New reliable and accurate extensometers are presented using a single amorphous core multivibrator with a dc output. These extensometers can detect strains as large as 40 percent for tensile test samples without any destruction due to the outstanding elasticity of the Metglas 2605A amorphous ribbon-wound cores. High linearity (0.18 percent full scale (FS)) is realized by annealing and applying 1000 times alternating stresses to the cote. Hysteresis against alternating strains is removed to less than 0.48 percent FS by the effect of a compensation spring. The amorphous core extensometer has advantageous points regarding reliability, temperature stability, and maximum operating temperature over conventional extensometers using semiconductor strain gauges.