50 BELL SYSTEM TECHNICAL JOURNAL 



Thus the high-frequency mode plates (AT and BT) might be tested in their 

 low frequency modes (corresponding roughly to the CT and DT modes, 

 respectively). A further discussion of this matter will be found in a later 

 chapter by I. E. Fair. 



5.8 Conclusions 



In the processing of quartz, consideration must be given to the nature of 

 twinning and to its characteristic distribution in the raw stone. There are 

 only two common types of twinning that need be considered, namely elec- 

 trical twinning and (true) optical twinning (''combined- twinning" and other 

 types are a rarity). Due to the characteristically large size (and the nature) 

 of electrical twins, a stone must be examined for electrical twinning (by the 

 etch technique) at an early stage of processmg so that the electrical twins 

 may be observed and cut apart before the angular cuts (AT, BT, CT, DT, 

 etc. slabs, bars, or wafers) are made. Otherwise, some of the large electrical 

 twins will be entirely cut up with the incorrect angular sense, and hence 

 wasted. 



On the other hand optical twins are characteristically small and inter- 

 layered, or small and scattered. The interlayered regions are entirely un- 

 usable. Hence processing labor will be saved by inspection of the raw stones 

 (by the polarized light means of Chapter IV), and of the first sections at 

 least (by the etch technique) for large regions of interlayered optical twin- 

 ning. 



Scattered optical twins and small electrical twins, or remnants of elec- 

 trical twins which have been cut apart, may be cut away in an intermediate 

 processing stage, or in a later stage plates containing such twinning may be 

 separated out. In either case the etch technique may be used to detect the 

 twinning. 



An alternative method of eliminating small electrical twins (or remanents 

 thereof) and of small optical twins (most of which are characteristically 

 very small) is by electrical tests on the linished plate. This method has 

 merit in that if the twins are sufficiently small, and not disadvantageously 

 placed in plate, they may not harmfully efifect the desired operating charac- 

 teristics of the plates. The degree of the effect depends not only upon the 

 size of the twin and its location in the plate, but upon whether the twinning 

 is electrical or optical; optical twinning being considerably less harmful than 

 electrical twinning. The effect of the twinning further depends upon the 

 type of plate being considered, i.e. its size and mode of operation, and use. 

 It is probable that twinning is more tolerable in low-frequency mode oscil- 

 lators (CT and DT) than in the high frequency modes (AT and BT), and of 

 course more tolerable in plates of low requirements on the operating charac- 

 teristics (activity, frequency and temperature-coefficient). Twinning is 



