Mechanical Interference in the Measurement of Strain: Notes on the Measurement of Displacement and Strain by means of Mechanical Interference with Reference to a Specialized Application

Details are given of a simple extensometer with a gauge length of 0·50 in. which uses the mechanical interference of two sets of parallel straight lines as the means by which to measure displacement and hence strain. The interference effect exhibited by two pieces of Dufaycolor reseau placed in surface contact has been employed for development purposes since the reseau provides a convenient and ready‐made source of fine parallel rulings, after the removal of the photographic emulsion. Some of the related aspects of mechanical interferometry and its possibilities are discussed by way of introduction.