The nib: two tines that open under loadThe feed: ink channel, comb of fins and air channelInk, in the cartridge and in the feedA fountain pen cut along its length and held at 55° over a moving paper tape, drawn at 4:1. Weights on the tray press it down; the loupe magnifies the nib. Ink is pulled forward along a narrow channel by capillary action, and each time the cartridge has lost about 0.6 µL a bubble of air is let in through the breather hole and travels back along the wider air channel. The numbers come from a simplified model of a soft nib and a water-based ink, not from a measured pen.
Line width

0.4 mm

Ink supply

6 of 6 µL in the fins

Reservoir

0 Pa below air

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How it works

  1. The pin opens the cartridge. The feed ends in a hollow pin that pierces the cartridge's plug. It gives ink a way out and air a way in.
  2. Capillary action draws ink forward. Along the feed's ink channel the gap is only 0.15 mm, and a gap that small pulls liquid with about 533 Pa, enough to lift ink against gravity.
  3. The paper takes it from the nib. The slit carries ink to the tipping. Paper pores, about 10 µm across, pull with about 8000 Pa, far harder than the slit, so ink moves into the page as the nib drags over it.
  4. Air has to come back. Every microlitre of ink that leaves thins the air above the ink, so the pressure in the cartridge falls. At 160 Pa below the room a bubble forces in at the breather hole, runs back along the wider air channel and rises to the top of the cartridge. Seal that channel and the suction builds toward 533 Pa until the feed can no longer pull.
  5. The fins are a reserve and a catch basin. The channel delivers at most 0.35 µL/s, so a bold, fast stroke would starve; the fins hold about 6 µL to draw on and refill when you pause. They also soak up the ink that a warming cartridge pushes forward, instead of letting it blot the page.
  6. The nib flexes. Press and the two tines spread, the slit opens and the line gets wider, 0.4 mm at rest and up to 1.6 mm in this soft nib. A wider line asks for more ink, which loops back to step 5.

Try: press 12 weights at 55 mm/s and watch the fins drain, then ease off and watch them refill. Press Remove the fins and repeat. Press Warm the pen with the fins on and off.

NibFeed, section, barrel, cap and cartridge shellInkThe same pen, pulled apart along its axis in the order you would take it apart: the nib and feed come out of the front of the section, the barrel unscrews and slides back off the cartridge, and the end cap comes away last. Every part is cut along the same plane as above. Select one to zoom in and read what it does and why the pen needs it.