Here’s an interesting post with a simple explanation of how faxing works.

The fax machine was invented by Alexander Bain in 1843, and the basic idea behind fax machines is quite simple.  It is possible to send a fax anywhere with a phone connection, including sending faxes internationally because the sheets of paper are transmitted in sound form (i.e. over phone lines). Here’s how it works:

Step 1: As the original sheet of paper is fed into the faxing machine, a photo sensor scans the page.  What the sensor is actually scanning is quite simple.  The photo scanner looks at a small area, roughly 0.01 inches squared, and decides if this area is white or black. Like black text on white paper, for example.

Step 2: There are two preset frequencies (the pitch of a certain sound tone) for black and white.  When the scanner senses a black area it sends one tone over the phone line, and when it senses a white area it sends the other tone over the phone line.

Step 3: The fax machine on the other end of the phone line “hears” the tone and then prints each area according to the sounds it hears. Since both computers know how big the standard scanning area is, and the pattern with which they scan the page, the fax machine on the other end can print out the paper.

Different methods for faxing have occurred during the 150 plus years since it has been invented, but that is the in-the-nutshell idea behind how a fax machine works. The original fax machines used tin foil, alcohol, and electrodes to read the paper.  Now, you can take a document or file on your computer and send it to a fax machine using an online international faxing service. However, the basic concept has remained the same; data transmission through sound.