THE EIGHTEENTH CENTURY
James Watt and the Steam-Engine
If you had visited the coal-mines of England and Scotland three hundred
years ago, you might have seen women bending under baskets of coal
toiling up spiral stairways leading from the depths of the mines. At
some of the mines horses were used. A combination of windlass and
pulleys made it possible for a horse to lift a heavy bucket of coal.
There came a time, however, when slow and crude methods such as these
could not supply the coal as fast as it was needed. The shallower mines
were being exhausted. The mines must be dug deeper. The demand for coal
was increasing. The supply of coal, it was thought, would not last until
the end of the century. The wood supply was already exhausted. It seemed
that England was facing a fuel famine.
There was only one way out of the difficulty. A machine must be invented
that would do the work of the women and horses, a machine strong enough
to raise coal with speed from the deepest mines. Then it happened that
two great inventors, Newcomen and Watt, arose to produce the machine
that was needed. When the world needs an invention it seldom fails to
appear. It is true of the world, as of an individual, that "Necessity is
the mother of invention."
In the mean time Torricelli had performed his famous barometer
experiment, and Otto von Guericke had astonished princes with proofs of
the pressure of the air. There was no apparent connection between these
experiments and the art of coal-mining, yet these discoveries made
possible the steam-engine which was to revolutionize first the
coal-mining industry and, later, the entire industrial world.
The First Steam-Engine with a Piston
The first steam-engine with a piston was made by Denys Papin, a
Frenchman. Papin had observed that, in Guericke's experiment,
air-pressure lifted several men off their feet. So he thought the air
could be made to lift heavy weights and do useful work. But how should
he produce the vacuum? His first thought was to explode gunpowder
beneath the piston. The gunpowder engine had been tried by others and
found wanting. He next turned his attention to steam, and discovered
that if the piston were forced up by steam and then the steam condensed,
a vacuum was formed beneath the piston, and air-pressure forced the
piston to descend. If the piston were attached to a weight by a rope
passing over a pulley, then, as the piston descended, it would lift the
weight. Papin's engine consisted simply of a cylinder and piston (Fig.
12). There was no boiler, but the water was placed in the cylinder
beneath the piston. A fire was placed under the cylinder and, as the
water boiled, the steam raised the piston. Then the fire was removed
and, as the cylinder cooled, the steam condensed, and the piston was
forced down by air-pressure. This was a slow and awkward method. The
engine required several minutes to make one stroke.
[Illustration: FIG. 12--PAPIN'S ENGINE
The first steam-engine with a piston. When the piston _B_ was forced
down by air-pressure, a weight was lifted by means of a rope _TT_
passing over pulleys.]
The principle of Papin's engine was first successfully applied by Thomas
Newcomen. Newcomen was a blacksmith by trade, and his great successor,
Watt, was a mechanic. Thus we see that great discoveries soon become
common property. The blacksmith and the mechanic soon learn to use the
discoveries of the scientist.
Newcomen's Engine
In the Newcomen engine the piston moved a walking-beam to which was
attached a pump-rod. Steam was used merely to balance the air-pressure
on the piston and allow the pump-rod to descend by its own weight. The
steam was condensed in the cylinder, and the pressure of the air forced
the piston down. Thus the work of raising water in the pump was done by
the air. Newcomen's first engine made twelve strokes a minute, and at
each stroke lifted fifty gallons of water fifty yards. He used this
engine in pumping water from the mines, and also made engines for
lifting coal.
At first the steam was condensed by throwing cold water on the outside
of the cylinder. But one day the engine suddenly increased its speed and
continued to work with unusual rapidity. The upper side of the piston
was covered with water to make the piston air-tight, and it was found
that this water was entering the cylinder through a hole that had worn
in the piston, and this jet of cold water was rapidly condensing the
steam. This was the origin of "jet condensation."
After this steam and water were alternately admitted to the cylinder
through cocks turned by hand. A boy, Humphrey Potter, to whom this work
was intrusted, won fame by tying strings to the cocks in such a way that
the engine would turn the cocks itself and the boy, Humphrey, was free
to play. This device was the origin of valve-gear.[1]
[1] Any device by which a steam-engine operates the valves
which admit steam to the cylinder is called "valve-gear." One form of
valve-gear is the link motion invented by Stephenson. This form will be
described in connection with the locomotive. A simple valve-rod, worked
by an eccentric such as is used on most stationary engines, is also a
form of valve-gear.
