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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 3 — Right and left-hand twinning in quartz as seen by polarized light. 



temperature, plates are cut with their long dimension 5° from the 

 mechanical axis. Tolerances in cutting and grinding to thickness, 

 length and width prior to calibrating are of the order of .001 mm., 

 requiring the use of technique similar to that employed in the manu- 

 facture of gage blocks. A few standard thicknesses, ranging from ,020 

 to .060 inch, are used for most crystal plates. Lengths vary from 0.5 

 to 2.0 inches while widths range from 0.15 to 1.5 inches. Because of 

 unavoidable waste in the cutting and grinding operations and the 

 rejection of quartz containing flaws, only a small portion of the material 

 entering the cutting room finds its way into finished plates. 



Up to this point the cutting and grinding are purely mechanical 

 operations, directed toward securing prescribed physical dimensions. 

 During final adjustment and in service, however, the crystal plate 

 must be connected as an electrical element. Electrodes are pro- 

 vided by coating the major surfaces of the plate with aluminum, 

 using a process of evaporation and condensation in a vacuum, similar 

 to that employed in the silvering of telescope mirrors. If the plate is 

 to be used in a balanced filter section which requires a pair of crystal 

 elements of the same frequency, as is frequently the case, the plating 

 on each face is then divided in half along the longitudinal axis. This 

 division, one-hundredth of an inch wide, must have a d.c. insulation 

 resistance of at least 100 megohms to insure proper operation in some 

 types of crystal filters. 



