EVOLUTION OF QUARTZ CRYSTAL CLOCK 531 



"The frequency of the fork in comparison with the N.P.L. Shortt clock can be measured 

 at any time with an accuracy of 5 parts in 10*. It is necessary to apply a correction for 

 the rate of the Shortt clock, and the ultimate accuracy with which the absolute value of 

 frequency is known depends on the accuracy of the time signals which are used to determine 

 the rate of the clock. The final frequency can, however, usually be ascertained with an 

 accuracy of ±1.5 parts in 10^. In its present condition the tuning fork maintains a 

 frequency stability of the order of 3 parts in 10^ over periods of a week or more." 



A considerable amount of efTort has been devoted to the improvement of 

 tuning forks, directed mostly toward stabilizing the fork itself. Patents 

 issued to H. H. Hagland^^, August Karolus-*- and Bert Eisenhour*-*^ have been 

 concerned with the reduction of temperature coefficient by various methods 

 of compensation in the alloy or in the mechanical structure of the fork. In 

 recent years, alloys have been produced from which forks with a zero coeffi- 

 cient of frequency can be machined. These alloys have neither a zero ex- 

 pansion coefficient nor a zero elastic coefficient, but the two coefficients are 

 so balanced that their effects cancel as they concern the frequency of a 

 tuning fork. 



One of the largest residual sources of error in a good fork is that caused by 

 the coupUng through the mounting. A fork which is efficient as a producer 

 of sound by coupling through the base would be quite useless as a precise 

 standard of rate due to the losses introduced in this manner. It has been 

 shown by S. E. Michaels^^ that the tines of a well-balanced fork can be so 

 shaped that practically no energy at fundamental frequency is transmitted 

 through the base. 



By making use of all that is known about materials, shapes and mountings 

 for tuning forks, and all that is known about stabilized vacuum tube cir- 

 cuits for driving them, it is quite possible that considerable further improve- 

 ment could now be obtained in such a standard. But another line of 

 development has shown greater promise in this field and the ultimate 

 accuracy of tuning fork oscillators has never been pursued. 



The Quartz Resonator 



During the same ten years that the greatest advances were being made 

 in the tuning fork art, the striking properties of the quartz crystal resonator 

 were reviewed and first applied in the construction of frequency and time 

 standards. Its use in primary standards for the most exacting measure- 

 ments of frequency and time is now almost universal in national and indus- 

 trial laboratories throughout the world. 



Quartz crystal is the most abundant crystalline form of silicon dioxide, 

 occurring, in some parts of the world, in large single crystals from which 

 mechanical resonators of useful dimensions can readily be formed. The 

 physical properties that make it eminently suitable for use in a standard of 

 rate or time are its great mechanical and chemical stability. Having a 



