EVOLUTION OF QUARTZ CRYSTAL CLOCK 561 



Applications of Quartz Clocks 



The many useful properties of the quartz crystal clock have been the 

 reason for its wide and expanding application for the precise measurement 

 of time and rate. 



First in historical order was the application to the measurement and con- 

 trol of frequency in communication. In this, the clock, through comparisons 

 with astronomical time, served as the means for determining the frequency 

 controlling it, the stability from the outset being great enough over intervals 

 of a day or more so that the average rate, as determined by daily checks with 

 time signals, was a very close approximation to the instantaneous rate at 

 any time intervening. The first of these clocks, already referred to^°, was 

 constructed in 1927 at the Bell Telephone Laboratories, in New York City, 

 primarily for use as an accurate standard of frequency. Since that first 

 experiment, three subsequent installations have been built in replacement 

 with progressively improved performance. The standard now in operation 

 (1947) was installed in 1937, using the first laboratory model GT crystals 

 and the first set of four bridge-stabilized oscillators, and has been in opera- 

 tion continuously since that time. Two of the four oscillators, mounted in a 

 temperature controlled booth, are shown in Fig. 29, and part of the auxiliary 

 equipment, including a clock dial, a spark chronograph and some monitoring 

 equipment, is shown in Fig. 30. This apparatus serves as the standard for 

 precise measurements of frequency and time throughout the Bell System and 

 is used to regulate the telephone Time of Day Service in New York City. 

 It is the standard of reference for the electric light and power services in 

 Metropolitan New York^^^, and is used for a number of other similar services, 

 distributed through the medium of a submaster installation^"^ maintained 

 by the Long Lines Department of the American Telephone and Telegraph 

 Company. The original oscillators in this submaster installation were 

 controlled by electrostatically-coupled 4000-cycle steel tuning forks in vacuo 

 but recently have been replaced by improved oscillators controlled by 4000- 

 cycle bi-morph quartz resonators. 



A clock shown in Fig. 31, which is on display in a window of the American 

 Telephone and Telegraph Company at 195 Broadway, is controlled from 

 this source. It is sometimes called "The World's Most Accurate Public 

 Clock". 



The facility with which standard frequency and time services can be 

 provided and distributed is an outstanding feature of the quartz clock 

 development. Such services, having the accuracy of the primary controlling 

 standard, may be provided anywhere that can be reached through a suitable 



