What are the "fins" on a fountain pen for?
What purpose do the "fins" on a fountain pen serve?
I tell you, you don't appreciate the vast edifice of modern technology until you start looking into stuff like fountain pens. I mean, geniuses sweated over these things! And all so you wouldn't get ink on your fingers.
Lest the Teeming Millions visualize a fountain pen equipped like a 1950s Buick, let me clarify that the fins we're talking about are those inky ribs or vanes just back of the writing tip. Pen engineers (no joke — one guy I talked to at Sheaffer Pen used to be in aerospace) call this the "comb feed." Basically it acts as a sponge to keep excess ink from dribbling out of the tip.
Suppose your ink cartridge is half full. When you pick up the pen, your hand's warmth makes the air in the cartridge expand, forcing more ink into the tip. Rather than get all over your page, the ink flows into the thin slots between the fins ("combs"), remaining there by capillary attraction. As ink flows out the tip during writing, the slots empty out, air manages to get up the fill tube into the cartridge, and more ink flows down to replenish the supply. Meanwhile you just scribble away, blissfully unaware of the technology that makes it all possible. On the scale of inventions that have enhanced the quality of life, I'm not saying it ranks up there with perforated toilet paper. But I'd still rather have it than not.