddle
to revolve by means of a falling weight, and thereby to churn a
quantity of water contained in a cylindrical vessel, the rotation of
the water being prevented by fixed vanes. In these experiments he
allowed for the work done outside the vessel of water or calorimeter,
for the buoyancy of the air on the descending weight, and for the
energy still retained by the weight when it struck the floor. From the
results obtained he deduced 772 foot-pounds as the mechanical
equivalent of heat. Expressed in terms of the Centigrade scale,
Joule's equivalent, that is, the number of foot-pounds of work in the
latitude of Manchester, which, if entirely converted into heat, will
raise one pound of water 1 deg. C., is 1390.
Joule's experiments show that the same amount of energy always
corresponds to, and can be converted into, the same amount of heat,
and that no transformations, electrical or other, can ever increase or
diminish this quantity. Maxwell expressed this principle as follows:--
_The energy of a system is a quantity which can neither be increased
nor diminished by any actions taking place between the parts of the
system, though it may be transformed into any of the forms of which
energy is susceptible._
This is the great principle of the conservation of energy which is
applicable equally to all branches of science.
Another principle, almost equally general in its applicability, is
that of the dissipation of energy, for which we are indebted in the
first instance to Sir William Thomson. All forms of energy may be
converted into heat, and heat tends so to diffuse itself throughout
all bodies as to bring them to one uniform temperature. This is its
ultimate state of degradation, and from that state no methods with
which we are acquainted can transform any portion of it. When energy
is possessed by a system in consequence of the relative positions or
motions of bodies which we can handle, and whose movements we may
control, the whole of the energy may be employed in doing any work we
please; in fact, it is all _available_ for our purpose, or its
_availability_ may be said to be perfect. Energy in any other form is
limited in its availability by the conditions under which we can place
it. For example, the energy of chemical action in a battery may be
used to produce a current, and this to drive a motor by which
mechanical work is effected, but some of the energy must inevitably be
degraded into the form of heat by the resistance of the battery and of
the conductor, and this portion will be greater as the rate of doing
work is increased. The ratio of the quantity of energy which can be
employed for mechanical purposes with the means at our disposal, to
the whole amount present, is called the _availability_ of the energy.
All forms of energy may be wholly converted into heat, but only a
fraction of any quantity of heat can be transformed into higher forms
of energy, and this depends on the temperature of the source of heat
and of the coldest body which can be employed as a condenser, being
greater the greater the difference between the temperatures of the
source and condenser, and the lower the temperature of the latter. In
every operation which takes place in nature there is a degradation of
energy, and though some portion of the energy may be raised in
availability, another portion is lowered, so that on the whole the
availability is diminished. Thus, in the case of the heat-engine, work
can be obtained from heat only by allowing another portion of the heat
to fall in temperature; and, as originally stated by Sir William
Thomson, "it is impossible, by means of inanimate material agency, to
obtain mechanical effect from any portion of matter by cooling it
below the temperature of the coldest of the surrounding objects," and
to leave the working substance in the same condition in which it was
at the commencement of the operations. Accepting this principle,
Professor James Thomson showed that increase of pressure must lower
the freezing point of water, for otherwise it would be possible to
construct an engine which, working by the expansion of water in
freezing, would continue to do work by cooling a body below the
temperature of any other body available, and he calculated the amount
of pressure necessary to lower the freezing point through one degree.
