EVOLUTION OF QUARTZ CRYSTAL CLOCK 547 



The second class comprises properties of the means for sustaining oscilla- 

 tions in such a resonator which relate the resulting actual rate to variations 

 in the electrical circuits, in the power voltages, in vacuum tubes and other 

 Hke effects. In the limit, it is the hope that the net result of all such effects 

 can be eliminated so that the stability of the quartz crystal alone will remain 

 the sole governing factor. This is the goal, and the inherent stability of the 

 substance, quartz crystal, is the limit toward which the stability of the 

 quartz crystal clock will approach but cannot exceed. 



The development of the quartz resonator and its mounting for numerous 

 applications is described in some detail by Raymond A. Heising and his 

 collaborators^^ in their recent book, "Quartz Crystals for Electrical Circuits". 

 Of all the types of resonator described in this work the one having the most 

 extensive use at the present time, for quartz clock installations and for 

 other applications of comparable accuracy, is the GT crystal resonator 

 developed by W. P. Mason^^. This resonator is cut from quartz crystal in 

 such a way that the positive and negative coefficients are effectively neutral- 

 ized over a range of about 100 degrees C, so that in any part of this range the 

 resulting temperature coefficient of frequency is not more than one part in a 

 million per degree C. With suitable precautions in manufacture, the 

 tangent at the point of inflection in the frequency-temperature curve may be 

 made horizontal, which means that the temperature coefficient may be made 

 substantially zero over a considerable range of temperature. 



The GT crystal resonator therefore introduces two significant advantages 

 in timekeeping, namely that greater accuracy of rate may be obtained with a 

 given accuracy of temperature control and that the value at which the 

 temperature is controlled may be chosen in a considerable range. In fact, 

 without any temperature control at all, the rate of a clock regulated by 

 such a crystal may be accurate to a tenth of a second a day over an ambient 

 range of 100 degrees C. Among the many quartz clock installations now 

 using the GT resonator, all or in part, are the Royal Observatory at Green- 

 wich, the British Post Office, the U. S. Naval Observatory and the U. S. 

 Bureau of Standards. 



One of the chief sources of variation in rate of quartz oscillators, in the 

 early stages of their development, was in the means for mounting and in the 

 electrical circuit connections. As mentioned previously, any variation in the 

 effective resistance or in the effective mass or stiffness of a resonator has a 

 direct effect upon its rate of oscillation. The problem reduces to that of 

 supporting the resonator so that the frictional losses are small and constant 

 and so that the coupling to the electrical circuit is as nearly as possible 

 invariable. 



The mounting of quartz crystal units is discussed at length by R. M. C. 



