506 BELL SYSTEM TECHNICAL JOURNAL 



In the method used for automatic interchecking a fourth oscillator 

 unit is used, identical with the other three except that the frequency 

 is maintained at a slightly different value. The difference between 

 the frequency of this oscillator and that of the other three is kept at 

 about 1 cycle in 10 seconds. The number of beats between the fourth 

 oscillator and each of the other three oscillators is recorded auto- 

 matically during each 1,000 second interval. The number of beats 

 thus recorded is approximately 100 during each interval. 



In 1,000 seconds each oscillator generates approximately one 

 hundred million waves. The numbers that are recorded are, therefore, 

 the number of parts in one hundred million by which oscillator No. 4 



Fig. 16 — Rotor of motor-generator. 



differs from each of the three primary oscillators during the interval. 

 If the numbers recorded during successive intervals remain the same, 

 the oscillators either did not vary, or they all varied in the same 

 direction by the same number of cycles. If the numbers recorded in 

 successive intervals vary by say 1, 2 or 5, it means that the oscillators 

 have drifted, relative to each other, by so many parts in one hundred 

 million. 



Designating the frequencies of the four oscillators by 



fl,f2js,U (1) 



the three numbers recorded are 



