How does a fax machine work?

A fax turns your page into 2 million dots, squeezes them 10 to 20 times with a clever code book, and sings them down a phone line. Scan your own drawing, read the real run codes, hear the handshake, and watch line noise smear a page until error correction fixes it.

Read how it works: How does a fax machine work? · The history of the fax machine · More boxes on Glassbox

Chapters in this interactive model

1. Inside a fax machine: A scanner, a printer and a modem, sharing one box and one phone line.
2. Turning a page into dots: One thin line at a time, every spot is judged black or white.
3. Squeezing the page: run-length codes: Why a page that is mostly white goes down the line in seconds.
4. Sending the page as sound: Beeps, a warble, then a hiss: two modems agreeing and talking fast.
5. Printing with heat, and fixing mistakes: A row of tiny heaters rebuilds the page, line by line, on paper that darkens when warm.
6. Fax today: over the internet, and why it survives: Phone lines went digital, and the fax learned to ride in packets.