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length, and it will at once give out a loud musical tone. Compare the pitches given by tubes of different lengths.

240. Sensitive Flames. - Demonstration. Select a piece of small glass tubing and draw it to a point in the Bunsen flame, leaving a fine opening. Connect the other end, by means of a rubber tube, to a gas supply, and if you have the right size of hole in the tube, and the right pressure of gas, which is generally greater than city gas mains supply, you will get a long line of flame, as in A (Fig. 217), that is just on the point of flaring. Make any kind of a sharp sound, and the flame will at once drop down to the form of B, and will keep flaring in that form as long as the sound continues. A shrill whistle, the rattle of keys, or any hissing sound will produce the same effect, showing that this is a very sensitive form of flame. Does the rapid change in pressure at the mouth of the tube, caused by the waves of condensation and rarefaction due to the high pitch of these sounds, explain the action of the flame? Test this flame by giving a shrill whistle outside of the room when the door is closed.

FIG. 217

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241. The Phonograph, invented by Edison, consists of a cylinder of specially prepared wax upon which the vibrations of a diaphragm are recorded by means of a fine metal point or chisel attached to the diaphragm. The waves of sound throw the diaphragm into vibration, this sets the point in motion, and as the wax cylinder is rotated the point cuts a series of spiral grooves. These grooves are made up of minute indentations which correspond to the condensations and rarefactions of the sound waves. By means of a special form of point which takes the place of the cutting tool, and follows in the groove which it has cut, the sound can be reproduced with remarkable fidelity.

Other instruments for the reproduction of sound make use of a disk for the reproducing surface.

242. Limit of Audibility. Every one knows that the range of voice differs for different people, one person singing tenor, another alto, and so on. There is a somewhat similar range in hearing, some ears being more sensitive to the high pitches, and some to the low.


Procure a Galton's whistle, which consists of a small brass whistle with a rubber bulb at one end and a screw for



adjusting the pitch at the other. Press the bulb when the screw is nearly out, and a rather low whistle will be heard. Turn in the

FIG. 218. Galton's Whistle

screw a little, and sound again. The pitch is higher. In this way make the pitch steadily higher and higher, and it will be found that first one member of the class and then another will be unable to hear the whistle.

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1. Does it change the pitch of a sonometer string to draw the bow more vigorously?

2. Which string on a violin is the smallest? Why?

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FIG. 219


4. What is the explanation of the cracking of a test tube by holding it at the top and drawing a damp towel from top to bottom?

5. Suppose Fig. 219 to represent the open end of a bell jar which is struck a light blow at A driving the rim toward the center. What will take place at the points B, C, and D, and where will the nodes be located?

3. Why is the G string on a violin wound with wire?

6. Suppose you wish to produce a low tone on an organ pipe and you want the pipe to be as short as possible. Would you use an open, or closed pipe?

7. In what respects does the sound produced by a wooden organ pipe 1 m. long differ from that given by a metal pipe of the same length? In what respects is it the same?

8. Where does the sand collect on a vibrating plate?


9. How would you establish the formation of a node in a vibrating plate?


1. If a sonometer string 1 m. long gives 128 vibrations per second, what must be the length of a similar string, stretched with the same weight, to give 192 vibrations? What tone will it give?

2. If the string of a sonometer when vibrating as a whole gives the tone c' or middle C of the piano, where must it be stopped with the finger to make it vibrate in thirds? What tone will it give?

3. The pitch given by a certain string is c when the spring balance by which it is stretched reads 1.5 lb. What will the balance read when the string is stretched so that it sounds e?

4. What will be the length in cm. of the part of a string 1 m. long that will give each tone of the major scale?

5. What is the wave length of the tone of a closed organ pipe 3 ft. long? How long must an open pipe be to give the same tone? 6. How long must an open organ pipe be to give 256 vibrations at 0° C.?

7. How long must it be to give 384 vibrations under the same conditions?

8. What must be the length of the short pendulum of Fig. 210 to make the ratio of vibrations 3:2 if the length of the pendulum as a whole is 1 m.? What must be its length to give the ratio C:E?

9. What is the length of the air column in the Galton whistle, Fig. 218, when it gives 16,000 vibrations per second, the temperature being 20° C.? What is the wave length of the sound produced?

10. The wave length of a certain tone given by the Galton whistle is 4 cm. How many vibrations does it give at 22° C.?




243. Heat a Form of Energy. According to the modern kinetic theory of heat, the molecules of all bodies are in a state of rapid vibration, and any increase of the rapidity of this motion, from whatever cause, increases the heat of the body, while the heat is decreased if this velocity is diminished.

Heat is a form of molecular energy which may be produced by other forms of energy, and is itself convertible into other forms.


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244. Temperature. The terms "hot" and "cold" are purely relative. Whether one body is hotter or colder than another depends upon whether it can itself impart heat to the second body, or receive heat from it. The condition of a body in this respect is called its temperature, and depends directly upon its molecular kinetic energy. If m is the mass of one of the molecules of a body, and v its average velocity at a certain temperature, the expression for its molecular kinetic energy (Formula 20) is mv2. Hence the temperature of a body is directly proportional to the square of the average velocity of its molecules.

If one body is put into contact with another, the one that has the higher temperature will lose some of its heat, and the

one that has the lower temperature will gain heat, until they both finally come to the same temperature.


Temperature must not be mistaken for quantity of heat. cup of hot water taken from a pailful will have the same temperature, but will contain very little heat in comparison with the water in the pail.

245. The Physical Effect of Heat upon Bodies. - There are two main results that may come from applying heat to a body. One is a change in its volume, and the other is a change in its physical condition.


FIG. 220

Demonstrations. - Make a piece of apparatus like that

shown in Fig. 220, as follows: Set two upright posts in a baseboard. Bore in each post, near the top, a hole large enough to take a brass wire in. in diameter. Fasten one end of the wire to one post by a screw in the top and let the wire pass loosely through the other post. Connect a battery and electric bell with the wire, and let the other end of the circuit be connected with a thin brass spring just beyond the movable end of the wire. Adjust the spring carefully and bring the flame of a Bunsen burner against the wire. The heat will expand the wire, which will make contact with the spring, when the electrical circuit will be completed and the bell will ring. Remove the flame; the wire will contract, the contact will be broken, and the bell will stop ringing.

Fit to the mouth of a test tube a rubber stopper with a single hole. Thrust a piece of glass tubing, about 30 cm. long, through the stopper. Fill the test tube with water and push in the stopper until the water stands at some point, as A (Fig. 221). Take the tube by the end


FIG. 221

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