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The most significant and far-reaching of the advances of the twentieth century, namely, man's conquest of the air after centuries of failure, was made when the Wright brothers first introduced the principle upon which all successful flight by heavier-than-air machines now depends, namely, control of stability by the warping of wings, or by ailerons (hinged attachments to wings), in connection with the use of a rudder. The upper panel shows one of the original gliders (Wilbur Wright inside) with which the Wrights first mastered the art of gliding (19001903) and made more than a thousand gliding flights, some of them 600 feet long, following in this work the principles of gliding flight first demonstrated by Lilienthal and a little later, much more completely, by Chanute of Chicago (18951897). The lower panel shows "the first successful power flight in the history of the world" (Orville Wright in the machine, Wilbur running beside it as it rose from the track). Four such flights were made on the morning of December 17, 1903, the longest of which lasted 59 seconds and covered a distance of 852 feet against a 20-mile wind

FIG. 338. The modern receiver

A modern telephone receiver is shown in Fig. 338. It consists of a permanently magnetized U-shaped piece of steel in front of whose poles is a soft-iron diaphragm which almost touches the ends of the magnet. Wound in opposite directions upon the two poles are coils of fine insulated wire in series with each other and the line wire. G is the earpiece, E the diaphragm, A the U-shaped magnet, and B the coils, consisting of many turns of fine wire and having soft-iron cores. When the rapidly alternating current from the secondary coil s (Fig. 337) flows through the coils of the receiver, the poles of the permanent magnet are thereby alternately strengthened and weakened in synchronism with the sound waves falling upon the diaphragm of the transmitter. The variations in the magnetic pull upon the diaphragm of the receiver cause it to send out sound waves exactly like those which fell upon the diaphragm of the transmitter.




Telephonic conversation can be carried on over great distances as rapidly as if the parties sat on opposite sides of the same table. An electrical impulse passes over the telephone wires from New York to San Francisco in about one fifteenth of a second. The cross section of a complete long-distance transmitter is shown in Fig. 339. The current traverses granular carbon held between solid blocks of carbon.

FIG. 339. Cross section of a long-distance telephone transmitter


1. Draw a diagram of an induction coil and explain its action. 2. Does the spark of an induction coil occur at make or at break? Why?

3. Explain why an induction coil is able to produce such an enormous E.M.F.

4. Why could not an armature core be made of coaxial cylinders of iron running the full length of the armature, instead of flat disks, as shown in Fig. 326 ?

5. What relation must exist between the number of turns on the primary and secondary of a transformer which feeds 110-volt lamps from a main line whose conductors are at 1100 volts P.D.?

6. Name two uses and two disadvantages of mechanical friction; of electrical resistance.

7. A transformer is so wound as to step the voltage of the lighting circuit from 2200 volts down to 110. Sketch the transformer and its connections, marking the primary and the secondary, and state the relative number of turns in each. If the house circuit uses 40 amperes, what current must flow in the primary?

8. Why does a "tungar" rectify an alternating current?

9. The same amount of power is to be transmitted over two lines from a power plant to a distant city. If the heat losses in the two lines are to be the same, what must be the ratio of the cross sections of the two lines if one current is transmitted at 100 volts and the other at 10,000 volts? (Power IE; heat loss is proportional to I2R.) 10. In telephoning from New York to San Francisco how far do you think the sound goes? What passes along the telephone wire?




377. Sources of sound. If a sounding tuning fork provided with a stylus is stroked across a smoked-glass plate, it produces a wavy line, as shown in Fig. 340; if a light suspended ball is brought into contact with it, the latter is thrown off with considerable violence. If we look about for the source of any sudden noise, we find that some object has fallen, or some collision has occurred, or some explosion has taken place,

in a word, that some violent motion of matter has been set up in some way. From these familiar facts we conclude that sound arises from the motions of matter.

378. Media of transmission. Air is ordinarily the medium through which sound comes to our ears, yet the Indians put their ears to the ground to hear a distant noise, and most boys know how loud the clapping of stones sounds under water. If the base of the sounding fork of Fig. 340 is held in a dish of water, the sound will be markedly transmitted by the water. These facts show that a gas like air is certainly no more effective in the transmission of sound than a liquid or a solid. Next let us see whether or not matter is necessary at all for the transmission of sound.

FIG. 340. Trace made by vibrating fork

*This chapter should be accompanied by laboratory experiments on the speed of sound in air, the vibration rate of a fork, and the determination of wave lengths. See, for example, Experiments 38, 39, and 40 of the authors' Manual.

Let an electric bell be suspended inside the receiver of an air pump by means of two fine springs which pass through a rubber stopper in the manner shown in Fig. 341. Let the air be exhausted from the receiver by means of the pump. The sound of the bell will be found to become less and less pronounced. Let the air be suddenly readmitted. The volume of sound will at once increase.

Since the nearer we approach a vacuum, the less distinct becomes the sound, we infer that sound cannot be transferred through a vacuum and that therefore the transmis

sion of sound is effected only through the FIG. 341. Sound not

transmitted through

a vacuum

agency of ordinary matter. In this respect sound differs from heat and light, which evidently pass with perfect readiness through a vacuum, since they reach the earth from the sun and stars.

379. Speed of transmission. The first attempt to measure accurately the speed of sound was made in 1738, when a commission of the French Academy of Sciences stationed two parties about three miles apart and observed the interval between the flash of a cannon and the sound of the report. By taking observations between the two stations, first in one direction and then in the other, the effect of the wind was eliminated. A second commission repeated these experiments in 1832, using a distance of 18.6 kilometers, or a little more than 11.5 miles. The value found was 331.2 meters per second at 0°C. The accepted value is now 331.3 meters. The speed in water is about 1400 meters per second, and in iron 5100 meters.

The speed of sound in air is found to increase with an increase in temperature. The amount of this increase is about 60 centimeters per degree centigrade. Hence the speed at 20° C. is about 343.3 meters per second. are equivalent to 1087 feet per second at per second at 20° C.

The above figures 0° C., or 1126 feet

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