There are various methods for measuring pressure, based on a variety of physical principles. They fall into two broad categories, direct pressure gauges and indirect pressure gauges. Direct pressure gauges include the piston manometer, in which the pressure directly causes the movement of a piston, with a pointer attached, along a calibrated scale, and liquid manometers. In liquid manometers, pressure is indicated by the displacement of a column of water or mercury in a U-shaped glass tube. Measurement with diaphragm pressure gauges and Bourdon tube gauges is based on the force with which the pressure deforms certain measuring elements. Another indirect method for gauging pressure is based on the relationship between pressure and certain electrical effects that are detected by pressure sensors.