552 BELL SYSTEM TECHNICAL JOURNAL 



The control of amplitude is obtained by the use of a resistance with positive 

 temperature coefficient in the bridge arm conjugate to the crystal, chosen so 

 as to have exactly the right value to balance the resistance of the crystal when 

 a specified current is flowing in the bridge. If larger than normal current 

 flows momentarily the resistance is increased, which decreases the feedback, 

 thus stabilizing the amplitude at some predetermined value. For the 

 highest stabihty it has been found advantageous to operate the crystal at a 

 very small fraction of the amplitude that normally would be used in a power 

 oscillator. In power oscillators the crystal sometimes is subjected to strains 

 near the fracture point, which is not a favorable condition for precision 

 control. The actual amplitude of motion of the crystal is of course extremely 

 small. In the GT crystal, as currently used, the maximum change of 

 dimensions during oscillation amounts to only about ±0.0006 per cent. 



The improvements in quartz resonators, and in their driving circuits, have 

 resulted in the construction of quartz crystal clocks that will keep time with 

 an accuracy better than 0.001 second a day, so that measurements of time 

 of great interest and value to astronomers and geophysicists can now be made 

 with an accuracy hitherto unattainable. 



Facility of Precise Time Measurement 



In making such precise measurements of time it is of importance, second 

 only to the inherent accuracy of the standards themselves, to have available 

 means whereby they can be carried out with facility and within a reasonable 

 time interval. The ease with which precise time measurements, and precise 

 rate comparisons, can be made is an outstanding feature of the quartz 

 crystal clock and already has an important bearing on the use of this type of 

 clock in astronomical observatories. This facility depends chiefly on two 

 properties of the oscillator clock: first, that continuous rotation of controlling 

 and measuring devices can be produced having the stability of the primary 

 control element; and, second, that the period of the control element, and 

 therefore of alternating current controlled by it, is of very short duration. 



The first of these, through simple devices controlled directly from the 

 electrical output of the crystal oscillator, with suitable frequency reducing 

 equipment, permits of ready comparison between any time phenomena in 

 the form of electric or light signals, and of the derivation of precisely con- 

 trolled time signals for radio transmission and for laboratory experiments. 



Of prime importance among these comes the means for rating crystal clocks 

 in terms of stellar observations using meridian transits or the photographic 

 zenith tube^. It is possible to control a mechanism in the time-star observ- 

 ing equipment so that the difference between a star position predicted from 

 the clock rate, and the actual star position, can be observed directly or re- 



