146 BELL SYSTEM TECHNICAL JOURNAL 



hundred cycle fork is kept in constant oscillation by a regenerative 

 method, the conditions being so controlled that the mean period of 

 the fork compares favorably with that of a good clock. 



The attraction of the second method lies in the possibility of obtain- 

 ing a sufficiently constant standard of frequency with nothing more 

 than a good clock and standard auxiliary apparatus easily capable of 

 application to any oscillating system. Such an outfit could be made 

 available in cases in which the expense incident to the installation 

 and maintenance of more elaborate equipment would not be justified. 



Requirements of a Clock-Controlled FREyuENCV Stand.vrd 



It is a comparatively simple matter to control or operate a fork, or 

 other oscillating system, by means of periodic impulses from a clock, 

 so that the total number of oscillations will be some definite multiple 

 of the number of impulses from the clock. However, the present 

 requirements are more severe than this. It is necessary to have the 

 oscillator operated so that each oscillation will be sensibly equal in 

 magnitude and duration to every other oscillation. In other words, 

 it is not sufficient that the clock and the oscillator keep in step over a 

 given period of time, but the instantaneous frequency of the fork must 

 not depart appreciably from the mean frequency. This requires a 

 form of control which will not be to any extent discontinuous, but 

 which M-ill change uniformly in proportion to the divergence of the 

 oscillator from the clock. Such a form of control in turn requires 

 that the frequency of the oscillator itself be sufficiently constant when 

 uncontrolled, to reduce all momentary fluctuations and rapid fre- 

 quency changes to a minimum. This requirement is best satisfied 1)\- 

 an oscillating system having a low decrement. Since a mechanical 

 system is usually far superior to an electrical system in this respect, 

 and since the most available mechanical oscillator for the range of 

 frequency in tjui-stion is a fork, our choice natur.iHy falk on tliis torni 

 of oscillator. 



A good fork maintained in continuous operation by some electric.d 

 means, such as regeneration, or a make and break contact and a dri\- 

 ing magnet, is a comparatively simple system and is capable of a high 

 degree of constancy.' It therefore satisfies all of the requirements 

 for our purpose, but there remains the devising of some control which 

 will be proportional to the divergence of the fork from the clock con- 

 trolling it. In order to use any such control it is practically neces- 

 sary to integrate the oscillations of the fork so that we may obtain a 



•H. M. Dadourian, Phys. Rev. 1.^, page 337, 1919, "On the Characteristics of 

 Electrically Operated Tuning Forks." 



