Artificial Double Tracking, or ADT, was invented by EMI at Abbey Road studios on 6 April 1966. In simple terms, the process involved taking an existing signal, creating a duplicate "image" of it and then aligning this secondary image either slightly behind or ahead of the original signal. If done properly it created the impression of two separate signals playing back almost, but not quite, in sync with each other. This system used two tape decks which were connected to the recording console, and to each other. As a vocal or instrument was being recorded onto the first tape machine the signal was fed from the record head of the first deck into the record head of the second deck, onto the tape, out from the playback head of the second deck and back into the record head of the first. If the playback heads of the two decks were precisely the same distance from their respective record heads, the voices/instruments would be recorded in perfect unison. However, the double-tracking effect relied on the almost inaudible millisecond delays between the guide vocal and the double-tracked vocal. By adjusting a Variable Speed Oscillator (VSO) that controlled the speed of the motor on the second tape deck, the tape would run slightly slower than on the first deck. With this slight delay now introduced, the signal coming out of the playback head on the first deck would be audibly 'doubled', but the delay was not enough to cause the vocals/instruments to be noticeably out of sync or out of tune. The use of ADT was used throughout the "Revolver" album by The Beatles.
Once you have ADT you can start producing other interesting effects. If the two "images" of sound were placed extremely close to each other (1 or 5 milliseconds delay) it was no longer possible to discern the presence of the two distinct signals. Of course, what you get is the comb-filter effect. However, what intrigued the EMI staff was the effect they got when experimented with ADT by very slightly moved the knob on the oscillator back and forth continually varying the delay between roughly 0 and 5 milliseconds. When they did this they invented "flanging". EMI then used this ADT/flanging setup and included one more refinement to deepen the effect; the signal from the tape unit was phase reversed creating a sound that gave the impression of sounds being momentary "ducked away" as the signal crossed the zero-delay point. Anyway, this was the use of ADT to create a new "effect" called "phasing". Flanging and phasing were used by The Beatles predominantly in the album "Sgt. Peppers Lonely Hearts Club Band". However, you have to have the mono version because most of the flanging/phasing effects are missing on the stereo version (hence you will not hear them on your CDs and MP3s).
EMI then experimented with this further and continuously varied the delay between 30 and 50 milliseconds and thus created when we know of as the "chorus" effect. If you listen to The Beatles album "Magical Mystery Tour", especially "I Am the Walrus" and you will hear this effect. The reason why I keep mentioning The Beatles as they were the ones who really drove the invention of these effects and where the first ones to use it and push it; so, that is why their name is always mentioned here.
Now, the oscillator used for the ADT-based effects was the Levell TG-150M crystal oscillator. This had a clear Perspex arm that indicated which frequency the unit was generating. It was discovered that even very slight motion of this frequency knob produced significant change in the output of the oscillator and therefore noticeable change in speed of the tape unit carrying the delayed signal. You could physically move this arm and "play" with the effect to match the music creating an interesting "wobble". This manual "wobble" operation could of course be applied to any of the ADT/flanging/phasing/chorusing process. Famously, this effect was used on the Eric Clapton solo on The Beatles "While My Guitar Gently Weeps". The wobbly/crying sound was created by someone manually wobbling the Levell oscillator attached to the ADT setup. That is why it is pretty impossible to replicate that same sound today.
Now, as mentioned above, there are quite a few flanger/phaser/chorus effects devices out there. However, all of them are based on the circuits invented in the 1970s and 1980s. As BJ pointed out, they are quite hard to "bond" with. That is because I think that none of them have gone back to how the original tape-based ADT/flanging/phasing/chorusing was first created. It would seem to me that if one replicated what was actually originally done at the time you should create a very, very interesting guitar effects pedal today. What would be interesting would be to take something like the Skreddy Echo device (
http://skreddypedals.com/skreddy_pedals_echo.htm) and find some way to (a) simulate processing the signal in the 1-50ms time period (b) give greater control over the oscillation variations. It seems to me that (a) all the existing flanging/phasing/chorusing pedals have not thought about replicating the old tape-based approach to producing these effects and (b) all the people creating tape-based echo devices (such as the new one by Skreddy) have not thought about how to use their basic technology to create these other effects. I don't know, it seems like a very interesting area to explore don't you think
For those who want more information that only thing I can find is the book "Recording The Beatles" by Brian Kehew and Kevin Ryan which is available at this
http://www.recordingthebeatles.com/ web site (see pages 294-302 on these effects). Basically, the technical details of how this was done was only recently "discovered" so this is the first and only book that details it at the moment. This is one of those old things that was kind of lost for a while. Anyway, could be fun