QUARTZ CR YSTAL A P PLICA TIONS 1 79 



The converse piezoelectric effect was predicted in 1881 by the French 

 physicist Lippmann on the basis of the principle of conservation of elec- 

 tricity. It was verified in the same year by the brothers Curie. In this 

 effect a crystal is strained when a voltage is applied to it. The effect can 

 be demonstrated by a model which consists of two thin pieces of Rochelle 

 salt poled so that one expands when the voltage is applied and the other 

 contracts. The result is — as in a bimetallic thermostat — the crystal bends. 

 For crystals 10 mil inches thick and 4 inches long, a ninety-volt battery 

 applied causes a displacement of a quarter of an inch or more of the end of 

 the unit. Reversing the voltage reverses the direction of the displacement. 

 The Curies constructed a bimorph unit of this type out of quartz and used it 

 practically to measure voltage by measuring the displacement of the end 

 of the crystal. By connecting the leads of an electrometer to the terminals, 

 they could measure force applied by measuring the amount of charge gen- 

 erated at the terminals. 



Outside of this use which was quite minor, the piezoelectric effect remained 

 a scientific curiosity until the war of 1914-1918. It did inspire, however, 

 considerable scientific speculation. Lord Kelvin in 1893 proposed a model 

 for explaining the piezoelectricity of quartz and was able to calculate 

 approximately the value of the piezoelectric constant. This model is 

 discussed briefly in the next section. He also constructed and demonstrated 

 a "piezoelectric pile" made from small spheres of zinc and copper, to illus- 

 trate the effect. At about the same time (1890-1892) Voigt published a 

 series of papers followed by a book "Lehrbuch der Kristall Physik" (1910) 

 in which the stresses, strains, fields and polarizations of piezoelectric crystals 

 are related in mathematical form. These mathematical expressions (which 

 are discussed further in the appendix) form a basis for the development of 

 the properties of oriented crystals as discussed in section l.vS. 



During the war of 1914-1918, Professor Langevin in Paris was requested 

 by the French Government to devise some way of detecting submarines 

 by acoustic waves they produce in water. After trying several devices he 

 finally found that piezoelectric quartz plates could be used for that purpose. 

 His device, which is shown in Fig. 1.1, consisted essentially of a mosaic of 

 quartz which has the property that when a voltage is applied the crystal 

 will expand and send out a longitudinal wave. Similarly, if a wave strikes 

 it, the wave will set the quartz in vibration and generate a voltage which 

 can be detected by vacuum tube devices. Langevin did not get his device 

 perfected till after the war so it was not used at that time to detect sub- 

 marines. Similar devices have, however, been used in this war. Langevin's 

 original apparatus was used extensively as a sonic depth finder. In this 

 use a pulse is generated which is recorded directly on a moving record and 



