An instrument for measuring strains; e.g., in concrete linings to shafts or roadways. It contains a length of fine wire under tension, the tension being varied by the strain to which the gage is subjected. The measurement made is that of the frequency of vibration of the wire when it is plucked by means of an electromagnetic impulse, and this measurement can be made with great accuracy. The gage is highly stable, and readings can be made over a period of years without any fear of zero drift.