524 BELL SYSTEM TECHNICAL JOURNAL 



chain of events which led eventually to the crystal clock followed a course 

 quite independent of pendulum clock development, although parallel with 

 it, and meeting it from time to time on the way. From the start, it in- 

 volved the use of resonant elements whose frequencies do not depend upon 

 gravity for controlling the frequency of oscillations in a positive feedback 

 amplifier. From a rather simple beginning, taking advantage of a series of 

 discoveries and inventions through about a century of progress, there has 

 evolved a clock whose stability is comparable with that of astronomical 

 time itself, as heretofore defined in terms of the earth's rotation, and having 

 a versatility far exceeding all other existing means for the precision measure- 

 ment of time. 



Electric Oscillators 



The first recorded experiments that relate directly to this development 

 were those of Jules Lissajous^ who, in 1857, showed that a tuning fork can 

 be sustained in vibration indefinitely by electrical means, using an electro- 

 magnet and an interrupter supported by one of the prong^ The idea of 

 using an interrupter to sustain vibration was not new with Lissajous, but 

 had been invented by C. G. Page^ and described by him as early as April 

 1837, to obtain a regularly interrupted electric current. Credit for this 

 important invention is often given to Golding Bird^° or Neeff^^ who evidently 

 were working along similar lines concurrently although quite independently 

 of each other. Page, Golding Bird and Neeff were all medical doctors and 

 evidently were interested in their devices more for their therapeutic interest 

 than for the general scientific value, since ^'galvanic" electricity was at- 

 tributed at that time with marvelous healing powers. 



Lissajous was probably the first to make use of the idea for accurate 

 measurements of rate, being a prolific experimenter in mechanics end 

 acoustics, and the originator of the famous method bearing his name for 

 the study of periodic motions. Indeed, the electrically operated fork was 

 developed especially for use as a standard to be used in studying the rates 

 of other vibrators. In principle, the electrically operated fork is like the 

 pendulum drive of Alexander Bain, except that the rate of vibration in this 

 case is not a function of gravity but for the most part is controlled by the 

 effective mass and elastic stiffness of the vibrating member. 



The tuning fork itself was invented in 1711 by John Shore, a trumpeter in 

 Handel's orchestra'^, and was developed to a high state of perfection by 

 the great instrument maker and physicist of Paris, Rudolph Konig. To 

 establish an accurate standard of pitch for calibrating these forks Konig 

 developed what he termed an "absolute" method for the determination of 

 frequency. This consisted of a tuning fork having a frequency of 64 vjbra- 



