258 OUR PHYSICAL WORLD 



clicks at audio frequency, producing a short buzz. A dash will 

 be a long buzz. 



The telephone receiver usually employed consists of two 

 watch-case receivers mounted on a head band in such a manner 

 that one receiver will be pressed on each ear. Such a piece of 

 equipment is called the head set (Fig. 117). The ordinary tele- 



FIG. 117. Radio room of the SS. "Leviathan." (Courtesy of the Radio 

 Corporation of America.) 



phone receiver is not sensitive enough for the faint radio signals, 

 and, therefore, much more sensitive receivers with very thin 

 diaphragms and a resistance of around 1,500 ohms are employed. 

 In order that signals of different frequencies may be picked 

 up, the receiving equipment must include apparatus for varying 

 the inductance and capacity of the circuit. By variation, the 

 receiving circuit may be tuned to respond to the vibrations of 

 the sending station. It will be recalled that the violin in the 



