Part 54 (1/2)
Induced currents; 3 laws, ill.u.s.trations.
Construction, action, and uses of--magneto, dynamo, induction coil, transformer, motor, telephone. Mercury arc rectifier.
Terms--primary, secondary, for coils and currents, armature, commutator, slip ring, brush, rectifier, open core, series, shunt, and compound connections for dynamos.
CHAPTER XV
SOUND
(1) SOUND AND WAVE MOTION
=317. What is a Sound?=--This question has two answers, which may be ill.u.s.trated as follows: Suppose that an alarm clock is set so that it will strike in one week and that it is placed upon a barren rock in the Pacific Ocean by sailors who immediately sail away. If when the tapper strikes the bell at the end of the week no ear is within a hundred miles, is any sound produced? The two view-points are now made evident, for some will answer ”no” others ”yes.” Those answering ”no” hold that sound is a _sensation_ which would not be produced if no ear were at hand to be affected. Those answering ”yes” understand, by the term sound, _a mode of motion capable of affecting the auditory nerves_, and that sound exists wherever such motions are present. This latter point of view is called the _physical_ and is the one we are to use in this study.
[Ill.u.s.tration: FIG. 314.--The tuning fork is vibrating.]
=318. Source of Sound.=--If we trace any sound to its source, it will be found to originate in a body in rapid motion usually in what is called a state of _vibration_. To ill.u.s.trate, take a tuning fork, strike it to set it in vibration and place its stem firmly against a thin piece of wood; the sound will be strengthened materially by the vibration of the wood. If now the vibrating fork is placed with the tips of the p.r.o.ngs in water, the vibration is plainly shown by the spattering of the water (Fig. 314). When one _speaks_, the vibrating body is in the _larynx_ at the top of the windpipe. Its vibration may be plainly felt by the hand placed upon the throat while speaking.
=319. Sound Media.=--Usually sounds reach the ear through the air. The air is then said to be a _medium for sound_. Other substances may serve as a sound medium, for if the head is under water and two stones, also under water, are struck together a sharp sound is heard. Also if one end of a wooden rod is held at the ear and the other end of the rod is scratched by a pin, the sound is more plainly perceived through the wood than through the air. Think of some ill.u.s.tration from your own experience of a solid acting as medium for sound. If an electric bell is placed in a bell jar attached to an air pump, as in Fig. 315, on exhausting the air the loudness of the sound is found to diminish, indicating that in a perfect vacuum no sound would be transmitted. This effect of a vacuum upon the transmission of sound is very different from its effect upon radiation of heat and light. Both heat and light are known to pa.s.s through a vacuum since both come to the earth from the sun through s.p.a.ce that so far as we know contains no air or other matter.
Sound differs from this in that it is always transmitted by some material body and cannot exist in a vacuum.
[Ill.u.s.tration: FIG. 315.--Sound does not travel in a vacuum.]
=320. Speed of Sound.=--Everyone has noticed that it takes time for sound to travel from one place to another. If we see a gun fired at a distance, the report is heard a few seconds after the smoke or flash is seen. The time elapsing between a flash of lightning and the thunder shows that sound takes time to move from one place to another. Careful experiments to determine the speed of sound have been made. One method measures accurately the time required for the sound of a gun to pa.s.s between two stations several miles apart. A gun or cannon is placed at each station. These are fired alternately, first the one at one station and then the one at the other so as to avoid an error in computation due to the motion of wind. This mode of determining the speed of sound is not accurate. Other methods, more refined than the one just described have given accurate values for the speed of sound. The results of a number of experiments show that, at the freezing temperature, 0C., the speed of sound in air is 332 meters or 1090 ft. a second. The speed of sound in air is affected by the temperature, increasing 2 ft. or 0.6 meter per second for each degree that the temperature rises above 0C.
The speed decreases the same amount for each degree C. that the air is cooled below the freezing point. The speed of sound in various substances has been carefully determined. It is greater in most of them than in air. In water the speed is about 1400 meters a second; in wood, while its speed varies with different kinds, it averages about 4000 meters a second; in bra.s.s the speed is about 3500 meters; while in iron it is about 5100 meters a second.
=321. The Nature of Sound.=--We have observed that sound originates at a vibrating body, that it requires a medium in order to be transmitted from one place to another, and that it travels at a definite speed in a given substance. Nothing has been said, however, of the _mode_ of transmission, or of the _nature_ of _sound_. Sounds continue to come from an alarm clock even though it is placed under a bell jar. It is certain that nothing material can pa.s.s through the gla.s.s of the jar.
If, however, we consider that _sound is transmitted by waves through substances_ the whole matter can be given a simple explanation. In order to better understand the nature of sound a study of waves and wave motion will be taken up in the next section.
Important Topics
Sound: two definitions, source, medium, speed, nature.
Exercises
1. Give two ill.u.s.trations from outside the laboratory of the fact that sound is transmitted by other materials than air.
2. Name the vibrating part that is the source of the sound in three different musical instruments.
3. Is sound transmitted more strongly in solids, liquids or gases? How do you explain this?
4. How far away is a steamboat if the sound of its whistle is heard 10 seconds after the steam is seen, the temperature being 20C.? Compute in feet and in meters.
5. How many miles away is lightning if the thunder is heard 12 seconds after the flash in seen, the temperature being 25C.?
6. Four seconds after a flash of lightning is seen the thunder clap is heard. The temperature is 90F. How far away was the discharge?
7. The report of a gun is heard 3 seconds after the puff of smoke is seen. How far away is the gun if the temperature is 20C.?