Vacuum gauges are devices for measuring vacuum or sub-atmospheric pressures. They use several different technologies. Manometers are usually made of a transparent U-shaped tube and partially filled with a liquid such as water, mercury or oil. The relative amount of liquid displacement between the legs of the U indicates the excess of pressure exerted on one side or the other. Thermocouple gauges measure changes in the thermal conductivity of a residual gas within a gauge tube. These devices include a filament, power supply for the filament, and moving coil meter for displaying the pressure. Hot cathode ionization gauges initiate a constant electron flow from the cathode or electron source to the anode or electron drain. These electrons hit a pressure-dependent quantity of gas molecules, which become positive ions and cause a pressure-related current on the ion collector. Cold cathode ionization gauges are also available. Because they do not have active components such as hot filaments, cold cathode gauges can withstand sudden or prolonged exposure to high-pressure gases.