590 



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, 



1 PM 



\\ 



10 



40 50 60 

 PARTS IN 108 



100 



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. 



