EVOLUTION OF QUARTZ CRYSTAL CLOCK 545 



Some of the outstanding properties of the quartz oscillator clock were dis- 

 cussed in 1932 by A. L. Loomis and W. A. Marrison^^, in relation to a series of 

 experiments comparing the performance of quartz clocks at Bell Telephone 

 Laboratories in New York and a set of synchronome free-pendulum clocks 

 operating in The Loomis Laboratory in Tuxedo Park, about fifty miles away. 

 The comparison wa$ effected through a circuit maintained between the two 

 laboratories over which a 1,000-cycle current controlled by a crystal in New 

 York was used to drive the Loomis Chronograph^^ in Tuxedo Park. During 

 part of the time, signals from the clocks were sent back over the same circuit 

 and recorded on the Bell Laboratories' Spark Chronograph^^. 



The quartz oscillator assembly at the Bell Telephone Laboratories at the 

 time of these experiments is shown in Fig. 19. The four ring crystals in their 

 individual temperature-controlled 'ovens' are mounted under hermetically 

 sealed bell jars to avoid the effects of ambient temperature and atmospheric 

 pressure changes. The vacuum tube oscillator circuits are immediately 

 below the bell jars; and the control, monitoring and power supply equipment 

 in the remainder of the space. 



One of the most interesting results of these cooperative experiments was 

 the measurement of a periodic variation in the rate of the pendulum clocks 

 in phase with the lunar daily cycle. The amount of this daily variation is 

 very small, being only a few tenths of a millisecond, but readily observable 

 in comparison with a stable rate standard that does not vary with gravity. 



Further Reiinemsnts in Quartz Clocks 



The spectacular results from the use of the quartz crystal clock up to this 

 time, about 1932, were due in part to its novelty and in part to the fact that 

 it is quite independent of some of the variable factors that affect conventional 

 precision clocks, including gravity itself upon which the rate of all pendulum 

 clocks depends. The remarkable stability of present day quartz oscillators 

 and clocks is the result of a series of developments and refinements extending 

 over a number of years. 



As mentioned previously, the factors that cause departure from constant 

 rate in the completed operating device fall into two distinct classes, namely 

 those which concern the inherent or natural frequency of the resonator itself, 

 and those which concern the means for driving it at that inherent rate. 



The first class comprises all those properties of the mounted resonator 

 which tend to relate its inherent rate to ambient conditions such as tempera- 

 ture, atmospheric pressure, change of position and vibration, and to the 

 passage of time — that is, aging. Since the final stabiUty cannot exceed the 

 inherent stability of the mounted resonator itself, its study is of prime 

 importance. 



