How Does a Fountain Pen Actually Work?
A fountain pen looks like it should leak constantly. There's no seal, no pump, and no button, just an open nib sitting at the end of a tube full of liquid, and yet ink only comes out when you're actually writing. The explanation is a genuinely elegant bit of everyday physics, and understanding it also explains most of what goes wrong when a pen misbehaves.
The three parts that make it work
A fountain pen has three functional parts. The reservoir (a cartridge, a converter, or a piston chamber) holds the ink supply. The feed, a small ridged piece usually made of plastic or ebonite sitting under the nib, is the actual engineering: it channels ink from the reservoir to the nib in a controlled amount, and just as importantly, lets air travel back up into the reservoir to replace the ink that left. The nib is where ink meets paper, split down the middle by a fine slit that uses capillary action to draw ink out to the tip.
Capillary action, in plain terms
Capillary action is the same effect that pulls water up a paper towel or a thin straw: a liquid moving through a narrow space because it's more attracted to the surface around it than gravity is pulling it down. The slit in a nib and the tiny channels in a feed are narrow enough for this effect to reliably pull ink forward whenever the nib touches paper, and to stop pulling ink forward the instant it doesn't.
Why it doesn't just pour out
The feed's air channel is the real trick. Every time a small amount of ink leaves the reservoir, an equal amount of air has to come back in to replace it, or the pen would create a vacuum that stops the flow entirely. The feed's fins and channels are engineered to let that air exchange happen at a steady, controlled rate, matched to how quickly ink is being used. That balance, ink out, air in, at the same pace, is what lets a fountain pen sit nib-down in a bag for hours without emptying itself, while still writing instantly the moment it touches paper.
Why this explains most pen problems
Almost every common complaint (skipping, hard starts, a pen that seems to flood) traces back to this same air-and-ink balance being disrupted, usually by dried ink partially blocking one of the feed's channels, or air pressure changes from altitude or temperature. Understanding the mechanism doesn't just satisfy curiosity; it's the fastest way to guess what's actually wrong when a pen stops behaving.