An instrument is described which measures the change in thickness of a rectangular crystal to 0.00025 of a millimeter and the change in length to approximately 0.001 of a millimeter. The instrument is designed to be used with a crystal of dimensions of 17.5×6×4 millimeters, the thickness dimension of 4 millimeters being parallel to the X axis and the length dimension being parallel to the Y axis; thus a strain as small as 0.00005 along Y and a strain as small as 0.00006 along X can be detected. The ratio of the compressional strain along Y to the extensional strain along X was found to be approximately unity over a fairly wide range of values for the strain along Y. This result is an approximate experimental verification of Voigt’s theory of the piezoelectric effect in quartz for static conditions. The device is constructed on a standard Michelson interferometer and a microscope of only medium power is necessary.
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