THE BRIDGE STABILIZED OSCILLATOR 



589 



between the oscillator under test and a reference bridge oscillator were 

 read upon a linear scale calibrated directly in terms of frequency 

 deviation. Full scale could be made one part in lO*, 10^, 10'^ or 10^ by 

 means of a simple switching operation. For most of the measurements 

 in this paper the full-scale reading was one part in a million, and the 

 resolution about ± 0.005 part in a million. 



By using a recording meter with this measuring set, continuous 

 frequency comparisons between two independent bridge oscillators 



-600 



-400 -200 200 400 



REACTANCE IN SERIES WITH CRYSTAL IN OHMS 



600 



Fig. 10 — Frequency of oscillator vs. adjusting reactance. 



were obtained over a period of several months. Figure 11 is a photo- 

 graph of a section of this record. It shows the short-time variations of 

 both oscillators plus a small amount of scattering caused by the meas- 

 uring equipment itself. The crystals were temperature-controlled in 

 separate ovens, and the power was supplied from separate sets of 

 laboratory batteries controlled to about ± 2 per cent in voltage. 

 Shielding was ample to avoid any tendency to lock in step. 



In addition to these small short-time variations, the oscillators 

 exhibited a very slow upward drift in frequency, attributed to aging 



