i 7 8 



Common Science 



FIG. 98. Making a phonograph record on 

 an old-fashioned phonograph. 



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 dia- 

 phragm, a flat disk which 

 is elastic and vibrates 

 back and forth very 

 easily. The air waves 

 from your voice would 

 not vibrate the needle it- 

 self 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 dia- 

 phragm 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. 



