ECONOMY in the use of engineering materials calls for a very accurate knowledge of the stresses arising in machinery and structural parts. The forces acting on the part are frequently impossible to determine and vary in their effect according to their static or dynamic nature. The surest method of determining the stresses in time and magnitude, is therefore to measure the resultant elastic deformation. Measurements by mechanical extensometer are, however, of limited usefulness, and, in particular, performed with difficulty on parts in rapid motion or vibration while running.