EVOLUTION OF QUARTZ CRYSTAL CLOCK 527 



With the vacuum tube oscillator controlled by electric circuit elements, 

 it would have been possible immediately to operate a clock by means of a 

 phonic wheel motor. Even if this had been done, however, the accuracy 

 would not have compared very favorably with that of good mechanical 

 clocks of the period. This is because the rate-controlling element of such 

 oscillators was subject to large changes due to temperature and aging, and 

 because means were not yet known for avoiding the effects of tube and other 

 variables on the resulting frequency. 



The next important step in our evolution was the use of the vacuum 

 tube to sustain the vibration of a tuning fork. This may be considered 

 either as an improvement on the contact-driven fork by the substitution of 

 a vacuum tube relay device instead of the contact, or as an improvement on 

 the vacuum tube oscillator by the substitution of a mechanical resonator 

 for the electrical resonant element. This achievement was first announced 

 by Professor W. H. Eccles^^ in April or May, 1919, and was followed on 

 June 20 by a note by Eccles and Jordan^^ in the London Electrician. Mean- 

 while, on June 16 of the same year, a similar announcement appeared in 

 Comptes Rendus by Henri Abraham and Eugene Block^^, showing that 

 parallel developments were in progress in both England and France. How- 

 ever, Eccles and Jordan in discussing their work at the National Physical 

 Laboratory stated: "Several instruments of this kind have been set up and 

 used during the past 18 months." From this, we may imply that they had 

 vacuum tube driven forks in operation early in 1918. 



One of the chief advantages of the use of the vacuum tube to sustain 

 oscillations in a mechanical system is that the variable friction of the contact 

 mechanism is avoided. Previously this had been one of the main causes of 

 instability. With the new method it became possible to operate in a wdde 

 frequency range, continuously, and at small amplitude, and to deliver 

 alternating currents of approximately sine wave form and having more 

 constant frequency than heretofore had been possible. The judicious use 

 of a vacuum tube in delivering power to sustain the vibration of a resonator 

 is analogous to the ideal of the so-called free pendulum but may be utilized 

 more effectively in freeing the resonator from disturbing influences associated 

 with the driving means. 



Another important advantage, which, however, was not realized im- 

 mediately, is the ease with which the phase of the driving force applied to 

 a mechanical vibrator can be adjusted for greatest frequency stability. 

 In a manner analogous to the pendulum, in which it was shown that the 

 rate is least affected when the driving impulse is applied at the instant of 

 maximum velocity, the current delivered to the driving electromagnet and 

 hence the force applied to the vibrating element, should be in phase with 



