AI Summary
5 min readThe Fluid Dynamics of Fountain Pens
Hannah Fry brought her fountain pen collection to the studio, and Michael Stevens—who admitted he'd used a fountain pen maybe once in his life—immediately guessed the most expensive one correctly: a limited-edition Jimi Hendrix pen that costs around a grand. But as Fry explained, expensive and special are not the same thing. Her favorite to write with is a Pelican, while her most special pen is a 1920s Waterman with a flexible 14-carat gold nib. The episode became a masterclass in the physics, history, and psychology of writing instruments.
Capillary Action and the Delicate Balance
The core mechanism that makes fountain pens work is capillary action—the same force that allows redwood trees to draw water hundreds of feet upward. When the nib touches paper, the paper absorbs the ink at the tip, which pulls more ink down from the chamber through a thin slit running down the center of the nib. A small hole at the top of that slit serves a counterintuitive structural purpose: it prevents the crack from spreading further, the same technique used on aircraft to stop cracks from propagating.
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What you'll learn
- 1 (00:01) **Opening and Hannah’s Fountain Pen Collection** - Michael and Hannah introduce the topic; Hannah brings out a range of pens and Michael guesses which is most expensive.
- 2 (05:30) **Hannah’s Favorites and a 1920s Waterman** - Hannah shares her two favorites: one for writing (Pelikan) and one for its mechanism (1920s Waterman).
- 3 (06:50) **The Physics of Fountain Pens: Capillary Action** - Hannah explains the core hydrodynamic principle: ink flows via capillary action, not gravity.
- 4 (09:35) **The Feed: Controlling Ink Flow** - Hannah describes the feed (the black part under the nib) that prevents ink flooding.
- 5 (11:50) **The Delicate Balance of Ink and Nib Design** - Hannah explains that pen designers match ink chemistry to nib structure for optimal flow.
- 6 (14:50) **Writing with the 1920s Waterman: A Wet Noodle** - Michael tries the Waterman and notes its soft, flexible nib produces variable line widths.
- 7 (16:40) **Filling Mechanisms: Lever, Button, and Piston** - Hannah demonstrates vintage filling systems: the Waterman uses a lever-compressed rubber bag; the Parker Duofold uses a push-button mechanism.
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Guests on this episode
Show Notes
The wait is finally over: Hannah Fry introduces Michael Stevens to her beloved fountain pen collection and the beautiful physics behind how fountain pens work.
From a £20 Lamy Safari to vintage Waterman and Montblanc pens, Hannah explains how capillary action, surface tension, air pressure and carefully engineered feeds control the flow of fountain pen ink. Michael tests the collection, chooses his favourite and discovers why the most expensive pen is not necessarily the nicest to write with.
Then Hannah and Michael answer your questions: which words are terrible at their jobs, how do aeroplane wings actually generate lift, and which moment in history would they most like to witness?
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