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Simple Science

301. Improvements
Modern Electrical Inventions:
The Sounder. Shortly after the invention of telegraphy, operators learned that they could read the message by the click of the marker against a metal rod which took the place of the tape. In practically all telegraph offices of the present day the old-fashioned tape is replaced by the sounder, shown in Figure. When current flows, a lever, L, is drawn down by the electromagnet and strikes against a solid metal piece with a click; when the current is broken, the lever springs upward, strikes another metal piece and makes a different click. It is clear that the working of the key which starts and stops the current in this line will be imitated by the motion and the resulting clicks of the sounder. By means of these varying clicks of the sounder, the operator interprets the message.

The Relay. When a telegraph line is very long, the resistance of the wire is great, and the current which passes through the electromagnet is correspondingly weak, so feeble indeed that the armature must be made very thin and light in order to be affected by the makes and breaks in the current. The clicks of an armature light enough to respond to the weak current of a long wire are too faint to be recognized by the ear, and hence in such long circuits some device must be introduced whereby the effect is increased. This is usually done by installing at each station a local battery and a very delicate and sensitive electromagnet called the relay. Under these conditions the current of the main line is not sent through the sounder, but through the relay which opens and closes a local battery in connection with the strong sounder. For example, the relay is so arranged that current from the main line runs through it exactly as it runs through M in Figure. When current is made, the relay attracts an armature, which thereby closes a circuit in a local battery and thus causes a click of the sounder. When the current in the main line is broken, the relay loses its magnetic attraction, its armature springs back, connection is broken in the local circuit, and the sounder responds by allowing its armature to spring back with a sharp sound.

FIG. - The sounder.
302. The Earth an Important Part of a Telegraphic System
Modern Electrical Inventions:
We learned that electricity could flow through many different substances, one of which was the earth. In all ordinary telegraph lines, advantage is taken of this fact to utilize the earth as a conductor and to dispense with one wire. Originally two wires were used; then it was found that a railroad track could be substituted for one wire, and later that the earth itself served equally well for a return wire. The present arrangement is shown in Figure, where there is but one wire, the circuit being completed by the earth. No fact in electricity seems more marvelous than that the thousands of messages flashing along the wires overhead are likewise traveling through the ground beneath. If it were not for this use of the earth as an unfailing conductor, the network of overhead wires in our city streets would be even more complex than it now is.

FIG. - Diagram of a modern telegraph system.
303. Advances in Telegraphy
Modern Electrical Inventions:
The mechanical improvements in telegraphy have been so rapid that at present a single operator can easily send or receive forty words a minute. He can telegraph more quickly than the average person can write; and with a combination of the latest improvements the speed can be enormously increased. Recently, 1500 words were flashed from New York to Boston over a single wire in one second.

In actual practice messages are not ordinarily sent long distances over a direct line, but are automatically transferred to new lines at definite points. For example, a message from New York to Chicago does not travel along an uninterrupted path, but is automatically transferred at some point, such as Lancaster, to a second line which carries it on to Pittsburgh, where it is again transferred to a third line which takes it farther on to its destination.
304. Compass
Magnets and Currents:
In the twelfth century, there was introduced into Europe from China a simple instrument which changed journeying on the sea from uncertain wandering to a definite, safe voyage. This instrument was the compass, and because of the property of the compass needle (a magnet) to point unerringly north and south, sailors were able to determine directions on the sea and to steer for the desired point.

Since an electric current is practically equivalent to a magnet, it becomes necessary to know the most important facts relative to magnets, facts simple in themselves but of far-reaching value and consequences in electricity. Without a knowledge of the magnetic characteristics of currents, the construction of the motor would have been impossible, and trolley cars, electric fans, motor boats, and other equally well-known electrical contrivances would be unknown.

FIG. - The compass.
305. The Attractive Power of a Magnet
Magnets and Currents:
The magnet best known to us all is the compass needle, but for convenience we will use a magnetic needle in the shape of a bar larger and stronger than that employed in the compass. If we lay such a magnet on a pile of iron filings, it will be found on lifting the magnet that the filings cling to the ends in tufts, but leave it almost bare in the center. The points of attraction at the two ends are called the poles of the magnet.

