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BELL SYSTEM TECHNICAL JOURNAL 



In order to determine extremely small relative variations in fre- 

 quency between two oscillators a method of measurement is used in 

 which the duration of individual beats between two oscillators may 

 be measured with an accuracy of about one part in 10,000. This is 

 equivalent to saying that the average beat frequency during each 10 

 second interval is measured with an accuracy of one part in ten 

 thousand. Since the percentage error in the beat frequency is one 

 million times greater than the percentage error in the primary fre- 

 quency, to measure the beat frequency with an accuracy of one part 



A-COUNTER MECHANISM. D - IOOOTH SECOND CAM. 



B- RESET MECHANISM. M - MASTER RESET RELAY. 



C- MAGNET OPERATED CLUTCH. BFI- CONTACT ON BEAT 



FREQUENCY INDICATOR. 



Fig. 21 — Electrical circuit of one element of beat counter. 



in ten thousand is equivalent to intercomparing the primary fre- 

 quencies with a precision of one part in ten thousand million. By 

 such a precise method of measurement a great deal can be learned 

 about the nature of the variations that do occur. 



The circuit of this device is shown in Fig. 22. Fig. 23 is a photo- 

 graph of the actual apparatus used in obtaining the data given in 

 Fig. 24. A circular transparent scale S, having 100 numbered di- 

 visions, is driven at 10 revolutions per second by a 1,000-cycle syn- 

 chronous motor controlled by one of the crystals. A modulator 

 operates the balanced relay at the difference frequency between the 

 crystal controlled oscillators. This relay discharges condenser Ci 



