170 LIGHT AND SOUND 



192. Noise and Tone. We distinguish clearly between 

 noises, which come from the confused mingling of sound 

 waves, and tones, which are the result of a rapid succession 

 of waves, all of the same sort, and coming at regular in- 

 tervals. It is this regularity that makes musical sounds 

 pleasant to the ear. By striking hard bodies, such as 

 dishes, stones, wires, car wheels, etc., we find that each 

 has its own definite tone. 



By the pitch of a tone we mean whether it is high or low. 

 Pitch is caused by the frequency of the vibrations of what- 

 ever causes the sound. " Middle C " of a piano is brought 

 about by a wire that vibrates 256 times in a second. The 

 range of the human voice is from about 80 vibrations 

 (lowest bass) to about 1000 vibrations (high soprano) in a 

 second. 



We can hear sounds due to a much greater frequency than we can 

 produce. Thus, the high notes of a violin, the noise of a cricket, and 

 the whistle of a locomotive are due to thousands of vibrations in a 

 second. When a sound comes from more than 40,000 vibrations per 

 second, the human ear can no longer hear it. 



193. The Telephone. The principle according to . 

 which the telephone " works " is shown in Fig. 166. M 

 and N are two permanent magnets placed inside the wire 

 coils B and C. The circuit is through the coils, the wire 

 connecting them, and the earth. Before each magnet 

 there is a thin, iron disk (D and E). If we make a sound 

 in front of D, the sound waves in the air make the disk 

 vibrate. As D moves back and forth in the " lines of 

 force " (c/. 138) of its magnet (M), it induces currents 

 (rf. 160) in the coil B. These currents are carried along 

 the wire, and appear in the coil C. Here they cause move- 