If a delicately made magnet is suspended as in Figure, and is allowed to swing freely, it will always assume a definite north and south position. The pole which points north when the needle is suspended is called the north pole and is marked N, while the pole which points south when the needle is suspended is called the south pole and is marked S.

A freely suspended magnet points nearly north and south.

A magnet has two main points of attraction called respectively the north and south poles.

FIG. - A magnet.

FIG. - The magnetic needle.
306. The Extent of Magnetic Attraction
Magnets and Currents:
If a thin sheet of paper or cardboard is laid over a strong, bar-shaped magnet and iron filings are then gently strewn on the paper, the filings clearly indicate the position of the magnet beneath, and if the cardboard is gently tapped, the filings arrange themselves as shown in Figure. If the paper is held some distance above the magnet, the influence on the filings is less definite, and finally, if the paper is held very far away, the filings do not respond at all, but lie on the cardboard as dropped.

The magnetic power of a magnet, while not confined to the magnet itself, does not extend indefinitely into the surrounding region; the influence is strong near the magnet, but at a distance becomes so weak as to be inappreciable. The region around a magnet through which its magnetic force is felt is called the field of force, or simply the magnetic field, and the definite lines in which the filings arrange themselves are called lines of force.

The magnetic power of a magnet is not limited to the magnet, but extends to a considerable distance in all directions.

FIG. - Iron filings scattered over a magnet arrange themselves in definite lines.
307. The Influence of Magnets upon Each Other
Magnets and Currents:
If while our suspended magnetic needle is at rest in its characteristic north-and-south direction another magnet is brought near, the suspended magnet is turned; that is, motion is produced. If the north pole of the free magnet is brought toward the south pole of the suspended magnet, the latter moves in such a way that the two poles N and S are as close together as possible. If the north pole of the free magnet is brought toward the north pole of the suspended magnet, the latter moves in such a way that the two poles N and N are as far apart as possible. In every case that can be tested, it is found that a north pole repels a north pole, and a south pole repels a south pole; but that a north and a south pole always attract each other.

The main facts relative to magnets may be summed up as follows: -

a. A magnet points nearly north and south if it is allowed to swing freely.

b. A magnet contains two unlike poles, one of which persistently points north, and the other of which as persistently points south, if allowed to swing freely.

c. Poles of the same name repel each other; poles of unlike name attract each other.

d. A magnet possesses the power of attracting certain substances, like iron, and this power of attraction is not limited to the magnet itself but extends into the region around the magnet.

FIG. - A south pole attracts a north pole.
308. Magnetic Properties of an Electric Current
Magnets and Currents:
If a current-bearing wire is really equivalent in its magnetic powers to a magnet, it must possess all of the characteristics mentioned in the preceding Section. We know that a coiled wire through which current was flowing would attract iron filings at the two ends of the helix. That a coil through which current flows possesses the characteristics a, b, c, and d of a magnet is shown as follows: -

a, b. If a helix marked at one end with a red string is arranged so that it is free to rotate and a strong current is sent through it, the helix will immediately turn and face about until it points north and south. If it is disturbed from this position, it will slowly swing back until it occupies its characteristic north and south position. The end to which the string is attached will persistently point either north or south. If the current is sent through the coil in the opposite direction, the two poles exchange positions and the helix turns until the new north pole points north.

c. If a coil conducting a current is held near a suspended magnet, one end of the helix will be found to attract the north pole of the magnet, while the opposite end will be found to repel the north pole of the magnet. In fact, the helix will
be found to behave in every way as a magnet, with a north pole at one end and a south pole at the other. If the current is sent through the helix in the opposite direction, the north and south poles exchange places.

If the number of turns in the helix is reduced until but a single loop remains, the result is the same; the single loop acts like a flat magnet, one side of the loop always facing northward and one southward, and one face attracting the north pole of the suspended magnet and one repelling it.

d. If a wire is passed through a card and a strong current is sent through the wire, iron filings will, when sprinkled upon the card, arrange themselves in definite directions. A wire carrying a current is surrounded by a magnetic field of force.

