Reed Switches are used in magnet extensometer probes to monitor settlement and heave in construction sites, dams, excavations, mines and tunnels. Access pipes are inserted into the earth, and magnets are provided at every joint and coupling. The reed switch probe, coupled to a measuring tape is inserted from above. Each time the probe comes near one of the magnets, an indication is given and the depth can be read off the tape. Suitable products: HB-2232 and LV-1925 reed switches with cropped leads, encapsulated in probes.