The conclusion was afterwards experimentally verified by Sir William
Thomson, and served to explain all the phenomena of regelation. Thus,
like the principle of the conservation of energy, the principle of the
dissipation of energy serves as a guide in the search after truth. But
there is this difference between the two principles--no one can
conceive of any method by which to circumvent the conservation of
energy; but Clerk Maxwell showed that the principle of dissipation of
energy might be overridden by the exercise of intelligence on the part
of any creature whose faculties were sufficiently delicate to deal
with individual molecules. In the case of gases, the temperature
depends on the average energy of motion of the individual particles,
and heat consists simply of this motion; but in any mass of gas,
whatever the average energy may be, some of the particles will be
moving with very great, and some with very small, velocities. By
imagining two portions of gas, originally at the same temperature,
separated by a partition containing trap-doors which could be opened
or closed without expenditure of energy, and supposing a "demon"
placed in charge of each door, who would open the door whenever a
particle was approaching very rapidly from one side, or very slowly
from the other, but keep it shut under other circumstances, he showed
that it would be possible to sort the particles, so that those in the
one compartment should have a great velocity, and those in the other a
small one. Hence, out of a mass of gas at uniform temperature, two
portions might be obtained, one at a high temperature and the other at
a low, and, by means of a heat-engine, work could be obtained until
the two portions were again at equal temperatures, when the services
of the "demons" might be again taken advantage of, and the operations
repeated until all the heat was used up.
Any theory which is brought forward to explain a phenomenon, or any
process which is proposed to effect any operation, must in the first
instance submit to the test of the application of these two principles
of conservation and dissipation of energy; and any proposal which
fails to bear these tests may be at once rejected. The essential
feature of the science of to-day is its quantitative character. We
must, for instance, not only know that radiant energy comes to us from
the sun, but we must learn how much energy is annually received by the
earth in this way; and, in the next place, how much energy is radiated
by the sun in all directions in the same time. When we have learned
this, we want to know what is the source of this energy; and no theory
of the sun which does not enable us to explain how this constant
expenditure of energy is maintained can be accepted. Last century it
was possible to believe, with Sir William Herschel, that the greater
part of the sun's mass is comparatively cool, and that it is
surrounded by only a thin sheet of flame. To-day such a theory would
be rejected at once, simply because the thin shell of flame could not
provide energy for the solar radiation for any considerable time. The
contact theory of the galvanic cell, as originally enunciated, fell to
the ground for a similar reason. The simple contact of dissimilar
metals could afford no continuous supply of energy to sustain the
current. Applied to the steam-engine, the doctrine of energy teaches
us, not only that, corresponding to the combustion of a pound of coal,
there is a definite quantity of work which is the mechanical
equivalent of the heat generated, and is such that no engine of which
we can conceive is capable of deriving from the combustion of the
pound of coal a greater amount of work, but it teaches us that there
is a further limitation fixed to the amount of work obtainable. This
limitation depends upon the range of temperature at our command; and,
when the range is known, we can express the amount of energy
realizable by a perfect engine working through that range as a
definite fraction of the whole energy corresponding to the heat of
combustion of the fuel. Thus, if we find that a particular engine
realizes only 15 per cent. of the energy of its fuel in work done, we
must not suppose that mechanical improvements in the engine would
enable us to realize any considerable portion of the other 85 per
cent.; for it may be that a theoretically perfect engine, working with
its boiler and condenser at the same temperatures as those of the
engine considered, could only realize 25 per cent. of the energy of
the fuel, reducing the margin for improvement from 85 to 10 per cent.,
as long as the range of temperature is unaltered. To improve the
efficiency beyond this limit, the range of temperature must be
increased, that is, generally, hotter steam must be used.
The principles of energy are thus guides, not only to the scientific
theorist, but to the practical engineer, and they have been
established only through careful measurement. The simple observation
of phenomena, and of the conditions under which they occur, could
never have led to the establishment of such principles; and, though
the carrying out of experiments which do not involve measurements is
of great value, it is the careful measurement, however simple, which
affords the highest training to the mind and hand, and without which
any course of instruction in experimental physics is of little value.
The Hindoos used to regard the earth as a vast dome carried on the
backs of elephants. The elephants themselves, however, required
support, and were represented as standing on the back of a gigantic
tortoise. It does not, however, appear that any support was provided
for the tortoise. In some respects this figure represents the
apparently perpetual condition of scientific knowledge. Phenomena are
investigated, and are shown to depend upon other actions which appear
simpler or more fundamental than the phenomena at first observed.
These, again, are found to obey laws which are of much wider
application, or appear to be still more fundamental; but it may be
that we are as far off as ever from discovering the great secret of
the universe, the ultimate nature of all things.