RADIO COMMUNICATION 



271 



by introducing into the receiving circuit a vacuum valve to act 

 as a generator of continuous waves. When two tuning forks of 

 slightly different pitch are sounded near together, a pulsating 

 sound is heard. This is due to the sound waves reinforcing each 

 other and interfering with each other at regular intervals. The 

 number of pulsations per second will be equal to the difference 



FIG. 125. Power tubes for transmission. (Photo by Radio Corporation of 

 America.) 



in the rates of vibration of the two notes. Identically the same 

 principle is used in the reception of continuous-wave telegraph 

 signals. The local oscillating tube generating the continuous 

 wave in the receiving circuit may be part of a separate circuit as 

 in the case of heterodyne reception. Or the detector tube may 

 be used for generating continuous waves as well as for acting as a 

 detector, and then we have autodyne reception. The rate of 

 oscillation of the receiving circuit may be varied, and the differ- 



