RADIO COMMUNICATION 



257 



the telephone receiver. The commonest type of detector in 

 use is the crystal detector. This piece of apparatus consists 

 of a piece of mineral, usually galena or iron pyrites imbedded in 

 a fusible alloy and so mounted that a fine wire may be adjusted 

 to touch the surface at one point. Since some points on the 

 mineral are more sensitive than others, the wire is made adjust- 

 able so that a sensitive point may be easily found while trying 

 to pick up signals caught by the antenna (Fig. 116). 



FIG. 1 1 6. The crystal detector. (Photo by the Radio Corporation of America . ) 



The action of this crystal type of detector as a rectifier is 

 much the same as that of a check valve in a pump. When the 

 oscillating current from the antenna, which is a back-and-forth 

 surge, attempts to pass through the crystal from the wire point, 

 the back-surge may be stopped so that current in one direction 

 only will pass through to the telephone receiver, and so on to the 

 ground, completing the circuit. The effect is that of a pulsating 

 direct current of audio frequency which will produce one click 

 in the telephone receiver for each wave-train. Since a dot in 

 the telegraph code is a short series of wave-trains, it will be 

 reproduced in the telephone receiver by a short succession of 



