Barrage balloon, c 1939.
© Science Museum / Science & Society Picture Library
Model (scale 1:25). Barrage balloons were used to protect land-based targets from low-flying aircraft. This forced the enemy aircraft to fly at heights of over 5000 feet where they could be attacked by anti-aircraft guns and fighter aircraft. The balloons, approximately 20,000 feet in capacity, were divided internally into two compartments by a fabric diaphragm, the lower compartment being the air space. As they ascended, the hydrogen in the upper compartment expanded, depresing the diaphragm.