EVOLUTION OF QUARTZ CRYSTAL CLOCK 



555 



The importance of the convenient methods for measuring time and time 

 interval inherent to the crystal clock is emphasized by the fact that some 

 observatories employed crystal clock mechanisms in connection with stellar 

 observations and in the transmission of time signals before they were used in 

 the actual time keeping departments^. 



The second property contributing greatly to the convenience of precise 

 time measurements is the relatively very short period of the quartz clock 

 control element. The chief advantage lies in the extreme accuracy with 

 which the rates and indicated times can be compared by electrical methods. 

 An example will suffice to illustrate this point. 



4-------Q— -^ 



^ 



« ipr"^ 



R^a^ ' i_J 



O, - OSCILLATOR 100,000.0 CYCLES. 

 O4- OSCILLATOR 100,000.1 CYCLES. 

 S- CIRCULAR TRANSPARENT SCALE. 

 G - SPARK GAP. 



A -FILM ADVANCING DEVICE. 

 R- RELAY. 



SMG-SUBMULTIPLE GENERATOR. 

 BFI - BEAT FREQUENCY INDICATOR. 



Fig. 24 — Device for comparison of oscillator rates accurate to 1 part in 10,000,000,000. 

 (From "High Precision Standard of Frequency", 1929.) 



Since the rate of a crystal clock is the rate of oscillation of the crystal or of 

 the current driving it, it is only necessary, in comparing clock rates, to 

 measure the relative frequencies of the oscillators concerned. This can be 

 done by any of the standard methods for frequency comparisons^ but, in the 

 case of quartz clocks, since in general the primary frequencies are high and 

 are nominally the same, special methods of extreme accuracy can be em- 

 ployed. The apparatus first designed for the ultra-precise comparison of 

 quartz oscillators and capable of an accuracy of one part in 10^° was de- 

 scribed by Harrison in 1929.^^* *^' *^ The principle of its operation is 

 shown in Fig. 24, reproduced from the paper "High Precision Standard of 

 Frequency". 



Two oscillators to be compared were adjusted so as to differ by about one 

 cycle in ten seconds. The problem reduces to that of measuring the beat 

 frequency, nominally 0.1 cycle per second, with as great accuracy as possible. 