Newcomen's engine was extensively used. The tin and copper mines of
Cornwall were deepened. Coal-mines were sunk to twice the depth that had
been possible. But as the mines were deepened the cost of running the
engines increased. The largest engines consumed about $15,000 worth of
coal per year. The Newcomen engine required about twenty-eight pounds of
coal per hour per horse-power, while a modern engine consumes less than
two pounds. Again, because of increased cost, mines were being
abandoned. Such was the situation when James Watt came into the field of
action.
Watt had learned the mechanic's trade in one year in a London shop, and,
because he had not passed through an apprenticeship of seven years, the
Guild of Hammermen, a labor-union of his time, refused him admission,
and this refusal meant no employment. He found shelter, however, in the
University of Glasgow, and was there provided with a small workshop
where he could make instruments for sale.
Watt's Engine
A small Newcomen engine belonging to the University of Glasgow was out
of repair. London mechanics had failed to make it work. The job was
given to Watt. That he might do a perfect piece of work on this engine,
he made a study of all that was then known relating to steam (Fig. 13).
[Illustration: FIG. 13--THE NEWCOMEN ENGINE, IN REPAIRING WHICH WATT WAS
LED TO HIS GREAT DISCOVERIES
Preserved in the University of Glasgow.]
He saw that there was a great loss of heat in admitting cold water into
the cylinder to condense the steam, and that, to prevent this loss, the
cylinder must be kept always as hot as the steam that enters it. While
thinking upon this problem the idea came to him that, if connection were
made between the cylinder and a tank from which the air had been
pumped out, the steam would rush into the tank, and might there be
condensed without cooling the cylinder. This was the origin of the
condenser.
We have seen that, in the Newcomen engine, the steam acted only on the
under side of the piston, air acting on the upper side. It occurred to
Watt that the steam should act on both sides of the piston. So he
proposed to put an air-tight cover on the cylinder with a hole and
stuffing-box for the piston to slide through and to admit steam to act
upon it instead of air. Thus he was led to invent the double-acting
engine. The action in the cylinder of Watt's engine was the same as that
of the modern engine.
To save the power of steam, Watt arranged the valve in his engine in
such a way that the steam was cut off from the cylinder when the piston
had made about one-fourth of a stroke. The steam in the cylinder
continues to expand and drive the piston. This device more than doubles
the amount of work that the steam will do (Fig. 14).
[Illustration: FIG. 14--CYLINDER OF WATT'S STEAM-ENGINE
Arrows show the course of the steam.]
Horse-Power of an Engine
When horses were about to be replaced by the steam-engine at the mines,
the question was asked: "How many horses will the engine replace?" Tests
were made by Watt and others before him of the rate at which a horse
could work in pumping water or in lifting a weight by means of a pulley.
Watt's experiments showed that "a good London horse could go on lifting
150 pounds over a pulley at the rate of 2-1/2 miles an hour or 220 feet
per minute, and continue the work eight hours a day." This would be
equal to lifting 33,000 pounds one foot high every minute. This rate of
doing work he called a horse-power. It is more than the average horse
can do, but this number was used by Watt that he might give good measure
in his engines. The horse-power of an engine at that time meant the rate
of work in lifting water or coal. Now it means the rate of work done by
the steam upon the piston, so that to find the useful horse-power of an
engine we must deduct the work wasted in friction.
The indicator for measuring the pressure of steam in the cylinder and
the fly-ball governor are also inventions made by Watt (Fig. 15). The
fly-ball governor replaced the throttle-valve which was at first used by
Watt to regulate the speed of his engines. The throttle-valve is still
used on locomotives.
[Illustration: FIG. 15--A FLY-BALL GOVERNOR
The balls as they rotate regulate the admission of steam to the
cylinder by means of the lever _L_ and the rod _R_.]
