An instrument used to measure a material’s elastic deformation produced by stresses created when loads are applied to the material.

A popular type of strain gauge is the Berry strain gauge, a hand-held instrument with a prepared gauge showing length before and after a known load is applied. Based on the relationship of the strain of deformation to the stress in the test material, the stress in the test structure is determined when the difference in gauge length is found.

The usual gauge length is 50.8 mm (2in.) for standard test specimens. Instruments adjusted to 203 mm (8 in.) are almost as common, and for structural tests on long members, a 508 mm (20 in.) instrument is used. All of them indicate changes of length in 0.025 mm (.001in.).