A magnetic needle held under a current-bearing wire turns on its pivot and finally comes to rest at an angle with the current. The fact that the needle is deflected by the wire shows that the magnetic power of the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by Oersted of Denmark less than a hundred years ago; but since that time practically all important electrical machinery has been based upon one or more of the magnetic properties of electricity. The motors which drive our electric fans, our mills, and our trolley cars owe their existence entirely to the magnetic action of current electricity.

FIG. - A helix through which current flows always points north and south, if it is free to rotate.

FIG. - A wire through which current flows is surrounded by a field of magnetic force.
309. The Principle of the Motor
Magnets and Currents:
If a close coil of wire is suspended between the poles of a strong horseshoe magnet, it will not assume any characteristic position but will remain wherever placed. If, however, a current is sent through the wire, the coil faces about and assumes a definite position. This is because a coil, carrying a current, is equivalent to a magnet with a north and south face; and, in accordance with the magnetic laws, tends to move until its north face is opposite the south pole of the horseshoe magnet, and its south face opposite the north pole of the magnet. If, when the coil is at rest in this position, the current is reversed, so that the north pole of the coil becomes a south pole and the former south pole becomes a north pole, the result is that like poles of coil and magnet face each other. But since like poles repel each other, the coil will move, and will rotate until its new north pole is opposite to the south pole of the magnet and its new south pole is opposite the north pole. By sending a strong current through the coil, the helix is made to rotate through a half turn; by reversing the current when the coil is at the half turn, the helix is made to continue its rotation and to swing through a whole turn. If the current could be repeatedly reversed just as the helix completed its half turn, the motion could be prolonged; periodic current reversal would produce continuous rotation. This is the principle of the motor.

It is easy to see that long-continued rotation would be impossible in the arrangement of Figure, since the twisting of the suspending wire would interfere with free motion. If the motor is to be used for continuous motion, some device must be employed by means of which the helix is capable of continued rotation around its support.

In practice, the rotating coil of a motor is arranged as shown in Figure. Wires from the coil terminate on metal disks and are securely soldered there. The coil and disks are supported by the strong and well-insulated rod R, which rests upon braces, but which nevertheless rotates freely with disks and coil. The current flows to the coil through the thin metal strips called brushes, which rest lightly upon the disks.

When the current which enters at B flows through the wire, the coil rotates, tending to set itself so that its north face is opposite the south face of the magnet. If, when the helix has just reached this position, the current is reversed - entering at B' instead of B - the poles of the coil are exchanged; the rotation, therefore, does not cease, but continues for another half turn. Proper reversals of the current are accompanied by continuous motion, and since the disk and shaft rotate with the coil, there is continuous rotation.

If a wheel is attached to the rotating shaft, weights can be lifted, and if a belt is attached to the wheel, the motion of the rotating helix can be transferred to machinery for practical use.

The rotating coil is usually spoken of as the armature, and the large magnet as the field magnet.

FIG. - The coil turns in such a way that its north pole is opposite the south pole of the magnet.

FIG. - Principle of the motor.
310. Mechanical Reversal of the Current
Magnets and Currents:
The Commutator. It is not possible by hand to reverse the current with sufficient rapidity and precision to insure uninterrupted rotation; moreover, the physical exertion of such frequent reversals is considerable. Hence, some mechanical device for periodically reversing the current is necessary, if the motor is to be of commercial value.

The mechanical reversal of the current is accomplished by the use of the commutator, which is a metal ring split into halves, well insulated from each other and from the shaft. To each half of this ring is attached one of the ends of the armature wire. The brushes which carry the current are set on opposite sides of the ring and do not rotate. As armature, commutator, and shaft rotate, the brushes connect first with one segment of the commutator and then with the other. Since the circuit is arranged so that the current always enters the commutator through the brush B, the flow of the current into the coil is always through the segment in contact with B; but the segment in contact with B changes at every half turn of the coil, and hence the direction of the current through the coil changes periodically. As a result the coil rotates continuously, and produces motion so long as current is supplied from without.

FIG. - The commutator.


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