Tension Test Setup  
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The figure shows a test specimen fixed between upper and lower grips. The knife edges of an extensometer contact the specimen at two points and the distance between these two points is the gage length.

An instrument called an extensometer is used to measure the elongation in the tension test specimen. The extensometer has two knife-edges, which are clipped to the test specimen (clips not shown). The initial distance between knife-edges is called the gage length. As tension is applied, the extensometer measures the elongation that occurs in the specimen over the gage length.

Extensometers are capable of very precise measurements - elongations as small at 0.0001 in. or 0.002 mm. They are available in a range of gage lengths, with the most common models ranging from 0.3 in. to 2 in. (in US units) and from 8 mm. to 100 mm. (in SI units).