COMMON FREQUENCY BROADCASTING 



587 



merit by means of dustproof can covers which also serve to prevent any 

 accidental disturbance of the settings and adjustments. 



Fig. 6 — Control panel for remote control of the monitoring point receivers. 



With this equipment in commercial operation, a checking of the 

 frequency every ten minutes in connection with the regular routine 

 inspection of the transmitter has been sufficient to maintain the carriers 

 within an average of two cycles per minute of absolute isochronism. 

 Departures from isochronism of this order of magnitude are not de- 

 tectable within the normal service area of either station. 



While with an installation of this type one is primarily concerned 

 with frequency stability rather than permanence of calibration, the 

 Laboratories have measured the frequency of these stations period- 

 ically. It was found at the time of the installation, after the reas- 

 sembly of the equipment subsequent to its shipment from New York, 

 that the frequency was about two cycles per second different from that 

 measured before shipment. Measurements since that time have shown 

 that the frequency has varied over a period of time between seven 

 cycles above the assigned frequency and seventeen cycles below it. It 

 is known, however, that these variations were primarily due to the 



