Close-up of the metal tip of a plain ballpoint pen resting on white paper at the end of a smooth curved blue line
AI-generated image, not a photograph of a real person or place.
doodlespen

HOW A BALLPOINT WORKS

A tiny ball
does the writing.

≈ 1 mm ball in the tip

  • The ball rolls
  • The ink clings
  • The paper grips
  • Surface science in your pocket

Inside the tip is a ball about a millimetre across.

As it rolls, one side picks up ink from the refill and the other side lays it on the page. That simple idea explains why ballpoints rarely leak, why they skip, and why one story about pens in space is a myth.

Updated

Try a surface

THE SHORT ANSWER

Roll, coat,
transfer.

A ballpoint is a ball bearing that writes.
Its ink, its ball and the paper all have to work together.

HOW IT WORKS

A ball in a socket

Illinois Institute of Technology researchers describe a small ball sitting in the tip: one side touches the ink, and as it rolls it carries ink round to the paper.

Try a surface

IN SPACE

Pencils came first

The Smithsonian calls the story of a costly US space pen versus a Soviet pencil false: both sides used pencils at first, then bought pressurised pens.

The real story

KEEP IT WRITING

Refill, don't bin

Many people throw away a whole pen when it stops working well. The IIT article notes the ink cartridge can be replaced instead.

Simple habits

A QUICK SURFACE CHECK

Will it write
here?

Pick a situation to see how an ordinary ballpoint copes. The bar shows how likely it is to skip or stop.

Surface check

Choose a situation
Ordinary paperWrites well

Paper is fibrous, and the ball needs friction with the surface to keep turning. Ink is also made to stick to paper fibres.

In an aircraft cabinNo leaks

Ballpoints spread around the time of air travel, replacing fountain pens that often leaked in a cabin's low air pressure.

Pressurised refill, any angleKeeps going

A refill pressurised with nitrogen, using a gel-like ink that turns liquid under pressure, was designed to write in any direction.

Upside down for a long timeStarts to fail

Ink flow is driven mainly by capillary forces, but gravity still matters: an ordinary ballpoint begins to fail if used upside down for a long time.

Glass, plastic or waxed paperSkips

Slick surfaces are hard to write on, likely because there is too little friction to keep the ball spinning.

Sources: Reverdy, Final Project: Ballpoint pen (Princeton MAE 559, 2010); IIT, Wetting the Ball (2023); Smithsonian NASM, The Saga of Writing in Space.

Based on a Princeton fluid mechanics course paper, an Illinois Institute of Technology research summary and the Smithsonian National Air and Space Museum.

TWO NUMBERS WORTH KNOWING

Small ball,
pricey pencils.

One figure for the pen tip,
one from the early days of spaceflight.

≈ 1 mm

Typical ballpoint ball diameter

At this size, capillary effects dominate, so the ink is pulled along mainly by surface forces rather than by gravity.

THE BALL · PRINCETON

$128.89

Cost of each early NASA mechanical pencil

NASA paid $4,382.50 for 34 mechanical pencils. Later it bought 400 pressurised pens at $6 each for the Apollo 7 mission.

SPACE PENCILS · SMITHSONIAN

The Smithsonian says the Soviet Union bought 100 pressurised pens and 1,000 ink cartridges for its Soyuz flights; before that its crews used grease pencils.

THREE HABITS FOR A HAPPIER PEN

Keep the ball
rolling.

Two plain clear-barrelled ballpoint pens on a wooden desk, one showing its blue ink refill through the barrel
AI-generated image, not a photograph of a real person or place.

Each habit follows from how the pen works: the ball needs ink to coat it and a surface to grip.

  1. Write on paper, not gloss

    Slick surfaces such as glass, many plastics and waxed paper give the ball too little grip to turn.

  2. Keep the tip pointing down

    An ordinary ballpoint relies partly on gravity to keep ink at the ball, so long spells writing upside down make it fail.

  3. Swap the refill

    If the barrel is fine, replacing the ink cartridge keeps the pen going and avoids throwing the whole thing away.

PENS IN SPACE

The myth of
the costly pen.

A plain blank notepad on a grey surface with a thin cord looped round its binding and a mechanical pencil beside it
AI-generated image, not a photograph of a real person or place.

A popular story says the US spent billions on a space pen while Soviet crews simply used pencils. The Smithsonian's National Air and Space Museum says that is false: from Project Gemini, NASA used pencils tethered to the cabin wall, and Soviet crews used grease pencils.

Pressurised pens came from private development and were offered to NASA in 1965. Safety mattered: after the Apollo 1 fire, every item on board, even writing tools, had to be rethought so it would not feed a fire in a pure oxygen atmosphere.

Questions

Questions people ask

How does a ballpoint pen work?

A small ball sits in the tip. One side touches the ink in the refill and, as the ball rolls across the paper, it carries a thin film of ink round to the page.

Why does my ballpoint skip?

Researchers at the Illinois Institute of Technology found skipping happens when the ink does not lubricate the ball completely. They suggest adjusting the ink's viscosity or the ball's material so it wets evenly.

Why won't a ballpoint write upside down?

Ink flow is driven mainly by capillary forces, but gravity is not negligible: an ordinary ballpoint begins to fail if used upside down for an extended time. Pressurised refills were designed to get round this.

Who invented the ballpoint pen?

The Science Museum Group credits László Bíró with inventing it in about 1931, with his brother helping to develop the thick ink. The first Biros were made in the UK by an aircraft company in about 1943.

Did NASA spend millions on a space pen?

No. The Smithsonian says the story is false. NASA used pencils at first, then bought 400 pressurised pens at $6 each for Apollo 7. The pen itself was developed privately.

Sources

Where the facts come from

  1. Illinois Institute of Technology: Wetting the Ball, improving ballpoint performance (2023)
  2. Princeton University, MAE 559: Final Project, Ballpoint pen (Reverdy, 2010, PDF)
  3. Smithsonian National Air and Space Museum: The Saga of Writing in Space
  4. Science Museum Group Collection: Biro ballpoint pen, 1945-1948
  5. Science Museum Group Collection: Biro pen, sectioned to show inside

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