Quite a few imagine that the fountain pen can be schematized as a container with holes in it. A colored liquid would be poured inside the container which, through the small hole, would reach the tip with which the liquid is spread on the paper.
Things are actually a little more complicated, even if there is a container and a hole.
The principle behind the operation of a fountain pen is known as the law of capillarity. The law of capillarity or Jurin's law was already formulated in the early eighteenth century by Mr. Justin Jurin, an Englishman very interested in philosophia naturalis - as physics was then called - but it had been widely anticipated and made possible by the many empirical observations in the field of pressure, especially from the experiments of our Florentine Evangelista Torricelli.
Without going into too many technical details, suitable for those familiar with physics or its applications in fluid engineering (but for which we refer the more curious to: https://study.com/learn/lesson/capillary-action.html )
the law of Jurin or of capillarity establishes that in the presence of certain conditions a liquid can go back up a capillary tube, i.e. a very small tube, even in the presence of a force that would push it in the opposite direction. This is what happens with the fountain pen.
The feeder, that element on which the metal nib rests, is nothing more than a "sponge" capable of making the liquid contained in the tank or in the ink cartridge rise towards the nib. In turn, the nib with its central cut acts as an additional sponge that accompanies the liquid in its final stroke, up to the tip of the nib, which we thus always find moistened with ink and ready to leave a colored trace on the paper.
Many aspects contribute to the correct functioning of the fountain pen (characteristics of the feed channels, air recovery, materials in which the feeders are made, ink viscosity, width of the nib cut, etc.) but this is the underlying mechanism. Once you understand why it works, it will be relatively simple to solve the small problems or micro-adjustments that can make a fountain pen more performing or more pleasant to use for everyone.
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