Part 5 (1/2)
Faraday's Electrical Discoveries
Now we shall turn to Faraday's electrical discoveries and inventions.
Men had long known that, in houses that have been struck by lightning, steel objects such as knives and needles are sometimes found to be magnetized. s.h.i.+ps struck by lightning had found their compa.s.s-needles pointing south instead of north, or wandering in direction and worthless. Men had wondered how an electrical discharge could magnetize steel. They had tried the spark of the electrical machine with no definite result. Franklin, in his experiment of magnetizing a steel needle by pa.s.sing an electric spark through it, could not tell before the spark was pa.s.sed through the needle which end would be the north pole. There was no seeming connection between the direction of the electric discharge and the polarity of the needle. After the discovery of the electric battery, men tried to discover a relation between the electric current and magnetism.
Oersted and Electromagnetism
The first success in this direction was achieved by Hans Christian Oersted, a native of Denmark. Poverty impelled his father to take him from school at the age of twelve and place him in an apothecary's shop.
The boy, Hans, found delight in the chemical work of the apothecary. His eagerness to learn and the pressure of poverty led him to neglect the usual sports of boyhood and devote his leisure time to reading and study. Again he entered school, and, though paying his way by his own work, he graduated with honor from the University of Copenhagen. He was appointed Professor of Physics in this university, and here he made his first great discovery in electromagnetism.
After working for seven years to discover a relation between current electricity and magnetism, he made a discovery which proved to be the first step in the invention of the dynamo. He was using a magnetic compa.s.s, which is a small magnetic needle balanced on a steel point. The needle points nearly north and south unless disturbed by a magnet brought near it. He had tried to find if a wire through which a current is flowing would disturb the compa.s.s as a magnet does. He had tried placing the wire east and west, thinking the compa.s.s-needle would follow the wire as it does a magnet. One day, while lecturing to his students, it occurred to him for the first time to place the wire north and south over the compa.s.s-needle. He was surprised and perplexed as he did so to see the needle swing round and point nearly east and west (Fig. 21). On reversing the current the needle swung in the opposite direction. He had discovered the magnetic action of an electric current. It was learned soon afterward that a coil of wire with an electric current flowing through it acts like a magnet, and that a current flowing around a bar of soft iron makes the iron a magnet (Figs. 22 and 23).
[Ill.u.s.tration: FIG. 21--OERSTED'S EXPERIMENT An electric current flowing over the compa.s.s-needle toward the north causes the needle to turn until it points nearly west.
By permission of Joseph G. Branch.]
[Ill.u.s.tration: FIG. 22.--A COIL WITH A CURRENT FLOWING THROUGH IT ACTS LIKE A MAGNET The coil is picking up iron filings.]
[Ill.u.s.tration: FIG. 23--A BAR OF SOFT IRON WITH A CURRENT FLOWING AROUND IT BECOMES A MAGNET]
Ampere
The news of Oersted's discovery aroused great interest throughout Europe. Soon after its announcement in France, Andre Marie Ampere made a discovery of equal importance. Oersted had discovered electromagnetism.
Ampere discovered electrical power or motion produced by an electrical current.
The youth of Ampere was pa.s.sed amid the stormy scenes of the French Revolution. His father had moved from his country home to Lyons and become a justice of the peace. In the destruction of the city of Lyons during the Reign of Terror he lost his head under the guillotine.
The blow was too great for Ampere, then a youth of eighteen. He had been a precocious child, advanced beyond his years in all the studies of the schools. But now his strong mind failed. For a year he wandered about mechanically piling up heaps of sand or gazing upon the sky. Then his mental power returned, and he took up with eagerness the study of botany and poetry.
He became a professor in the Polytechnic School in Paris, and it was while teaching in this school that he made his great discoveries. He found that two coils of wire can be made to attract or repel each other by an electric current. If the current flows through the two coils in the same direction, they attract each other (Fig. 24). If the current flows in opposite directions through the coils, they repel each other (Fig. 25). This is not very strange to us, for we know that a coil with a current flowing through it acts just like a magnet. Each coil then has a north pole and a south pole. If the coils are placed so that the two north poles or the two south poles are together, they will repel each other. If the north pole of one coil is near the south pole of the other, they will attract each other.
[Ill.u.s.tration: FIG. 24--TWO COILS WITH CURRENTS FLOWING IN SAME DIRECTION ATTRACT EACH OTHER]
[Ill.u.s.tration: FIG. 25--TWO COILS WITH CURRENTS FLOWING IN OPPOSITE DIRECTIONS REPEL EACH OTHER]
Ampere believed that electric currents are flowing around within the earth, and that the earth has a north and a south magnetic pole for the same reason that a coil of wire has magnetic poles; that these poles are caused by the currents flowing around in the earth just as the poles of the coil are caused by the current flowing around in the coil.
We do honor to the name of Ampere whenever we measure an electric current, for electric currents are measured in ”amperes.”
Arago
Another important discovery was made by a young Frenchman, Francois Arago, within a year of the time when Oersted and Ampere made their discoveries. The three great discoveries of these men were made in the years 1819 and 1820. The youth of Arago was full of adventure. He had a.s.sisted in making a survey in the Pyrenees, the haunt of daring robber-bands. Twice in his cabin he was visited by a chief of a robber-band who claimed to be a custom-house guard. On the second visit he said to the robber: ”Your position is perfectly known to me. I know that you are not a custom-house guard. I have learned that you are the chief of the robbers of the country. Tell me whether I have anything to fear from your confederates.” The robber replied: ”The idea of robbing you did occur to us; but, on the day that we molested an envoy from the French, they would direct against us several regiments of soldiers, and we are not so strong as they. Allow me to add that the grat.i.tude which I owe you for the night's shelter is your surest guarantee.”
At a later time, when war between Spain and France was threatened, he was accused of being a spy, and a mob was formed to put him out of the way. He escaped in disguise through the midst of the mob and boarded a Spanish s.h.i.+p. He was carried to Morocco, ran the gantlet of bloodthirsty Mussulmans in Algiers, escaped death by a hair's-breadth, and through it all clung to the papers which recorded the results of the survey in the mountains, and delivered them in safety to the office of the Bureau of Longitude in Paris.
Arago made a discovery which, with those of Oersted and Ampere, prepared the way for Faraday's great electrical discoveries and the invention of the dynamo. He found that a plate of copper whirling above or below a magnetic needle will draw the needle after it (Fig. 26). He could make the speed of the whirling copper plate so great that the needle would whirl rapidly, following the copper plate. Faraday was the first to explain Arago's experiment.
[Ill.u.s.tration: FIG. 26--ARAGO'S EXPERIMENT When the copper plate whirls the magnet whirls also, though it does not touch the copper plate.]
Faraday's First Electric Motor
Faraday's first electrical discovery was made soon after that of Arago.
Oersted had proven that an electric current acts on a magnet. The magnet turns at right angles to the wire. Faraday saw that this is because the north pole of the magnet tries to go round the wire in one direction, and the south pole tries to go round in the opposite direction. He placed a magnet on end in a dish of mercury, with one pole of the magnet above the mercury, and found that the magnet would spin round a wire carrying a current. When the current acts on one pole of the magnet only, the magnet spins round the wire (Fig. 27). So Faraday's first electrical discovery prepared the way for the electric motor.