At o is a delivery valve, opening outwards, which is often a ball-valve but sometimes a flap-valve.
The ball-valve is supported by a suspension thread (T) from the roof of the bell (B).
The ball-valve ensures that while air can be sucked from both sides of the piston, it can be admitted to the lower side only.
Should the pressure increase, the bell (B) is raised and with it the ball-valve (V).
When the pressure falls again, the ball-valve is lowered and more gas passes through.
The communication between the train-pipe and the cylinder is controlled by a ball-valve, one form of which is shown in fig. 2.
ball valve n.
A valve regulated by the position of a free-floating ball that moves in response to fluid or mechanical pressure.