A Clock-Controlled Tuning Fork as a Source 

 of Constant Frequency 



By J. G. FERGUSON 



Notk: The art of electrical conimunication employs such a wide variety 

 of methods for the transmission of intelligence that it utilizes alternatin); 

 currents whose fre<iuencies cover the entire range l)ctwecn a few cycles per 

 second and several million. With the increasing use of these methods, it 

 iK'comes more and more imperative that determinations of the frequency 

 of any alternating current may Ik made with extreme accuracy. In piir- 

 ticular, recent developments in carrier current telephony and telegraphy 

 over wires have placed exceedingly rigorous limits on the frequency adjust- 

 ment of certain typxrs of apparatus. It is many times necessary to hold 

 such c<)uipment as oscillators or filters to within 0.1 [x;r cent, of given fre- 

 quency values under commercial o(x"rating conditions. This means that 

 calibrating devices usetl in the manufacture and maintenance of such cir- 

 cuits must be reliable to 0.01 percent, and that the primary standard should 

 be good to at>out 0.1X)1 per cent, or one part in KK),000. 



1 he present pajx-r discusses one of the methods recently developed in 

 the Bell System Lalwratory for obtaining a source of practically constant 

 frequency with which other frequencies may be compared. It consists of 

 a clock-controlled tuning fork making 50 vibrations per second and, as is 

 shown, the maximum deviation of its frequency from the mean is less than 

 one part in 50,000. 



A study has also been made of means for improving the constancy of the 

 control clock an<l a new type of clock mechanism consisting of an electrically 

 actuated (x^ndulum, the impulse of which is controlled by a photo-electric 

 cell, is suggested. — lii>lTOR. 



Introduction 



THE art of clock making is of such long standing that there have 

 been few improvements of note in the last fifty years tend- 

 ing to increase accuracy. The average rate of oscillation of a good 

 clock when taken over a sufficiently long period of time as, for in- 

 stance, a day, can be held constant to about one part in 1,000,000. 

 This accuracy is sufficiently high for all ordinary requirements in the 

 measurement of time, including the field of electrical cominunication. 



However, in electric measurements, the problems which present 

 themselves ordinarily require the accurate measurement of intervals 

 ven.- much shorter than a second which is usually the smallest interval 

 registered by the a\^rage clock. In solving these problems, we are 

 therefore forced to the alternative either of designing a clock to have 

 a period very much shorter than those of existing clocks or of using 

 some form of short pcricxl oscillator whose uniformity can be con- 

 trolled by the second impulses from a clock. 



The first method has been admirably worked out as described by 



other members of the staff of this laboratory.' In this system a 



' Paper by J. VV. Horton, N. H. Rickcr and W. A. Marrison, presented at the 

 annual convention of the American Institute of Electric Engineers, June, 1923. 



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