# Ball Pen Anatomy

By Numa

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/UCp8bLZULVOA)

## Guide

### 1. An everyday ball pen

This is a classic capped ballpoint pen, about 15 centimetres long\. It looks simple, but a few small parts work together every time you write\. Drag anywhere to turn it and look around\.

The ruler beside it is marked every 5 millimetres, starting at the end of the pen\.

### 2. The cap and its clip

The cap keeps the tip from drying out and from marking your pocket\. Its long clip is a springy plastic arm that grips your pocket\. The small hole in the top lets air through\.

That vent hole was added in 1991 as a safety feature: air can always pass through it\.

### 3. Six sides, not round

The barrel is the body you hold\. It has six flat sides like a pencil, so it is easy to grip and does not roll off the table\. A tiny pinhole in its side lets air in to replace the ink you use\.

On this famous design the barrel is clear plastic, so you can see how much ink is left\.

### 4. Taking it apart

Pull the plug out of the back and the refill slides free\. The refill is the real writing machine: a long thin tube of ink with a metal tip\. Everything else is there to hold and protect it\.

### 5. A tube of thick ink

Ballpoint ink is not runny like water\. It is a thick, oily paste, so it does not drip out of the tip\. A clear plug sits behind the ink and follows it down the tube as you use it up\.

Here the top of the tube is cut away so you can see the blue ink inside\.

### 6. The ball in its socket

At the very tip sits a tiny ball, about 1 millimetre across, made of very hard tungsten carbide\. A metal socket holds it, with a lip that lets just a little of the ball peek out\. Ink flows down a narrow hole and grooves to coat the back of the ball\.

The tip is shown cut in half here\. Real balls range from about 0\.5 to 1\.6 millimetres\.

### 7. Roll, and the ink follows

Press the tip on paper and drag\. Friction makes the ball roll, carrying ink from inside the socket out onto the paper\. The ball is both the roller and the lid: when it stops, it nearly seals the tip\.

Watch the lines on the ball turn as it rolls along\.

### 8. Back together

Refill in, plug in, cap on\. A ball, a socket, a tube of thick ink and a little air are all a ball pen needs\. Next time you write, picture that tiny ball rolling\.