Another truck houses a complete telephone exchange, since the observers in the balloon may wish to communicate with any one of a number of batteries which they are serving. Telephone communication is established by means of an insulated wire which forms the core of the cable, while the steel cable itself acts as the return wire to complete the circuit. In some cases, a separate copper cable is employed, using the steel cable as the return half of the circuit. In addition there is a truck for transporting the balloons, for the company must always have duplicate equipment at hand in case of the destruction of the balloon it is using or, as more frequently happens, damage of a nature that requires hours or days to repair. In addition to the balloon itself, there are covers and the ground cloth, as in inflating a balloon no part of its fabric must be allowed to touch the ground because of the danger of stones or sticks tearing rents in it. The balloon proper and its immediate accessories utilize at least one and sometimes two motor trucks. To hold the balloon on the ground when out of service, there are eighty sacks of sand weighing 25 pounds each, or an aggregate of 1 ton of ballast, in addition to which there are necessary a large number of steel screw stakes, spare ropes and parts, ladders and the like, besides the basket and its equipment. The stakes are employed to hold the balloon down in a heavy wind by "pegging" it in the same manner as a tent. Three or four trucks are required to carry the large supply of hydrogen necessary, which entails the transportation of 130 to 150 containers. Each container holds several thousand cubic feet of gas under high pressure, which is released through a reducing valve. Some of the other transportation units required are the "cook wagon," quartermaster’s stores truck, truck for carrying tents, blankets, and other impediments for the men, and the "doctor’s wagon" (ambulance).