Part 5 (2/2)

[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.]

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