. The Bell System technical journal . syn-thetically has been developed. Raw crystals suitable for use infilter manufacture must be unusually large and free from flaws. Theprincipal source is Brazil, the bulk of the quartz being brought in bynative prospectors to trading posts and shipped to this country viaRio de Janerio and other coastal cities. The crystals usually rangebetween 3 and 10 pounds in weight, with occasional pieces reaching100 pounds. The raw quartz passes through successive stages of inspection andselection, commencing at the trading post and culminating in carefulexaminations
. The Bell System technical journal . syn-thetically has been developed. Raw crystals suitable for use infilter manufacture must be unusually large and free from flaws. Theprincipal source is Brazil, the bulk of the quartz being brought in bynative prospectors to trading posts and shipped to this country viaRio de Janerio and other coastal cities. The crystals usually rangebetween 3 and 10 pounds in weight, with occasional pieces reaching100 pounds. The raw quartz passes through successive stages of inspection andselection, commencing at the trading post and culminating in carefulexaminations before and during the cutting operations. A concen-trated beam of light from an arc lamp (see Fig. 2) is used in locatinginternal flaws, which generally appear as small bubbles and inclusionsof foreign matter. Quartz takes two distinct forms, left-hand andright-hand, having opposite piezo-electric polarities. Portions of rawcrystals containing both forms are not usable. This condition, called MANUFACTURE OF QUARTZ CRYSTAL FILTERS 519. Fig. 2—Inspection of quartz crystals. An arc light beam aidsin the detection of internal flaws. twinning, appears as shown in Fig. 3 when observed with polarizedHght. For use in filters, quartz must be cut into rectangular plates properlyoriented with respect to the electrical, mechanical and optical axes of thecrystal, as shown in Fig. 4. A polariscope and an X-ray spectroscopeare used in locating these axes to an accuracy of ± degree. Forthe majority of applications the plate is cut in the plane of the me-chanical and optical axes, with the long dimension set at an angle ° from the mechanical axis. This orientation eliminates secondaryresonances in the completed crystal and makes the primary resonancefrequency relatively independent of slight errors in orientation. Forapplications requiring a low coefficient of resonance frequency versus 520 BELL SYSTEM TECHNICAL JOURNAL
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Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
