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 <i>X</i> axis and the length dimension being parallel to the <i>Y</i> axis; thus a strain as small as 0.00005 along <i>Y</i> and a strain as small as 0.00006 along <i>X</i> can be detected. The ratio of the compressional strain along <i>Y</i> to the extensional strain along <i>X</i> was found to be approximately unity over a fairly wide range of values for the strain along <i>Y</i>. 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.
M. Y. COLBY and SIDON HARRIS, "Measurement of a Strain Ratio in an X-Cut Quartz Crystal," J. Opt. Soc. Am. 24, 217-217 (1934)