A small xenon-filled tube with metal electrodes fused into the ends. The gas flashes brilliantly when a condenser is discharged through the tube. The duration of the flash is primarily dependent upon the capacitance of the condenser. As a rule of thumb, flash time is approximately equal in microseconds to the condenser capacity in microfarads. Sometimes the flash is made to recur at a specified frequency that may reach many thousands per second; in such a case the device is called a strobe unit, as it produces stroboscopic effects and makes rapidly moving parts appear to stand still.