Reading Blink over my lunch break, came across this passage during my lunch break which I found highly interesting. It's more or less a digression in the middle of the first chapter, which I'll have more to say about later (I hope):
...Morse code is made up of dots and dashes, each of which has its own prescribed length. But no one ever replicates those prescribed lengths perfectly. When operators send a message--particularly using the old manual machines known as the straight key or the bug--they vary the spacing or stretch out the dots and dashes or combine dots and dashes and spaces in a particular rhythm. Morse code is like speech. Everyone has a different voice.
In the Second World War, the British assembled thousands of so-called interceptors--mostly women--whose job it was to tune in every day and night to the radio broadcasts of the various divisions of the German military. The Germans were, of course, broadcasting in code, so--at least in the early part of the war--the British couldn't understand what was being said. But that didn't necessarily matter, because before long, just by listening to the cadence of the transmission, the interceptors began to pick up on the individual fists of the German operators, and by doing so, they knew something nearly as important, which was who was doing the sending. "If you listened to the same call signs over a certain period, you would begin to recognize that there were, say, three or four different operators in that unit, working on a shift system, each with his own characteristics," says Nigel West, a British military historian. "And invariably, quite apart from the text, there would be the preambles, and the illicit exchanges. How are you today? How's the girlfriend? What's the weather like in Munich? So you fill out a little card, on which you write down all that kind of information, and pretty soon you have a kind of relationship with that person."
The interceptors came up with descriptions of the fists and styles of the operators they were following. They assigned them names and assembled elaborate profiles of their personalities. After they identified the person who was sending the message, the interceptors would then locate their signal. So now they knew something more. They knew who was where. West goes on: "...And in a moment of crisis, when someone very high up asks, 'Can you really be absolutely certain that this particular Luftwaffe Fliegerkorps [German air force squadron] is outside of Tobruk and not in Italy?' you can answer, 'Yes, that was Oscar, that was absolutely sure.'"
The key thing about fists is that they emerge naturally. Radio operators don't deliberately try to sound distinctive. They simply end up sounding distinctive, because some part of their personality appears to express itself automatically and unconsciously in the way they work the Morse code keys. The other thing about a fist is that it reveals itself in even the smallest sample of Morse code. We have to listen to only a few characters to pick out an individual's pattern. It doesn't change or disappear for stretches or show up only in certain words or phrases... An operator's fist is stable.
Just an interesting piece of military history I'd never encountered previously.