Basic Idea Behind Fax Machines

Fax machines have been around in one form or another for more than a century - Alexander Bain patented the first fax design in 1843 (see Science Online: Alexander Bain and fax machine for more information). Looking back at some early designs, you can get a pretty good idea of how they work today. Most of the first designs in a rotating drum. To send a fax, you attach the piece of paper to the drum, with the impression outward faces. The rest of the machine worked something like this:

• There was a small photo sensor with a lens and a light.
• The photo sensor is attached to an arm and faced the sheet of paper.
• The arm can move down on the sheet of paper from one extreme to another as the road turn into the drum.

In other words, it worked something like a lathe.

The photo sensor was able to see and focus on a small piece of land on pieces of paper - perhaps an area of 0.01-inch-square (0.25 square millimeters). That small piece of paper would be either white or black. The drum would rotate so that the photo sensor could consider a line of the sheet of paper and then lose one line. He did so either in stages or in a long spiral.

To transmit information via a telephone line in early fax machines used a technique very simple: If the ground of the role that the photo cell was watching were white, the fax machine to send a tone, if it were black, would send a different tone (see How modems work for details). For example, it could have sent a 800-Hertz tone white and a 1300-Hertz pitch black.

On the receiving end, there would be a rotation similar mechanism drum, and a kind of pen to mark on paper. When the receiving fax machine heard a 1300-Hertz tone that would apply pen to paper, and when they heard a 800-Hertz tone to take the pen out of paper.