Crackmeters are used to monitor movement across surface cracks and joints in concrete structures or rock. The instrument assembly includes the crackmeter, mounting brackets, and anchors. The anchors are installed on opposite sides of the crack, and the mounting brackets are screwed onto the anchors. Then the crackmeter is attached to the mounting brackets so that it spans the crack. Opening or closing of the crack changes the output of the sensor. Readings are obtained with a portable readout or data logger.
Source:http://sisgeo-northamerica.com/products/crackmeters-jointmeters/
Link to report of this instrument: CME