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. Ships 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 ship. 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.

[Ill.u.s.tration: FIG. 27--ONE POLE OF A MAGNET SPINS ROUND A WIRE THROUGH WHICH AN ELECTRIC CURRENT FLOWS]

An Electric Current Produced by a Magnet

He had written in his note-book: "Convert magnetism into electricity."

An electric current would magnetize iron. Would not a magnet produce an electric current? This was his problem.

He connected a coil of wire to an instrument that would tell when a current was flowing, and placed a magnet in the coil. Others had claimed, and Faraday at first believed, that a current would flow while the magnet lay quiet within the coil. But Faraday was alert for the unexpected, and the unexpected happened. For an instant, as he thrust the magnet into the coil, his instrument showed that a current was flowing. Again, as he drew the magnet quickly from the coil, a current flowed, but in the opposite direction (Fig. 28). From this simple experiment has grown the alternating-current machinery by which the power of Niagara is made to light cities and drive electric cars at a distance of many miles.

[Ill.u.s.tration: FIG. 28--WHEN A MAGNET IS THRUST INTO A COIL OF WIRE IT CAUSES A CURRENT TO FLOW IN THE COIL, BUT THE CURRENT FLOWS ONLY WHILE THE MAGNET IS MOVING Drawing reproduced by permission of Joseph G. Branch.]

A friend of Faraday, on learning of this discovery, wrote the following impromptu lines:

"Around the magnet Faraday Was sure that Volta"s lightnings play.

But how to draw them from the wire?

He took a lesson from the heart: "Tis when we meet, "tis when we part, Breaks forth the electric fire."

A magnet will produce an electric current in a wire, but only when the magnet or the wire is in motion.

Detecting and Measuring an Electric Current

The instrument which Faraday used to detect a current was derived from Oersted"s experiment. When a current flows in a north-and-south direction over a compa.s.s-needle, the needle swings round. When the current stops flowing the needle swings back to the north-and-south position. The effect on the needle is stronger if the current flows through a coil of wire and the coil is placed in a north-and-south position around the needle (Fig. 29). The stronger the current flowing through the coil the farther the needle will turn from the north-and-south position.

[Ill.u.s.tration: FIG. 29--A COIL OF WIRE AROUND A COMPa.s.s-NEEDLE The needle tells when a current is flowing, and how strong the current is.]

The coil and the needle together are called a galvanometer, and may be used to tell when a current is flowing, and also to indicate the strength of the current.

An Electric Current Produced by the Magnetic Field of Another Current

Faraday had found that a current flowing around a piece of iron will make the iron a magnet, and that a magnet in motion will cause a current to flow in a wire. It seemed to him that a second wire placed near the first should have a current produced in it without the presence of iron.

He wound two coils of copper wire upon the same wooden spool. The wire of the two coils he separated with twine and calico. One coil was connected with a galvanometer, the other with a battery of ten cells, yet not the slightest turning of the needle could be observed. But he was not deterred by one failure. He raised his battery from ten cells to one hundred cells, but without avail. The current flowed calmly through the battery wire without producing, during its flow, any effect upon the galvanometer. During its flow was the time when an effect was expected.

Again the unexpected happened. At the instant of making contact with the battery there was a slight movement of the needle. When the contact was broken, another slight movement, but in the opposite direction to the first (Fig. 30). The current in one wire caused a current to flow in the other, but the current in the second wire continued for an instant only at the making and breaking of the contact with the battery. This was the beginning of the induction-coil used to-day in wireless telegraphy.

[Ill.u.s.tration: FIG. 30--FARADAY"S INDUCTION-COIL Starting and stopping the battery current in the primary coil causes a changing magnetic field, and this causes a current to flow in the secondary coil.

Drawing reproduced by permission of Joseph G. Branch.]

What was the secret of it? Simply this: that a current in one wire will cause a current to flow in another wire near it, but only while the current in the first wire is changing. That is, at the instant when the first wire is connected to the battery, or its connection broken, a current is induced in the second wire. There is no battery or other source of current connected to the second wire; but a current flows in this wire because it is near a wire in which a current is rapidly starting and stopping. When these two wires are wound in coils, together they form an induction-coil. The wire which we have called the first wire forms the "primary" coil, and the one we have called the second wire forms the "secondary" coil. By repeatedly making and breaking the circuit in the primary coil we get an alternating current in the secondary coil. Fig. 31 is from a photograph of some of the coils actually used by Faraday.

[Ill.u.s.tration: FIG. 31--HISTORICAL APPARATUS OF FARADAY IN THE ROYAL INSt.i.tUTION Some of Faraday"s transformer coils are shown here. The instrument on the left in a gla.s.s case is his galvanometer.]

Faraday"s Dynamo

To invent a new electrical machine was Faraday"s next aim. Arago"s disk of copper whirling near a magnet had a current induced in it, so Faraday thought. It was the action of this induced current which caused the magnet to follow the whirling disk. Could the current in Arago"s disk be collected and caused to flow through a wire? He placed a copper disk between the poles of a magnet. One galvanometer wire pa.s.sed around the axis of the disk, the other he held in contact with the edge. He whirled the disk. The galvanometer needle moved. A current was flowing in the disk as it whirled. The current from the whirling disk flowed through the galvanometer. Faraday had discovered the dynamo (Fig. 32).

[Ill.u.s.tration: FIG. 32--FARADAY"S FIRST DYNAMO A current flows in the copper disk as it whirls between the poles of the magnet.

By permission of Joseph G. Branch.]