Abstract: Adjustable apparatus (20, 20a) is provided, preferably in the form of an impact micrometer, which includes first and second interfitted bodies (22, 24, 22a, 24a) cooperatively defining therebetween an enclosed, constant volume internal fluid chamber (58, 58a). An adjuster (26,26a) is also provided which is connected to one of the first and second bodies (22, 24, 22a, 24a) and has an element (68, 68a) movable within the chamber (58, 58a). Preferably, the adjuster (26, 26a) is threadably coupled with the first body (22, 22a) through fine micrometer threading (36, 62, 36a, 62a). In use, the apparatus (20,20a be adjusted through rotation of the adjuster (26,26a) causing corresponding movement of the element (68, 68a) within the chamber (58, 58a); this in turn results in displacement of the fluid (59, 59a) to change the shape but not the volume of the chamber (58, 58a) and consequent movement of the first body (22, 22a) because of the incompressible nature of the fluid (59, 59a).