496 



BELL SYSTEM TECHNICAL JOURNAL 



parts remaining after holes of different diameters had been trepanned 

 in them. 



It is possible to make ring-shaped crystals having negligible tempera- 

 ture coefficients in a considerable range of frequencies, and, since the 

 ring shape permits of an improved method of mounting in which there 

 is very little friction on the holder, they have been adopted for use 

 in the present standard. Such a crystal having a frequency of 100,000 



Fig. 4 — Three 100,000-cycle low temperature coefficient rings used 

 in frequency standard. 



cycles is of substantial size and is reasonably easy to make and adjust. 

 Three of the crystals used in the present standard, adjusted to 100,000 

 cycles, and having temperature coefficients less than one part in a 

 million per degree C, are shown in Fig. 4. 



The variation of frequency with temperature for one of the ring- 

 shaped crystals is given in Fig. 5, showing that it is very small over 



UJ Z 



a. ui 



30 35 



TEMPERATURE - 



40 

 DEC. CENT. 



Fig. 5— Variation of frequency with temperature for a 100,000-cycle ring crystal 

 adjusted for low temperature coefficient. 



the usual room temperature range. All of the ring-shaped 100,000- 

 cycle crystals made thus far are alike in having a coefficient which is 

 small throughout this range.^ The temperature coefificient of a disc 

 of the same frequency having the same outside dimensions as the 

 100,000-cycle rings, is approximately 30 parts in a million per degree 

 C, more than thirty times that of the adjusted crystal. 



* Where an accuracy of the order of only one part in 100,000 is desired, as in some 

 portable standards, such a crystal could be employed without any form of tempera- 

 ture control. 