At the end of the eighteenth century the steam-engine was full grown. It
remained for the nineteenth century to apply the engine to locomotion on
sea and land, to develop the steam-turbine, and so to increase the power
of the steam-engine that, early in the twentieth century, a
68,000-horse-power engine should speed an ocean liner across the
Atlantic in five days.
The Leyden Jar
The first electrical invention of practical use was made by Benjamin
Franklin. In Franklin's time great interest in electricity had been
aroused by the strange discovery of a German professor, Pieter van
Musschenbroek, of the University of Leyden. This professor had tried
what he called a new but terrible experiment. He had suspended by two
silk threads a gun-barrel which received electricity from an electrical
machine. From one end of the gun-barrel hung a brass wire. The lower end
of this wire dipped in a jar of water. He held the jar in one hand,
while with the other he tried to draw sparks from the gun-barrel.
Suddenly he received a shock which seemed to him like a lightning
stroke. So violent was the shock that he thought for a moment it would
end his life.
Out of this experiment came the Leyden jar, which for a century and a
half was of no practical use, but which now forms an important part of
every wireless telegraph equipment. The Leyden jar is simply a glass
bottle or jar coated with tin-foil both inside and outside (Fig. 16).
When charged with electricity the jar will hold its charge until the two
coatings are connected by a metal wire or other good conductor of
electricity. A person may receive a strong shock by holding the jar in
one hand and touching a knob connected to the inner coating with the
other hand.
[Illustration: FIG. 16--A LEYDEN JAR]
Popular interest in electricity was aroused by this discovery. The
friction electrical machine and the Leyden jar were simple and easy to
make. People of fashion found them interesting and amusing, the more so
because of the shock felt on taking through the body the discharge from
the "wonderful bottle," and the fact that several persons could receive
the shock at the same instant. On one occasion the Abbé Nollet
discharged a Leyden jar through a line composed of all the monks of the
Carthusian Monastery in Paris. As the line of serious-faced monks a mile
in length jumped into the air, the effect was ridiculous in the extreme.
Conductors and Insulators
About this time other great electrical discoveries were made. Early in
the century, Stephen Gray discovered that some objects conduct
electricity and others do not. He discovered that, when a glass tube is
electrified by rubbing, it will attract and repel light objects. In the
same way a comb or penholder of rubber may be electrified by rubbing it
on the sleeve. A bit of paper which touches the comb becomes
electrified. Electricity can be transferred from one object to another.
Gray discovered further that contact is not necessary, that a hempen
thread or a wire will carry an electric charge from one object to
another. A silk thread will not carry the electric charge. "Some things
convey electricity," he said, "and some do not, and those which do not
can be used to prevent the electricity escaping from those which do."
Could this obscure inventor have seen a modern telegraph line with the
glass insulators on the poles, which prevent the electric current
escaping from the telegraph wire, he might have realized the importance
of his discovery. He set up a line of hempen thread six hundred and
fifty feet long, and with an electrical machine at one end of the line
electrified a boy suspended from the other end.
Two Kinds of Electric Charge
A Frenchman, DuFay, while carrying further the experiments of Gray, was
watching a bit of gold-leaf floating in the air. The gold-leaf had been
repelled after contact with his electrified glass tube. Thinking to try
the action of two electrified objects on the gold leaf, he rubbed a
piece of gum-copal and brought it near the leaf. To his astonishment the
leaf, which was repelled by the glass tube, was attracted by the
gum-copal. He repeated the experiment again and again, and each time the
leaf was repelled by the glass and attracted by the gum. He concluded
from this that there are two kinds of electricity, which he named
"vitreous" and "resinous." The two kinds of electric charge were called
by Franklin "positive" and "negative."
Franklin made a battery of Leyden jars, connecting the inner coating of
one to the outer coating of the next throughout the series. In this way
he could get a much stronger spark than with a single jar. On one
occasion he nearly lost his life by taking a shock from his battery of
Leyden jars. He magnetized and demagnetized steel needles by passing
the discharge from his Leyden jars through the needles.
