QUARTZ CRYSTAL APPLICATIONS 185 



])ositions in the formation of a crystal. But \vc must suppose, or we may supi)ose, 

 the mutual forces of attraction (or chemical afiinity), between the silicon of one 

 crystalline molecule and the oxyj:i;en of a neighboring crystalline molecule, to be 

 influential in determining the orientation of each crystalline molecule, and in 

 causing disturbance in the relative positions of the atoms of each molecule, when 

 the crystal is strained by force applied from without. 



"Imagine now each double atom of oxygen to be a small negatively electrified 

 particle, and each atom of silicon to be a particle electrified with an equal quantity 

 of positive electricity. Suppose now such pressures, positive and negative, to 

 be applied to the surface of a portion of crystal as shall produce a simple elongation 

 in the direction perpendicular to one of the three sets of rows. This strain is 

 indicated bv the arrow heads in Fig. 1.5A and is realized to an exaggerated extent 

 in Fig. 1.5B. 



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(A) (B) 



Fig. 1.5 — Kelvin's model of quartz molecules 



"This second diagram shows all the atoms and the centers of all the crystalline 

 molecules in the positions to which they are brought by the strain. Both diagrams 

 are drawn on the supposition that the stiffness of the relative configuration of 

 atoms of each molecule is slight enough to allow the mutual attractions between 

 the positive atoms and the negative atoms of neighboring molecules to keep them 

 in line through the centers of the molecules, as Fig. 1.5A shows for the undisturbed 

 condition of the systems, and Fig. 1.5B for the system subjected to the supposed 

 elongation. Hence two of the three diameters through atoms of each crystalline 

 molecule are altered in direction, by the elongation, while the diameter through the 

 third pair of atoms remains unchanged, as is clearly shown by Fig. 1.5B compared 

 to Fig. 1.5A. 



"Remark, first that the rows of atoms, in lines through the centers of the crystal- 

 line molecules, perpendicular to the direction of the strain, are shifted to parallel 

 positions with distances between the atoms in them unchanged. Hence the atoms 

 in these rows contributed nothing to the electrical effect. But in parallel to these 

 rows, on each side of the center of each molecule, we find two pairs of atoms whose 

 distances are diminished. 



