The experiment which follows will show that we usually must have air to do the vibrating to carry the sound.
Experiment 56. Make a pad of not less than a dozen thicknesses of soft cloth so that you can stand an alarm clock on it on the plate of the air pump. The pad is to keep the vibrations of the alarm from making the plate vibrate. A still better way would be to set a tripod on the plate of the air pump and to suspend the alarm clock from the tripod by a rubber band. Set the alarm so that it will ring in 3 or 4 minutes, put it under the bell jar, and pump out the air. Before the alarm goes off, be sure that the air is almost completely pumped out of the jar. Can you hear the bell ring? Distinguish between a dull trilling sound caused by the jarring of the air pump when the alarm is on, and the actual ringing sound of the bell.
Fig. 97. When the air is pumped out of the jar, you cannot hear the bell ring.
The experiment just completed shows how we know there would be no sound on the moon, since there is practically no air around it. The next experiment will show you more about the way in which phonographs work.
Fig. 98. Making a phonograph record on an old-fashioned phonograph.
Experiment 57. Put a blank cylinder on the dictaphone, adjust the recording (cutting) needle and diaphragm at the end of the tube, start the motor, and talk into the dictaphone. Shut off the motor, remove the cutting needle, and put on the reproducing needle (the cutting needle, being sharp, would spoil the cylinder). Start the reproducing needle where the recording needle started, turn on the motor, and listen to your own voice.
Notice that in the dictaphone the air waves of your voice are all concentrated into a small space as they go down the tube. At the end of the tube is a diaphragm, a flat disk which is elastic and vibrates back and forth very easily. The air waves from your voice would not vibrate the needle itself enough to make any record; but they vibrate the diaphragm, and the needle, being fastened rigidly to it, vibrates with it.
In the same way, when the reproducing needle vibrates as it goes over the track made by the cutting needle, it would make air vibrations too slight for you to hear if it were not fastened to the diaphragm. When the diaphragm vibrates with the needle, it makes a much larger surface of air vibrate than the needle alone could. Then the tube, like an ear trumpet, throws all the air vibrations in one direction, so that you hear the sound easily.
Experiment 58. Put a clean white sheet of paper around the recording drum, pasting the two ends together to hold it in place. Put a small piece of gum camphor on a dish just under the paper, light it, and turn the drum so that all parts will be evenly smoked. Be sure to turn it rapidly enough to keep the paper from being burned.