SPECIFYING QUARTZ CRYSTAL ORIENTATION 



225 



might expect to find two kinds of quartz that are related to each other as 

 one's right hand is related to his left. We do find them and call them 

 right-hand quartz and left-hand quartz respectively. They are illustrated 

 in Fig. 2.4. These conventional figures are shown in many texts but no one 

 has seen such perfect quartz crystals. They are drawn possessing just 

 these faces and no others merely to illustrate the symmetry of quartz and 

 its occurrence in right-handed and left-handed forms. 



These figures are also useful in defining how a blank shall be cut from 

 one kind of quartz. It is found that if a crystal be compressed with forces 



Fig. 2.1 — Hexagonal axis system 



parallel to a pair of sides of the hexagon an electric polarization takes place 

 in the direction of the forces. The edge "modified" by the presence of 5 

 and X faces becomes negative. If we allow these charges to leak ofif and 

 then suddenly release the mechanical forces the "modified" edge becomes 

 positively charged as the crystal expands. This is true for both right- 

 hand and left-hand crystals. 



Let us now talk about right-hand quartz. Since expansion is con- 

 sidered as a positive strain (contraction as negative) it is decided to take 

 the positive end of the electric axis as pointing towards the modified edge. 

 This gives us a positive charge at the positive end of the electric axis when 

 a positive stress (tension) is applied along this axis. This positive direc- 



