The first fax machine was invented in the 1840s by Alexander Bain. Used to send information from one place to another, it was, before the advent of personal computers, essential equipment for many offices.

Early fax machines were made up of a rotating drum and a photosensor (a device that reacts to light) which focused on a very small area. The document to be faxed was scanned by the photosensor, which could detect whether each small area was black or white. The machine would send a different audio tone depending on whether the area examined was black or white. The receiving fax would have a similar rotating drum, and when it received one tone it would print a mark, and when it received the other tone it would leave a blank.

This can be easily demonstrated in the classroom. First give each student a grid of small squares. Ask them to create a picture by filling in some of the squares on the grid. Show them some examples, explaining that the squares need to be filled completely or left blank.

Next, get your students to work in pairs to see if they can do the job of the fax machine. One child will “send” a fax, and the other will “receive” it. The receiver needs a matching blank grid and the sender needs to encode the picture grid, singing a high note for a blank and a low note for a filled square. They work along the squares, with the receiver filling a square when there is a low tone and leaving a space when there is a high one. This needs some concentration, especially if the sender is working at speed. How accurately can the children send and receive faxes?

Alternatively, choose a message to send yourself and get your students to try to receive it. Which student can recognise the message quickest?

Sophie Duncan is project manager for science at the BBC www.bbc.co.ukscience