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a pressure which deforms the crystal lattice causes a separation of the cen- 

 ters of gravity of the positive and negative charges thus generating a dipole 

 moment (product of the value of the charges by their separation) in each 

 molecule. How this separation can cause a coupling to an electrical circuit 

 is illustrated by Fig. 1 .4 which shows a crystal with metal electrodes normal 

 to the direction of charge separation. If we short-circuit these electrodes 

 and apply a stress which causes the centers of gravity of the charges to 

 separate, free negative charges in the wire will be drawn toward the electrode 

 in the direction of positive charge separation, and free positive charges in 

 the wire will be drawn to the electrode in the direction of negative charge 

 displacement until the crystal appears to be electrically neutral by any test 

 conducted outside the crystal. When the stress is released the charges in 

 the wire will flow back to their normal position. If, during the process, 

 we connect an oscillograph in the short-circuited wire, there will be a pulse 

 of current in one direction when the stress is applied and a pulse in the oppo- 



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Fig. 1.4 — Method for transforming mechanical energ\' into electrical energy in a crystal 



site direction when the stress is released. By putting a resistance in the 

 connecting wire and applying a sinusoidal stress to the crystal, an alternating 

 current will flow through the load and consequently mechanical power will 

 be changed into electrical power. Using the converse effect, a source of 

 alternating voltage in the electrical circuit will produce an alternating stress 

 in the crystal, and if this is working against a mechanical load, the electrical 

 energy will be changed into mechanical energy. 



To apply this concept to quartz let us consider Fig. 1.5, which represents 

 the approximate arrangement of molecules in a quartz molecule. Lord 

 Kelvin's explanation of the piezoelectricity of quartz is the following: 



"The diagram (Fig. 1.5A) shows a crystalline molecule surrounded by si.x 

 nearest neighbors in a plane perpendicular to the optic axis of a quartz crystal. 

 Each silicon atom is represented b>- -|- (plus) and each oxygen double atom — 

 (minus). The constituents of each cluster must be supposed to be held together 

 in stable equilibrium in viture of their chemical affinities. The different clusters, 

 or crystalline molecules, must be supposed to be relatively mobile before taking 



