Sound 



177 



FIG. 97. When the air is pumped out of the jar, you cannot hear the bell ring. 



sound caused by the jarring of the air pump when the alarm 

 is on, and the actual ringing sound of the bell. 



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. 



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 dicta- 

 phone. 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. 



