The Story of Great Inventions Part 3
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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 pa.s.sing 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.
[Ill.u.s.tration: 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_ pa.s.sing 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.
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 c.o.c.ks turned by hand. A boy, Humphrey Potter, to whom this work was intrusted, won fame by tying strings to the c.o.c.ks in such a way that the engine would turn the c.o.c.ks itself and the boy, Humphrey, was free to play. This device was the origin of valve-gear.
 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 pa.s.sed through an apprentices.h.i.+p 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.
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).
[Ill.u.s.tration: 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).
[Ill.u.s.tration: 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.
[Ill.u.s.tration: FIG. 15--A FLY-BALL GOVERNOR The b.a.l.l.s 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 bra.s.s 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 gla.s.s 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 k.n.o.b connected to the inner coating with the other hand.
[Ill.u.s.tration: 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 fas.h.i.+on 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 Abbe 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 gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s tube, was attracted by the gum-copal. He repeated the experiment again and again, and each time the leaf was repelled by the gla.s.s 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 pa.s.sing 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 pa.s.sed. 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 ident.i.ty of lightning and electricity.
[Ill.u.s.tration: FIG. 17--FRANKLIN'S KITE EXPERIMENT Taking electricity from the clouds.]
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 s.h.i.+ps' masts, called by sailors "Saint Elmo's fire," and perhaps, also, the observation of Caesar 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 pa.s.sing 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 superst.i.tion 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 quant.i.ty 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 pa.s.sed, 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.
The Story of Great Inventions Part 3
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