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BELL SYSTEM TECHNICAL JOURNAL 



facture of the crystal. Within the range using the same crystal 

 thickness the impedance and frequency differences, called for by the 

 design of the different filters, can be provided by variations in width 

 and length of the crystals. The ratio of width of the crystals to their 

 length ranges from about 1/2 to 4/5. 



The major surfaces of the crystals are plated with a thin layer of 

 aluminum deposited by an evaporation process. This plating is 

 divided along the center line lengthwise of the crystals to form the two 



ELECTRICAL AXIS = X 



MECHANICAL AXIS=Y 



OPTICAL AXIS = Z 



Fig. 6 — The crystal elements are out with their major surfaces perpendicular to 

 the electrical axis of the natural quartz, with their side surfaces making a small angle 

 with the mechanical axis, and with their end surfaces making a small angle with the 

 optical axis. 



electrically independent crystals. Since the crystals vibrate longi- 

 tudinally with a node across the middle, they are clamped at this node 

 in mounting. Figure 6 shows the orientation of the crystal plates 

 with respect to the natural axes of the quartz from which they are cut. 

 The plates are cut as accurately as is practicable to the dimensions 

 computed making a small allowance in length and then the crystal is 

 finally adjusted in an electric circuit by grinding the end of the crystal 

 until the resonant frequency falls within five or ten cycles of that 

 desired. 



