You can walk into nearly any office today, big or small, hi-tech or low tech, and you will find a fax machine. Connected to a normal phone line, a fax machine allows you to transmit pieces of paper to someone else instantly! Even with FedEx and e-mail, it is nearly impossible to do business without one of these machines today. In this article, you'll learn how a fax machine works its magic!

The Basic Idea: If you look back at some of the early designs, you can get a very good idea of how they work today. Most of the early designs involved a rotating drum. To send a fax, you would attach the piece of paper to the drum, with the print facing outward. The rest of the machine worked something like this: There was a small photo sensor with a lens and a light. The photo sensor was attached to an arm and faced the sheet of paper. The arm could move downward over the sheet of paper from one end to the other as the sheet rotated on the drum. In other words, it worked something like a lathe. The photo sensor was able to focus in and look at a very small spot on the piece of paper perhaps an area of 0.01 inches squared (0.25 millimeters squared). That little patch of paper would be either black or white. The drum would rotate so that the photo sensor could examine one line of the sheet of paper and then move down a line. It did this either step-wise or in a very long spiral. To transmit the information through a phone line, early fax machines used a very simple technique: If the spot of paper that the photo cell was looking at were white, the fax machine would send one tone; if it were black, it would send a different tone. For example, it might have sent an 800-Hertz tone for white and a 1,300-Hertz tone for black. At the receiving end, there would be a similar rotating-drum mechanism, and some sort of pen to mark on the paper. When the receiving fax machine heard a 1,300-Hertz tone it would apply the pen to the paper, and when it heard an 800-Hertz tone it would take the pen off the paper.

Modern Fax Machines: A modern fax machine does not have the rotating drums and is a lot faster, but it uses the same basic mechanics to get the job done:

• At the sending end, there is some sort of sensor to read the paper. Usually, a modern fax machine also has a paper-feed mechanism so that it is easy to send multi-page faxes.

• There is some standard way to encode the white and black spots that the fax machine sees on the paper so that they can travel through a phone line.

• At the receiving end, there is a mechanism that marks the paper with black dots.

A typical fax machine: that you find in an office is officially known as a CCITT (ITU-T) Group 3 Facsimile machine. The Group 3 designation tells you four things about the fax machine:

• It will be able to communicate with any other Group 3 machine.

• It has a horizontal resolution of 203 pixels per inch (8 pixels/mm).

• It has three different vertical resolutions:

• Standard: 98 lines per inch (3.85 lines/mm)

• Fine: 196 lines per inch (7.7 lines/mm)

• Super fine (not officially a Group 3 standard, but fairly common): 391 lines per inch (15.4 lines/mm)

• It can transmit at a maximum data rate of 14,400 bits per second (bps), and will usually fall back to 12,000 bps, 9,600 bps, 7,200 bps, 4,800 bps or 2,400 bps if there is a lot of noise on the line.

Receiving the Fax: The bits for the scanned document travel through the phone line and arrive at a receiving fax machine. The bits are decoded, uncompressed and reassembled into the scanned lines of the original document. There are five common ways to print the fax, depending on the type of machine that receives it:

• Thermal paper When fax machines started infiltrating offices en mass in the 1980s, most of them used thermal paper. The paper is coated with chemicals that react to heat by turning black. Thermal paper has several big advantages:
• It is very inexpensive to build a thermal printer.
• Thermal printers have no moving parts except for the paper-feed mechanism.
• There are no expendables like ink or ribbons because the paper contains the ink.
• Thermal printers are nearly indestructible.

The only disadvantage is that the paper discolors over time, and it turns completely black if you leave it in a hot car.