The feed is a conceptually simple object– it’s just a way of trading air for liquid in the pen’s reservoir. Actually managing this trick takes a pretty clear sense of fluid dynamics, though, since too much liquid leads to disaster and too little air leads to stoppages (all of which in more depth elsewhere).
The feed has evolved over the years. They started as quite simple objects, aiming only at the exchange noted above, but later addressing the problem of too much ink getting outside the reservoir. Early efforts in this direction seemed to focus on an efficient drain-back, as was notably the object of Parker’s “Lucky Curve” feed (touching the side of the reservoir as a means of making capillary action work inwards as well as outwards), but a more successful approach was to add complexity to the feed to give the ink a halfway house to hang around in between reservoir and page.
A "51" showing off its collector
The Waterman "Spoon Feed", with the little ink-gathering cut-outs.
At first, these external buffers took the shape of a simple cut-out to either side of the feed channel, but as time went on they developed into teeth, crenellations and combs, until a final expression in the collector of the Parker “51”. Sheaffers of the same era had slightly more traditional feeds, but of nearly the same surface area. Most modern pens, apart from some extremely affordable atavisms, have feeds in this pattern– a multiplicity of fine comb-like baffles to hold ink between the reservoir and the point. Indeed, even rollerball pens appear with an interior structure similar to the Parker collector. Have a look at the Pilot Petit 1, or the interior of the Parker 25, for rather inexpensive examples of the serious convolutions of even low-priced modern feeds.
