f For direct burial the cable is armoured with layers of steel wire or the tube. At each traverse of the burner the percentage of germanium tape appropriate to the installation conditions. In open country cable dopant can be modified and the refractive index of the glass ploughs are employed which open the trench, lay the cable and changed. Other dopants are phosphorous and fluorine. A hundred or backfill in one operation. more passes are typically involved and the whole process is com- Two techniques are used to resist water penetration through a puter controlled. Finally the burner temperature is raised and the damaged sheath. The cable core can be saturated with petroleum support tube collapses into a solid rod or preform. jelly or a similar compound. The disadvantage is the difficulty of It will be apparent that an advantage of this technique is that as cleaning all the cable elements when joining and it is not appropri- well as pure starting materials, the complicated deposition and ate to air cored coaxial cables. The second technique is by air collapse process take place in a closed environment, minimising the pressure, either continuous flow or static. Static pressure systems problems of contamination by handling or from the atmosphere. rely on total air integrity of the cable. Regularly spaced pressure In the next operation, the preform is fitted in a pulling tower and sensitive switches e.g. at each joint, close across a monitor control its end heated to a temperature at which a fibre filament can be pair when pressure drops. Resistance measurements from the termi- drawn off, at about 2200°C. At the bottom of the tower is the take nal stations identify the location. Continuous flow systems tolerate up drum the speed of which is controlled by a diameter monitor just some leakage from the cable and damage is sensed by an increase below the drawing furnace. Following the diameter monitor, the in flow rate. Cables to be used with either of these systems must fibre is drawn through a primary coating vessel, 