Information collected from the Internet
… with small interventions of mine (in brackets)… where I couldn’t resist.
— Early pens did not have an effective means to allow in the right amount of air to take the place of the departing ink, so the ink flow was consequently not well controlled.
— In the 1870s, L.E. Waterman developed his three-channel feed. During writing, due to alteration of writing pressure, the nib touched and moved away from the feed. This allowed small amounts of air escalating up the feed into the sac. (very interesting design)
— Later, cavities were added below the nib, which were able to absorb excess ink, thus preventing blots.
— Later, for the same purpose pens of the 1940s and onward used internal cavities as a collector.
— The feed is probably the most important single part of the pen. Without an effective feed, even the finest pen made from the best materials will drip and leak constantly or else not give up any ink at all.
— The ink is fed to the nib through a feed via a combination of gravity and capillary action.
— Tests of ink flow within the feed (don’t ask me how they did them) have shown that the fluid pressure varies in a “zigzag” fashion over time as you write, indicating that you are actually “pumping?” ink from inside the pen as you write.
— Careful design and execution of the feed channels (including variation in their depth) are required to reinforce this pumping behaviour as well as to deal with excess ink flow that may happen during long writing sessions.