by Chris Woodford. Last updated: November 3, 2016.
When Alexander Graham Bell (1847–1922) spoke the immortal words "Mr. Watson! Come here! I want to see you!" into his primitive telephone in March 1876, he became one of the founding fathers of the modern age of telecommunications. But just suppose for a moment that he'd wanted to send his colleague a picture instead of spoken words—how exactly would he have done it? Most people assume fax machines (which send documents down phone lines) are newer than the telephone lines they use, but the first fax (Alexander Bain's "chemical telegraph") was actually patented decades before the phone in the 1840s. Today, the Internet has largely made faxing obsolete, but many businesses still rely on trusty old fax technology. Let's take a closer look at how it works!
Photo: A simple, affordable, personal fax machine. Fax is short for "facsimile": the idea is to create a replica of an original document at the other end of the telephone line. Photo by Michelle Kinsey Bruns published on Flickr under a Creative Commons Licence.
Imagine you're a fax machine...
Suppose you have an urgent contract you want me to sign and you need to get it to me as quickly as possible. You could mail it, of course, but that will take at least a day to reach me and another day for me to return it. You could use a courier—but, unless we live near one another, we're still talking about a turnaround time of hours. Or you could send the contract down the phone with a fax machine in a minute or so.
Let's imagine for a moment that fax machines haven't been invented, but you still want to use the phone. Suppose you need to transmit a one-page document to me. What can you do? Let's make the problem really easy. Let's say the document can be either a totally black page or a totally white one. Now transmitting the document is really easy. You simply pick up your phone, dial my number, wait for me to answer, and then say either "black" or "white".
Okay, let's make the problem a bit harder. Suppose the document is a single page divided into four squares and each of the four areas can be either black or white. Again, it's fairly easy for you to transmit this document. You ring me up and just say "Black black white black", "White black white white" or whatever—and I can instantly recreate the document in my mind's eye at the other end.
Now let's make the problem much harder. You have to send to me an entire page covered in black-and-white, computer-printed words. Actually, this problem isn't as hard as it looks. All you have to do is divide the page into thousands of grid squares and then read out, from left to right and from top to bottom, whether each square is black or white. Suppose I'm sitting at the other end of the phone with a piece of paper ruled with an identical grid of squares. As you read out "black", "white", "white", "black", I just need to shade in all the black squares with a pencil and skip the white ones. By the time you get to the bottom of the page, my shaded-in page will look just like yours. If we make the squares small enough, so each one is slightly bigger than a pinhead, I will magically end up with an exact, readable copy of your page. Simply speaking, this is how fax machines work.
How real fax machines work
Well, okay, it's not exactly how they work! A fax machine is designed to both send and receive documents so it has a sending part and a receiving part. The sending part is a bit like a computer scanner, with a CCD (charged-coupled device) that scans only one line of a document at a time, and only in black and white. Crudely simplified, it looks at each line separately, detects the black areas and the white areas, and transmits one kind of electric pulse down the phone line to represent black and another to represent white (just like saying "black" and "white", in fact). The phone line transmits this information almost instantly to a fax machine at the other end. It receives the electrical pulses and uses them to control a printer. If the receiving fax hears "black", it draws a tiny black dot on the page; if it hears white, it moves along slightly, leaving a white space instead. It takes about a minute or so to transmit a single page of writing (or a complex drawing) in this clumsy but very systematic way.