ball joint (exhaust system). A flexible expansion joint used in an aircraft engine exhaust system to allow relative movement between the parts as heat causes them to expand and contract.
ball peen hammer. A hammer with one side of its head shaped like a ball. The ball is used to peen, or flatten, the heads of rivets driven in commercial sheet metal.
ballast (aircraft balance). A weight permanently installed in an aircraft to move the center of gravity to a location within its allowable limits.
ballast lamp (electrical equipment). An incandescent lamp installed in series with an electrical load to maintain a constant current.
If the current increases, the filament gets hotter and its resistance increases. This brings the current back to its original value.
ballistic missile. A self-propelled missile which is guided as it goes upward. However, as it comes down, it follows a ballistic trajectory, its natural curved path.
balloon (aerostatic device). A lightweight aerostatic machine, usually spherical in shape and neither powered nor steerable. When the balloon is filled with hot air or with a gas that is lighter than air, it displaces more air than it weighs, and the resulting aerostatic lift causes it to rise.