A tiny rolling ball, a slit thinner than a hair, and ink that climbs by itself. At the tip sits a tiny steel or tungsten-carbide ball, 0.5 to 1 mm across, held by a brass socket so it can spin but not fall out.
A tiny rolling ball, a slit thinner than a hair, and ink that climbs by itself. Take apart ballpoints and fountain pens in 3D.
PenClearOpened 7 Jul 202612 min to playFree · no sign-up
In 60 seconds
A ballpoint is a ball in a socket
At the tip sits a tiny steel or tungsten-carbide ball, 0.5 to 1 mm across, held by a brass socket so it can spin but not fall out. Behind it is a tube of thick, oily ink.
Rolling carries the ink
Drag the pen and friction with the paper makes the ball roll. Its top is coated in ink inside the pen, and as it turns that ink is laid down on the page. Thick ink makes only a thin film, so it rarely smudges.
Gravity and air keep it flowing
Ordinary ballpoints need gravity to keep ink against the ball, and a tiny air hole so air can replace the ink that's used. Point one at the ceiling and it runs dry. Pressurized 'space pen' refills push ink out with gas instead.
Fountain pens use a split nib
A fountain pen has no ball. Its nib is split into two tines, and the slit between them is narrow enough to draw ink along it by capillary action. Press harder and the tines spread, making a wider line.
Ink out, air in
The black feed under the nib sends ink down one channel while air bubbles back up another into the reservoir. That swap stops a vacuum forming, and the feed's fins hold spare ink so the pen doesn't flood.
Capillary action does the lifting
Liquids that cling to a surface climb narrow gaps by themselves. Halve the gap and they climb twice as high. The same force pulls ink through a nib, into paper fibres and up a felt-tip's wick.
A tube of thick ink, a tiny ball, a spring, and a clever button.
Pull a click pen apart and there are only a handful of parts. The refill is a thin plastic tube full of thick, oily ink. At its end sits a brass tip holding a tiny steel or tungsten-carbide ball, usually 0.5 to 1 mm across.
The ball is the whole trick. It's held by the tip so it can spin but can't fall out. When it rolls across paper, it carries ink from the tube down to the page.
The spring pushes the refill back into the barrel. The button and a little rotating cam hold it out when you click, and let it go when you click again.
It picks up ink on top and lays it down underneath.
The ball sits in a socket just big enough to let it turn. Inside the pen, its top half is bathed in ink. As you drag the pen, friction with the paper makes the ball roll, and the inked surface comes round to the bottom and touches the page.
Ballpoint ink is thick and oily, so only a thin film clings to the ball, and it barely soaks into paper. That's why ballpoints rarely smudge or bleed through. Gel and rollerball inks are runnier, so they lay down wetter, bolder lines.
The line width depends on the ball: a 0.7 mm ball draws a line only about 0.3 mm wide, because just a small part of the ball touches the paper.
Point it at the ceiling and the ink walks away from the ball.
An ordinary ballpoint relies on gravity to keep ink sitting against the back of the ball. Write on a wall for a while, or on a notepad held above your head, and the ink column slides back up the tube. The ball runs dry and the line fades.
There's a tiny air hole at the back of the refill. As ink leaves through the ball, air comes in behind it. Block the hole and a partial vacuum builds up that holds the ink back, so the pen stops writing even pointing down.
A pressurized refill, the idea behind the Fisher Space Pen (1965), seals the tube and puts compressed nitrogen behind a plug of ink. The gas pushes ink to the ball at any angle, underwater, in freezing cold and in space.
Ink runs down a slit; air bubbles up to take its place.
A fountain pen has no ball. Its metal nib is split down the middle into two tines. The slit between them is so narrow that ink is drawn along it by capillary action, the same force that pulls water up a paper towel.
Under the nib sits the feed, a black comb with tiny channels. It does two jobs at once: ink runs down one channel towards the nib, while air bubbles back up another into the reservoir. Without that swap, a vacuum would build and the ink would stop.
The fins of the feed hold spare ink, like a sponge, so the pen doesn't flood when your hand warms the air inside. Press harder and the tines spread: the slit widens and the line gets wider.
Water molecules are attracted to glass more than to each other, so water creeps up a glass wall. In a narrow tube, that creep lifts the whole column. The narrower the tube, the higher it climbs. This is capillary action.
