Artificial Double-Tracking
Singers and instrumentalists often overdub a second take that can be mixed with the original to produce a chorus effect. As long as the pitch and timing differences between the two takes are subtle, double-tracking, as it's called, will generally improve the quality of the performance.
Working with the Beatles, Abbey Road engineer Ken Townsend, thought of a way to create the sound of double-tracking without actually having to record a second take.
The Studer J37 4-track recorders they had at Abbey Road Studios were unique in that they had separate amplifiers and separate outputs for both the sync repro heads, (which were also the record heads), and playback heads. All other multi-tracks have a switch to select one or the other output, but not both at once, like these particular Studer J37s.
Ken Townsend took the sync head output, which was upstream, and therefore earlier than the playback head, ran it to a second tape recorder, recorded it, and then output the signal from that machine's playback head. Then, by mixing this with the signal from the J37's playback head, he was able to create a very slight delay.
This delay could be adjusted, by using an oscillator that fed a power amp that was used to supply AC power to the second machine. The oscillator mimicked the 50 Hz line frequency, and by adjusting the frequency of the oscillator he was able to speed up or slow down that machine's capstan. By slowly changing the tape speed of this second recorder, he could not only change the timing of the delay, but could also subtly modulate the pitch. Combining this with the natural speed changes, due to the wow and flutter of the two machines, enhanced the chorus effect.
This technique, that was widely used by the Beatles and many others, is known as ADT, Artificial Double-Tracking.