The duct head

The usual method is to attach an apparatus containing an air seal for the cable and a connector for the duct at the duct entrance. The duct is then pressurised, typically at about 12 atmospheres, by means of a compressor that pumps air into the apparatus. At the other end, the duct is open, and the pressure there is one atmosphere. Consequently there is a pressure gradient pointing from the open end to the pressurised end. It should be noted that this pressure gradient is generally not constant. It causes a buoyancy force on the cable, acting towards the exit.