The height follows Jurin’s law: h = 2γ·cos θ / (ρ·g·r). Here γ is surface tension, θ how well the liquid wets the wall, ρ its density, g gravity, and r the tube's radius. Halve the radius and the liquid climbs twice as high.
A fountain pen's slit is a capillary a few hundredths of a millimetre wide. That's why ink runs to the tip by itself, and why paper, a mesh of tiny fibres, soaks it up. Mercury does the opposite: it's more attracted to itself than to glass, so it's pushed down.
A tiny rotating cam that remembers whether the pen is out or in.
Press the button of a retractable pen and three parts meet: the plunger you push, a cam ring with slanted teeth, and ribs moulded inside the barrel.
The plunger's teeth push the cam down and slide it sideways along their slopes, turning it a little. When you let go, the spring pushes back and the cam lands on the ribs. Every other click it lands on top of a rib, which holds the tip out. On the next click it turns a little further and drops into a groove, so the tip goes in.
Each click turns the cam one eighth of a turn. Nothing needs to remember the state, because the cam's angle is the memory.
A tiny rolling ball. The ball rolls: ink sticks to it inside the pen and is laid down on the paper as it turns.
Why doesn’t ballpoint ink leak out of the tip?
It’s thick and oily, and the ball seals the tip. Thick ink plus a tight ball-and-socket act like a seal until the ball turns.
What does the spring do?
Pulls the refill back in when the pen is retracted. The spring retracts the refill; the button and cam hold it out.
Why does the ball roll instead of sliding?
Friction with the paper turns it as you move the pen. The paper grips the bottom of the ball, so dragging the pen makes it roll.
A 0.7 mm ball draws a line about how wide?
0.3 mm. Only a small part of the ball touches the paper, so the line is narrower than the ball.
Why do ballpoints smudge less than rollerballs?
Their ink is thick and oily and barely soaks in. Thick oil-based ink sits in a thin film that dries fast.
Why does an ordinary ballpoint stop writing upside down?
Gravity pulls the ink away from the ball. Without gravity holding it at the tip, the ink column slides back and the ball runs dry.
What does the tiny hole at the back of a refill do?
Lets air replace used ink. Air must replace used ink, or a vacuum holds the ink back.
How does a space pen write in zero gravity?
Compressed gas pushes the ink to the ball. Pressurized nitrogen behind the ink does gravity’s job.
What pulls ink down the nib’s slit?
Capillary action. A very narrow gap draws liquid along it by itself.
Why do bubbles rise into a fountain pen as it writes?
Air must replace the ink that flows out. Air replaces used ink, or a vacuum would stop the flow.
What happens when you press harder on a flexible nib?
The tines spread and the line gets wider. Spreading tines widen the slit and let out a wider stripe of ink.
Make a glass tube half as wide. What happens to the water column?
It rises twice as high. Height is inversely proportional to radius: h ∝ 1/r.
Why does mercury sink in a narrow glass tube?
It’s more attracted to itself than to glass. Mercury doesn’t wet glass (contact angle above 90°), so the curve pushes it down.
What moves ink from a fountain pen’s feed to the tip?
Capillary action in the slit. The nib’s narrow slit is a capillary.
How far does the cam turn each click?
An eighth of a turn. Its slanted teeth step it one eighth of a turn per click.
What decides whether the tip stays out?
Whether the cam lands on a rib or drops into a groove. The cam’s angle alternates between resting on a rib and sliding into a groove.
What pulls the tip back in?
The spring. When the cam drops into a groove, the spring pushes the refill back into the barrel.
Words worth knowing
Tungsten carbide
A very hard material used for most ballpoint balls because it barely wears down.
Viscosity
How thick a liquid is. Ballpoint ink is about as thick as honey; fountain pen ink is nearly as thin as water.
Capillary action
A liquid rising in a narrow gap because it clings to the walls more than to itself.
Surface tension
The pull between molecules at a liquid's surface that makes it behave like a stretched skin.
Nib
The metal point of a fountain pen, split into two tines by a thin slit.
Feed
The part under a fountain pen's nib that sends ink out and lets air in.
Cam
A shaped part that turns a push into a rotation, like the one inside a click pen.
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