EVOLUTION OF QUARTZ CRYSTAL CLOCK 571 



other thatt sidereal. For small and slowly changing effects this could be taken 

 care of very simply by means of electrical circuits now well known for adding 

 or subtracting small changes in the control frequency. 



The other apphcation refers to a suggestion made by the author a few 

 years ago^^^ for the measurement of gravity, and changes in gravity, by 

 comparison of the forces Mg and Mcc'^R. The proposal was based on the 

 idea that co can be measured or produced with an accuracy two or more 

 orders greater than required, and that the problem reduces to that of balanc- 

 ing two forces and of measuring a linear displacement. The physical set-up 

 would be some form of conical pendulum driven at constant angular velocity 

 about the vertical axis under control of a crystal. Some such arrangements 

 are shown in the reference. 



Future Possibilities 



It is part of the nature of a scientist to extrapolate ahead of any current 

 development and to wonder what lies beyond. That feeling is certainly 

 justified in the field of time measurement, for the major advances have 

 taken place in so short a period and so recently, as compared with the 

 thousands of years during which Man has been time-conscious in some 

 degree, that it is reasonable to expect continued advancement for many years 

 to come. Such advancement may come as improvements and refinements 

 in existing techniques, or radically new methods may be developed with 

 inherently more stable potentiaHties. 

 Accuracy of Rate 



In the first place, it is not reasonable to suppose that the final accuracy 

 that can be attained with the quartz crystal clock has been reached; in view 

 of the rapid current progress indicated in the chart of Fig. 1, it is much too 

 soon to assume this, and there is considerable evidence that improvements 

 could be made by making fuller use of some of the stable properties of quartz 

 crystal and of refinements in the mounting and sustaining circuits. The 

 quartz oscillator assemblies in most general use at the present time embody 

 some compromises which it would not be necessary to make if an all-out 

 effort were being made to construct a few clocks having the highest attain- 

 able stability under the most favorable conditions of operation. 



The first of these concerns the shape and size of the resonator itself and is 

 related to the frequency of oscillation. From the standpoint of stability of 

 operation, the actual frequency that is used in the oscillator is of little con- 

 cern because it is now a very simple matter to obtain low frequencies, suitable 

 for the operation of mechanisms, starting with any frequency that can be 

 controlled by a crystal resonator. The choice of 100,000 cycles for the first 

 zero-coefficient resonator was made because, as a standard of frequency, 



