General Science - Bertha May Clark (children's books read aloud TXT) 📗
- Author: Bertha May Clark
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In Section 299 we learned that substances differ very greatly in the resistance which they offer to electricity, and so it will not surprise us to learn that while it takes 300 feet of iron telegraph wire to give 1 ohm of resistance, it takes but 39 feet of number 24 copper wire, and but 2.2 feet of number 24 German silver wire, to give the same resistance.
NOTE. The number of a wire indicates its diameter; number 30, for example, being always of a definite fixed diameter, no matter what the material of the wire.
If we wish to avoid loss of current by heating, we use a wire of low resistance; while if we wish to transform electricity into heat, as in the electric stove, we choose wire of high resistance, as German silver wire.
CHAPTER XXXV HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE318. The Dynamo. We have learned that cells furnish current as a result of chemical action, and that the substance usually consumed within the cell is zinc. Just as coal within the furnace furnishes heat, so zinc within the cell furnishes electricity. But zinc is a much more expensive fuel than coal or oil or gas, and to run a large motor by electricity produced in this way would be very much more expensive than to run the motor by water or steam. For weak and infrequent currents such as are used in the electric bell, only small quantities of zinc are needed, and the expense is small. But for the production of such powerful currents as are needed to drive trolley cars, elevators, and huge machinery, enormous quantities of zinc would be necessary and the cost would be prohibitive. It is safe to say that electricity would never have been used on a large scale if some less expensive and more convenient source than zinc had not been found.
319. A New Source of Electricity. It came to most of us as a surprise that an electric current has magnetic properties and transforms a coil into a veritable magnet. Perhaps it will not surprise us now to learn that a magnet in motion has electric properties and is, in fact, able to produce a current within a wire. This can be proved as follows:—
FIG. 237.—The motion of a magnet within a coil of wire produces a current of electricity.
Attach a closely wound coil to a sensitive galvanometer (Fig. 237); naturally there is no deflection of the galvanometer needle, because there is no current in the wire. Now thrust a magnet into the coil. Immediately there is a deflection of the needle, which indicates that a current is flowing through the circuit. If the magnet is allowed to remain at rest within the coil, the needle returns to its zero position, showing that the current has ceased. Now let the magnet be withdrawn from the coil; the needle is deflected as before, but the deflection is in the opposite direction, showing that a current exists, but that it flows in the opposite direction. We learn, therefore, that a current may be induced in a coil by moving a magnet back and forth within the coil, but that a magnet at rest within the coil has no such influence.
An electric current transforms a coil into a magnet. A magnet in motion induces electricity within a coil; that is, causes a current to flow through the coil.
A magnet possesses lines of force, and as the magnet moves toward the coil it carries lines of force with it, and the coil is cut, so to speak, by these lines of force. As the magnet recedes from the coil, it carries lines of force away with it, this time reducing the number of the lines which cut the coil.
320. A Test of the Preceding Statement. We will test the statement that a magnet has electric properties by another experiment. Between the poles of a strong magnet suspend a movable coil which is connected with a sensitive galvanometer (Fig. 237). Starting with the coil in the position of Figure 228, when many lines of force pass through it, let the coil be rotated quickly until it reaches the position indicated in Figure 238, when no lines of force pass through it. During the motion of the coil, a strong deflection of the galvanometer is observed; but the deflection ceases as soon as the coil ceases to rotate. If, now, starting with the position of Figure 238, the coil is rotated forward to its starting point, a deflection occurs in the opposite direction, showing that a current is present, but that it flows in the opposite direction. So long as the coil is in motion, it is cut by a varying number of lines of force, and current is induced in the coil.
The above arrangement is a dynamo in miniature. By rotation of a coil (armature) within a magnetic field, that is, between the poles of a magnet, current is obtained.
In the motor, current produces motion. In the dynamo, motion produces current.
321. The Dynamo. As has been said, the arrangement of the preceding Section is a dynamo in miniature. Every dynamo, no matter how complex its structure and appearance, consists of a coil of wire which can rotate continuously between the poles of a strong magnet. The mechanical devices to insure easy rotation are similar in all respects to those previously described for the motor.