Franklin's Kite Experiment
The conjecture that lightning is of the same nature as the spark from
the Leyden jar or the electrical machine had gained a hold on the minds
of others before Franklin. In France sparks had been drawn from a rod
ninety-nine feet high, but this did not reach into the clouds. Franklin
determined to send a kite into a thunder-cloud, thinking electricity
from the cloud would follow the string of the kite and could be stored
in a Leyden jar, and used like the charge from an electrical machine. He
had felt the power of a Leyden-jar discharge, and through it had nearly
lost his life. He knew that lightning is far more powerful than any
battery of Leyden jars, and yet to test the truth of his theory, that
lightning is an electrical discharge, he was about to draw the lightning
to his hand. He knew little of conductors of electricity. Whether the
cord would draw little or much of the "electric fire" he knew not. So
far as he knew he was toying with death.
The kite was made of two light strips of cedar placed crosswise, and a
large silk handkerchief fastened to the strips. A sharp wire about a
foot long was fastened to one of the strips. To the lower end of the
cord he attached a key and a silk ribbon. By means of the ribbon he held
the cord to insulate it from his hand. The kite soared into the clouds,
and Franklin and his son stood under a shed awaiting the coming of the
"electric fire" (Fig. 17). Soon the fibres of the cord began to
bristle up. He approached his knuckles to the key. A spark passed. He
brought up a Leyden jar and charged it with electricity from the cloud,
and found that with this charge he could do everything that could be
done with electricity from his machine. He had proved the identity of
lightning and electricity.
[Illustration: FIG. 17--FRANKLIN'S KITE EXPERIMENT
Taking electricity from the clouds.]
The Lightning-Rod
Some time before, he had discovered the action of a point in discharging
electricity. He said: "If you fix a needle to the end of a gun-barrel
like a little bayonet, while it remains there the gun-barrel cannot be
electrified so as to give a spark, for the electric fire continually
runs out silently at the point." In the dark you may see a light gather
upon the point like that of a firefly or glow-worm. If the needle is
held in the hand and brought near to an object charged with electricity,
the object is quietly discharged, and a light may be seen at the point
of the needle. This action of points explains the light sometimes seen
on the tops of ships' masts, called by sailors "Saint Elmo's fire," and
perhaps, also, the observation of Cæsar that, in a certain African War,
the spears of the Fifth Roman Legion appeared tipped with fire.
The lightning-rod was the outcome of Franklin's observations, and this
was the first practical invention relating to electricity. A building
may be electrified by an electrified cloud passing over it. If the
building is protected by pointed rods, the electric charge will quietly
escape from the points. The lower ends of the rods must be in the moist
earth below the surface. The lightning-rod has not proved so great a
protection as Franklin supposed it would. He supposed that a
lightning-stroke is a discharge in one direction only; but we now know
that it is a rapid surging back and forth, and this fact accounts for
the failure of the lightning-rods to furnish perfect protection. In
surging back and forth, the lightning may skip from the lightning-rod to
some metal object within the building, as a stove or radiator. The
lightning-rod robbed the thunder-storm of its terrors to the timid, and
in time dispelled the superstition of people who believed that thunder
and lightning are evidence of the wrath of the Deity.
Franklin was the first to propose an answer to the question: What is
electricity? He believed electricity to be a subtle fluid existing in
all objects. If an object has more than a certain amount of this fluid,
it is positively electrified; if less than this amount, it is negatively
electrified.
The "one-fluid" theory of Franklin was soon met by the "two-fluid"
theory proposed by Robert Symmer, for Franklin's theory had failed to
explain why two bodies negatively electrified should repel each other.
According to Symmer, an uncharged body contains an equal quantity of two
different electrical fluids. An excess of one of these produces a
positive charge, an excess of the other a negative charge.
Symmer's experiments are almost ludicrous. He wore two pairs of silk
stockings, and found that white and black silk worn together became
strongly electrified. When the two stockings worn on one foot were
pulled off together, and then separated, they were found to be
electrified, and attracted each other so strongly that a force of about
one pound was required to separate them. The two charges, negative and
positive, could, however, be separated. He thought, therefore, that
there are "two electrical powers," not one, as Franklin believed. His
belief was strengthened by examining a quire of paper through which an
electric spark had passed, and finding that "the edges of the holes were
bent two different ways, as if the hole had been made in the quire by
drawing two threads in contrary directions through it."
