General Science Part 27

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.

CHAPTER x.x.xIII

MAGNETS AND CURRENTS

304. 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 compa.s.s (Fig. 221), and because of the property of the compa.s.s 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.

[Ill.u.s.tration: FIG. 221.--The compa.s.s.]

Since an electric current is practically equivalent to a magnet (Section 296), 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.

305. The Attractive Power of a Magnet. The magnet best known to us all is the compa.s.s needle, but for convenience we will use a magnetic needle in the shape of a bar larger and stronger than that employed in the compa.s.s. 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 (Fig. 222). The points of attraction at the two ends are called the poles of the magnet.

[Ill.u.s.tration: FIG. 222.--A magnet.]

If a delicately made magnet is suspended as in Figure 223, and is allowed to swing freely, it will always a.s.sume 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.

[Ill.u.s.tration: FIG. 223.--The magnetic needle.]

306. The Extent of Magnetic Attraction. 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 224. 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.

[Ill.u.s.tration: FIG. 224.--Iron filings scattered over a magnet arrange themselves in definite lines.]

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

307. The Influence of Magnets upon Each Other. 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 (Fig. 225). 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.

[Ill.u.s.tration: FIG. 225.--A south pole attracts a north pole.]

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.

308. Magnetic Properties of an Electric Current. 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 saw in Section 296 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.

[Ill.u.s.tration: FIG. 226.--A helix through which current flows always points north and south, if it is free to rotate.]

_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.

[Ill.u.s.tration: FIG. 227.--A wire through which current flows is surrounded by a field of magnetic force.]

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 pa.s.sed 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 (Fig. 227). 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.

[Ill.u.s.tration: FIG. 228.--The coil turns in such a way that its north pole is opposite the south pole of the magnet.]

309. The Principle of the Motor. If a close coil of wire is suspended between the poles of a strong horseshoe magnet, it will not a.s.sume any characteristic position but will remain wherever placed.

If, however, a current is sent through the wire, the coil faces about and a.s.sumes 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.

[Ill.u.s.tration: FIG. 229.--Principle of the motor.]

It is easy to see that long-continued rotation would be impossible in the arrangement of Figure 228, 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 229. 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.

310. Mechanical Reversal of the Current. _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.

[Ill.u.s.tration: FIG. 230.--The commutator.]

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.

311. The Practical Motor. A motor constructed in accordance with Section 309 would be of little value in practical everyday affairs; its armature rotates too slowly and with too little force. If a motor is to be of real service, its armature must rotate with sufficient strength to impart motion to the wheels of trolley cars and mills, to drive electric fans, and to set into activity many other forms of machinery.

The strength of a motor may be increased by replacing the singly coiled armature by one closely wound on an iron core; in some armatures there are thousands of turns of wire. The presence of soft iron within the armature (Section 296) causes greater attraction between the armature and the outside magnet, and hence greater force of motion. The magnetic strength of the field magnet influences greatly the speed of the armature; the stronger the field magnet the greater the motion, so electricians make every effort to strengthen their field magnets. The strongest known magnets are electromagnets, which, as we have seen, are merely coils of wire wound on an iron core. For this reason, the field magnet is usually an electromagnet.

When very powerful motors are necessary, the field magnet is so arranged that it has four or more poles instead of two; the armature likewise consists of several portions, and even the commutator may be very complex. But no matter how complex these various parts may seem to be, the principle is always that stated in Section 309, and the parts are limited to field magnet, commutator, and armature.

[Ill.u.s.tration: FIG. 231.--A modern power plant.]