Abstract: A lateral displacement measuring apparatus by which the circumference of an extrudable solid specimen may be continuously measured while the specimen is subjected to compressive forces along an axis. In a preferred form, the apparatus includes a pair of platens, one fixed and one movable, adapted to compressively load the specimen to its functional or performance limit. A first end of a tensile member is coupled to a displacement transducer, the tensile member being in contacting lateral engagement with the specimen, the other end of the tensile member being secured rigidly to an adjustable but fixed support. As the platens are loaded together, thereby squeezing the specimen, the transducer produces a signal corresponding to the magnitude of lateral expansion of the specimen.