Abstract: An extensometer for measuring changes in the vertical distance between the roof and floor of a mine opening which comprises; a pair of rigid telescoping members, a tension spring securing said members to each other in a telescoping relationship, a hook for attaching a non-extensible cable to each of said members, and an indicator for determining telescopic movement of said members. The extensometer of this invention is utilized by securing one of said telescoping members to the roof of the mine and the other telescoping member to the floor of the mine with the axis of said members being disposed along the vertical. The telescoping members are secured, under tension, to the mine roof and floor by means of a non-extendible cable such as a steel cable. The indicator for determining the telescopic movement of the members may be a pair of lugs or flanges or other indicating marks, each member of which pair is affixed to a separate telescoping member.