Each fountain pen has its own feed designed for one type of ink.
In order to use today’s ink in vintage pens, the feed needs to be adapted to the viscosity of modern inks which is considerably different than that for which it was designed.
For this purpose, a special “flow regulating filter” is placed in the feed so that the right amount of ink flows. This avoids ink leaks and the possible formation of condensation inside the cap once the pen is closed.
The nib feed:
The feed carries the ink from the ink reservoir to the nib.
It controls the ink flow to the nib ensuring a balance between the passage of the ink and the air that replaces it.