EVOLUTION OF QUARTZ CRYSTAL CLOCK 529 



Telephone Laboratories' headquarters in New York City '^•^^. A photo- 

 graph of the original records is reproduced in Fig. 6. This is beUeved to be 

 the first time that a vacuum tube oscillator type of time standard was ever 

 used in the service of astronomy. 



During the following ten years a great number of improvements were 

 made in tuning fork oscillators and they became widely used as precise 

 frequency standards. The Bell Laboratories' 100-cycle fork standard was 

 mounted in a container which could be sealed at constant pressure or vac- 

 uum. It was carefully temperature controlled and provision was made to 

 keep the amplitude within prescribed limits. In describing this improved 

 standard^, comprising a synchronous motor geared directly to a clock 

 mechanism, the authors Horton and Marrison made the following statement: 



"During tests on this frequency standard, it was found that it constituted a far more 

 reliable timekeeper than the electrically maintained pendulum clock which was used to 

 obtain the data already published. The pendulum clock was, therefore, dispensed with 

 and all measurements of the rate of the fork are now made by direct comparison with the 

 mean solar day as defined by the radio time signals sent out by the U. S. Naval 

 Observatory." 



In all fairness to the pendulum clock in question, it should be stated that 

 the laboratory was situated on the seventh floor of a building adjoining a 

 busy street and so was continually subject to vibration from traffic, wind, 

 and other changing conditions. Disturbances of this sort have little or no 

 effect on standards of the electric oscillator type but seriously impair the 

 performance of most high precision pendulum clocks. The relative im- 

 munity of the oscillator standard to change of position and shock has an 

 important bearing on its value in many applications. 



Probably the most precise tuning fork controlled time and frequency 

 standards ever constructed were those developed in the National Physical 

 Laboratory at Teddington, as a continuation of the work begun there by 

 Professor Eccles and carried forward by Dr. Dye and his staff. A report 

 by D. W. Dye and L. Essen in the Royal Society Proceedings in 1934^^ 

 described a number of refinements in the fork and method of use some of 

 which had been suggested by Dr. Dye as a result of his studies ten years 

 earlier. Among these was the use of elinvar in the construction of the forks 

 in order to reduce the effect of variable temperature on the frequency. 

 Elinvar is a nickel steel containing about twelve per cent of chromium, 

 which on proper treatment has a small or zero temperature coefficient of 

 elasticity. It was invented by Charles Edouard Guillaume^^-^ and was 

 further studied by P. Chevenard'^^- ^°. The excellence of the N.P.L. fork 

 standard can be appreciated readily from the conclusion of the 1934 report 

 which states in part: 