FIG. 239.—A modern electrical machine.
The current obtained from such a dynamo alternates in direction, flowing first in one direction and then in the opposite direction. Such alternating currents are unsatisfactory for many purposes, and to be of service are in many cases transformed into direct currents; that is, current which flows steadily in one direction. This is accomplished by the use of a commutator. In the construction of the motor, continuous motion in one direction is obtained by the use of a commutator (Section 310); in the construction of a dynamo, continuous current in one direction is obtained by the use of a similar device.
322. Powerful Dynamos. The power and efficiency of a dynamo are increased by employing the devices previously mentioned in connection with the motor. Electromagnets are used in place of simple magnets, and the armature, instead of being a simple coil, may be made up of many coils wound on soft iron. The speed with which the armature is rotated influences the strength of the induced current, and hence the armature is run at high speed.
FIG. 240.—Thomas Edison, one of the foremost electrical inventors of the present day.
A small dynamo, such as is used for lighting fifty incandescent lamps, has a horse power of about 33.5, and large dynamos are frequently as powerful as 7500 horse power.
323. The Telephone. When a magnet is at rest within a closed coil of wire, as in Section 319, current does not flow through the wire. But if a piece of iron is brought near the magnet, current is induced and flows through the wire; if the iron is withdrawn, current is again induced in the wire but flows in the opposite direction. As iron approaches and recedes from the magnet, current is induced in the wire surrounding the magnet. This is in brief the principle of the telephone. When one talks into a receiver, L, the voice throws into vibration a sensitive iron plate standing before an electromagnet. The back and forth motion of the iron plate induces current in the electromagnet c. The current thus induced makes itself evident at the opposite end of the line M, where by its magnetic attraction, it throws a second iron plate into vibrations. The vibrations of the second plate are similar to those produced in the first plate by the voice. The vibrations of the far plate thus reproduce the sounds uttered at the opposite end.
FIG. 241.—Diagram of a simple telephone circuit.
324. Cost of Electric Power. The water power of a stream depends upon the quantity of water and the force with which it flows. The electric power of a current depends upon the quantity of electricity and the force under which it flows. The unit of electric power is called the watt; it is the power furnished by a current of one ampere with a voltage of one volt.
One watt represents a very small amount of electric power, and for practical purposes a unit 1000 times as large is used, namely, the kilowatt. By experiment it has been found that one kilowatt is equivalent to about 1-1/3 horse power. Electric current is charged for by the watt hour. A current of one ampere, having a voltage of one volt, will furnish in the course of one hour one watt hour of energy. Energy for electric lighting is sold at the rate of about ten cents per kilowatt hour. For other purposes it is less expensive. The meters commonly used measure the amperes, volts, and time automatically, and register the electric power supplied in watt hours.
INDEXof gases by charcoal, 57.
of light waves, 135-138.
Accommodation of the eye, 123.
Acetanilid, 259.
Acetylene, as illuminant, 152-153.
manufacture of, 152-153.
properties of, 220.
Acid, boric, 253.
carbolic, 152, 251, 252.
hydrochloric, 55, 80, 227, 238, 241.
lactic, 230.
oxalic, 247, 248.
salicylic, 253. sulphuric, 55, 80, 240, 241, 307.
sulphurous, 242.
Acids, action on litmus, 220.
Adenoids, 51.
Adulterants, detection of, 16.
Air, characteristics of, 81-83, 86, 189.
compressibility of, 91.
expansion of, 10-11.
humidity, 38, 39.
pumps, 201-205.
transmits sound, 269.
weight of, 86.
See Atmosphere.
Alcohol, 234.
in patent medicines, 260.
Alizarin, 248.
Alkali, 222.
Alternating current, 351.
Alum, 247.
in baking powder, 230.
Ammeter, 341, 343.
Ammonia, 152.
a base, 221-222.
in bath, 226.
in manufacture of ice, 98.
neutralizing chlorine, 240.
Ampere, 342.
Anemia, 259.
Angle, of incidence, 110.
of reflection, 110.
of refraction, 114.
Aniline, 152, 245.
Animal charcoal, 58.
Animal transportation,
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