360 BELL SYSTEM TECHNICAL JOURNAL 



known that blue needles may not be used in many other types of oscillators. 

 It is likely that smokiness is less objectionable than blue needles (very 

 lightly-smoky material not recognized as such has been widely used). 



A further important factor, often disregarded, is related to high-frequency- 

 mode plates and their method of manufacture, and more specifically to the 

 method of finally adjusting the dimensions to give the required activity, fre- 

 quency, and temperature characteristic. When such plates are made by the 

 PRE-DIMENSIONING technique, which requires very small tolerances 

 on the machined dimensions and orientation, they are finally finished by 

 hand adjustment of only one dimension, the thickness. On the other hand, 

 when the plates are machined to only moderate accuracy of dimension and 

 orientation, they must be finally hand adjusted on all three dimensions to 

 obtain satisfactory characteristics. By this method of adjustment it is 

 possible to correct, not only for misdimensioning and misorientation, but 

 for small defects in the quartz itself. However, with pre-dimensioning and 

 a single dimensional adjustment practically no correction may be made for 

 errors, or quartz defects. Thus, higher quality quartz may be necessary 

 for manufacture by the pre-dimensioning technique than otherwise. 



The conclusions that may be drawn from these considerations are: (1) 

 only by a quantitative statistical study can it be determined whether a 

 given type of defect will be permissible in a given type of finished plate, (2) 

 known usability of a given type of defect in a given type of plate does not 

 prove its usability in a different type of plate (the type includes size, mode 

 of vibration, and required electrical operating characteristics), and (3) the 

 method of manufacture is also related to the usability of defective quartz 

 (i.e., pre-dimensioning vs. non-predimensioning) . 



Since in the past very little defective quartz has been used in the manu- 

 facture of piezoelectric elements, especially oscillators, there is little manu- 

 facturing experience that may be used as a guide to its introduction now. 

 The quickest means of obtaining this information, and of making use of the 

 reservoir of defective quartz, would seem to result from trial manufacture 

 first of the most likely to succeed types of plates, from quartz with the most 

 likely tolerable types of defects. If and when this utilization is found to be 

 practical the less likely cases may be examined, while at the same time de- 

 fective material is being used and experience is being gained in grading the 

 raw quartz into usably defective and non-usably defective. This special 

 grading of quartz will be difficult to control exactly. It will be easier to 

 grade into types of imperfections than into quantities of defects per-unit- 

 volume. Further, it will be easier in manufacturing trials to determine 

 whether a given type of defect is permissible if the defects appear in large 

 quantities, than if in very small quantities (where they may actually be 

 absent in some finished plates). For these two reasons it will be preferable 



