At Somplago, in the heart of a mountain placed in the Carniche Pre-Alps seismic zone, it has been installed a new extensometer, which can take and register (between two points of the rock at 4 meters distance) relative movements, lower than 10^-7 cm. In this work, you will find the detailed description of this new instrument, both for what concerns the theory and the technical details. By this instrument, it will be possible to investigate the rock alterations brought by any efforts, and of course, also those which are connected to the local seismic activity.