There was a long controversy regarding the two theories, and neither
quite gained possession of the field. Each contained some truth, and
each had its weak points. The two had more in common than men at that
time thought.
Galvani and the Electric Current
Franklin had proven that there is electricity in the atmosphere, and
that lightning is an electric discharge. A widespread interest in the
electricity of the atmosphere followed this discovery. Aloisio Galvani,
a physician in Bologna, Italy, in attempting to learn the effect of
atmospheric electricity on the nerves and muscles of the human body,
made a discovery which led to the electric battery and a knowledge of
electric currents.
Having dissected a frog, he laid it on a table on which stood an
electrical machine. When one of his assistants touched lightly the nerve
of the thigh with the point of a knife while a spark was drawn from the
electrical machine, the muscles contracted violently, as if they were
attacked by a cramp. When he held the knife by the bone handle, there
was no convulsion as there was when he held it by the steel blade.
He next thought it important to find out if lightning would excite
contraction of the muscles. He stretched and insulated a long iron wire
in the open air on the housetop and, as a storm drew near, hung on it a
dissected frog. To the feet he fastened another long iron wire, which
was allowed to dip in the water in the well. "The result," he said,
"came about as we wished. As often as the lightning broke forth, the
muscles were thrown into repeated violent convulsions, so that always,
as the lightning lightened the sky, the muscle contractions and
movements preceded the thunder and, as it were, announced its coming. It
was best, however, when the lightning was strong, or the clouds from
which it broke forth were near the place of the experiment."
He describes his greatest experiment as follows: "After we had
investigated the power of atmospheric electricity in storms, our hearts
burned with the desire to investigate the daily quiet electricity of the
atmosphere. Therefore, as the prepared frogs, hung on an iron railing
which surrounded a hanging garden on our house, with brass hooks
inserted in the spinal cord, fell into convulsions not only when it
lightened, but when the sky was calm and clear, I thought that the cause
of these contractions was the changes in the electricity of the
atmosphere. Then for hours, yes, even days, I observed the animals, but
almost never a movement of the muscles could be seen. At last, tired
with such fruitless waiting, I began to press the brass hooks, which
were fastened in the spinal cord, against the iron railing to see if
such a trick would cause the muscles to contract, and if instead of
changes in the atmospheric electricity any other changes would have any
influence. I observed, indeed, vigorous contractions, but none which
could be caused by the condition of the atmosphere."
It was pressing the brass hook against the iron railing, thus forming an
electric battery, that caused electricity to pass through the muscles of
the frog. Galvani did not know that he had discovered a new source of
electricity. He never arrived at a correct explanation of his results,
and never knew the value of his discovery.
Volta and the Electric Battery
It was left for Alexander Volta to show that, in Galvani's experiment,
the muscles of the frog, together with the brass hook and the iron
railing, formed an electric battery. Volta showed that an electric
charge can be produced merely by bringing two different metals into
contact. He found that, if he placed copper and zinc in sulphuric acid,
or a solution of common salt, he could, produce a continuous flow of
electricity (Fig. 18).
[Illustration: FIG. 18--VOLTA EXPLAINING HIS ELECTRIC BATTERY TO
NAPOLEON BONAPARTE
From a painting. Photo by Dubray.]
In the beginning of the year 1800 Volta made the first electric battery
(Fig. 19). It was made of copper and zinc disks placed alternately, with
a piece of wet cloth above each pair of disks. With his column of disks
he could obtain a strong shock; indeed, many shocks, one after the
other. This first battery of Volta's was a form of "dry battery." Later
Volta devised his "crown of cups," a form of wet battery similar to
some batteries in use to-day. Each cup contained a strip of copper and a
strip of zinc in dilute sulphuric acid.
[Illustration: FIG. 19--THE FIRST ELECTRIC BATTERY
No. 1--A battery of one hundred pairs of copper and zinc disks.
No. 2--Two such batteries connected.
By permission of the Italian Institute of Graphic Arts, Bergamo.]
Volta did not know the real use of the liquid in his battery, nor that
the strength of the current depends on the rate at which the metal is
dissolved by the acid; but he had discovered the electric current, and
with this discovery began a new era in electrical invention.