| Part of a fountain pen that fills using a sack mechanism. The pressure bar is a flat piece of metal (usually brass) that sits between the sack and the barrel of the pen. It is manipulated from an input on the outside of the pen (typically a button at the end of the barrel, or a lever on the side of the pen).
This manipulation either moves or bows the pressure bar, which, in turn, compresses the sack. Everything in the sack is expelled, including ink and air (be careful where you point that thing!). Once the pressure on the bar is removed, the sack is allowed to expand. A vacumm is created, drawing in whatever the surrounds the nib (again, ink, air, or water).
As mentioned buttons and levers are the most common methods of manipulating a pressure bar.
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