EVOLUTION OF QUARTZ CRYSTAL CLOCK 541 



variations of considerably larger magnitude. It was natural, then, to 

 see what could be done about these effects. 



Zero Temperature Coefficient of Frequency 



With the knowledge that X-cut resonators had negative coefficients, 

 frequently as large as thirty parts in a million per degree C, and that Y-cut 

 resonators in general had positive coefficients, often in excess of a hundred 

 parts in a million per degree, the author undertook to make resonators of 

 such shape that the oscillations would occur in both modes simultaneously, 

 and so combine the coefficients, in the hope that the resultant could be 

 made zero.^^ 



The first experiments, made on two series of resonators both yielded 

 encouraging results. The first was a series of rectangular X-cut plates of 

 varying thickness shown in Fig. 13. The second was a series of three circular 

 discs of different diameters, all being cut with the large surfaces in the plane 

 of the Y and Z axes. The three discs were made from the same material, 

 each smaller one being trepanned from the previous one after complete 

 measurements had been made upon it. The set of circular crystals remain- 

 ing after these tests were completed is shown in Fig. 14 and th^e slab from 

 which they were cut is shown assembled with the original large crystal in 

 Fig. 15. 



Subsequent tests showed that the annular pieces could be designed for a 

 low or zero coefficient and such a shape shown in Fig. 16 was employed for 

 a number of years in the Bell System Frequency Standard in New York 

 City^^. As described in this reference, the reason for using the ring in 

 preference to the solid disc or rectangular plate was in the convenience of 

 mounting. The rings were formed with a ridge in the central plane of the 

 hole so that they could be supported on a horizontal pin thus providing a 

 one-point support at a position where the vibration is very small. The rings 

 used in this first application of zero coefficient quartz resonators have been 

 called "doughnut" crystals for obvious reasons. In Fig. 17, George Hecht 

 is shown making a final adjustment, by "lapping" with fine abrasive, on one 

 of the four original zero-coefficient ring crystals. Mr. Hecht made all four 

 of these resonators, as well as many others of various shapes and sizes used 

 in the early experiments in this work. 



Supported as described, the rings hang in a vertical plane and, as first 

 used, they were supported freely between solid electrodes rather closely 

 spaced to the flat surfaces. The small amount of free motion relative to the 

 electrodes, inherent in this sort of mounting, caused occasional changes in 

 frequency if the support were disturbed, which at times would be as large 

 as one part in ten million. To avoid this difficulty, other ring crystals were 



