EVOLUTION OF QUARTZ CRYSTAL CLOCK 567 



at Washington, where time determinations of great accuracy are made by 

 means of a Photographic Zenith Tube and a set of quartz clocks. A con- 

 tinuous precise check is maintained between these organizations by radio 

 communication so that the Naval Observatory time signals sent out from 

 NSS at Annapolis and other Navy stations, and from WWV the Bureau 

 of Standards radio transmitting station at Beltsville, Md., as well as all the 

 carrier frequencies from Beltsville, are very accurately determined and 

 maintained in agreement throughout. 



The time studies of the U. S. Naval Observatory up to 1937 are described 

 in two important articles by J. F. Hellweg, then Superintendent of the 

 Observatory. The first of these^^^ in 1932 describes the state of the art just 

 before the quartz clock entered the scene, and the second^^ in 1937, already 

 referred to, tells of some of the first improvements brought about by its use 

 including the elegant method for making direct photographic time-star 

 checks of the crystal clock rate by means of the Photographic Zenith Tube. 

 Many of the advances involving the use of quartz clocks at the Naval 

 Observatory have not as yet been published. 



The British Post Office and the National Physical Laboratory with labora- 

 tories at Dollis Hill and Teddington respectively, in cooperation with the 

 Royal Observatory at Greenwich, have done much the same sort of 

 thing in England in relation to time and frequency measurements and broad- 

 cast services as has just been described. Considering the number of crystal 

 units among these organizations and the precise nature of the intercompari- 

 sons maintained between them, this is probably the most extensive and 

 elaborate quartz clock system in the world. In connection with Greenwich 

 Observatory alone, the complete installation includes eighteen or more such 

 clocks used in deriving the best possible mean rate from steller observations 

 at Greenwich and from studies of other time observatories throughout the 

 world. 



An outline description of the quartz clocks of Greenwich Observatory, and 

 of their function there, has been discussed by Humphry M. Smith in Elec- 

 trical Times^^^ (London) in March 1946. These clocks employ for the most 

 part the GT cut crystal, first described by W. P. Mason, the bridge stabilized 

 oscillator circuit developed by L. A. Meacham, and the regenerative modula- 

 tor type of frequency dividers similar to those first developed by R. L. 

 Miller. 



The accuracy of the quartz clocks exceeds that of the best pendulum 

 clocks with the result that quartz clocks are now used exclusively in the most 

 precise measurements of time. Some of the considerations^^^ leading up to 

 the adoption of quartz clocks at Greenwich were discussed in 1937 by H. 

 Spencer Jones, Astronomer Royal. Since then, reports have appeared from 

 time to time by the Astronomer RoyaP*-"^ and others"* concerning the 



