An instrument for monitoring a physical parameter consisting of temperature or pressure, includes an elastic sensor displaceable as a function of the physical parameter being monitored, a belt extending over at least two spaced rollers and coupled to the elastic sensor in such a manner as to be displaced by an amount proportional to the displacement of the elastic sensor, the belt having a characteristic which changes along the length thereof, and sensing means responsive to the characteristic of the belt for producing an output signal which varies as a predetermined function of the physical parameter being monitored.