Anti aircraft fire was the first line of defence against the German bombers. The guns were a 3.7 inch calibre. They fired a substantial shell, fused to explode at a given height. The Ack-Ack barrages around London did succeed, occasionally, in downing a German aircraft. But they proved to be relatively inefficient. However, one of their primary achievements was the boost civilian morale. The din they made at night was terrific.
The Anti Aircraft Command was part of the army, but the General who was in command, Sir Frederick Pile, had his headquarters bang next door to Dowding’s at Bentley Priory. Pile played a useful part in the Battle of Britain by becoming a close friend and confidant of Dowding. The latter would often, in the evening, expound for an hour or more his thoughts on the Battle. Pile was a very good listener and didn’t interrupt. It was a great help to Dowding to let off steam in this way.