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BELL SYSTEM TECHNICAL JOURNAL 



of the mounted crystals. This aging decreased in a regular manner 

 with time, the mean drift of one of the crystals being less than one part 

 in ten million per month after three months of continuous operation, 



10 20 30 40 50 60 70 80 90 100 

 PARTS IN 108 



Fig. 11 — Record of frequency comparison between two bridge stabilized oscil- 

 lators. Full scale one part in a million. Variations less than ± 2 parts in one 

 hundred million. 



and about a third of this amount after seven months. In most appli- 

 cations, gradual frequency drift is not objectionable even though the 

 required accuracy is very high, for readjustment is merely a matter of 

 setting a calibrated dial. 



