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Nicholas DiFabbio 
nicholasdifabbio.com
On Tuesday October 6, 1964 during The Beatles recording session for the song Eight Days A Week, John Lennon developed the guitar riff that would inspire the writing of the song I Feel Fine. John said, ‘I want to write a song around this riff', and the others said, ‘yes you go away and do that’.

Twelve days later, during a hectic day of recording at Abbey Road Studios crammed in during a UK concert tour which had begun on October 9, John told Ringo he had finished the song but that it was, as he said ‘lousy'.  But they decided to try it and immediately realized it was an A side single, I Feel Fine. John Lennon on lead vocal and lead and rhythm guitar, Paul McCartney bass and harmony vocal, George Harrison lead and rhythm guitar and harmony vocal, and Ringo Starr, drums. Lennon and Harrison played the opening riff together, John on his electrified Gibson J160 acoustic, and George on a Gretsch Tennessean electric.

But there is a unique element in the introduction on the final recording; a guitar string feeding back. It had been an accidental discovery in the studio. John leaned his Gibson J160 acoustic on an amp leading it to feeding back. On the beginning of I Feel Fine you can hear Paul striking an ‘A’ on his bass to trigger the sympathetic vibration of the A string on John's guitar. The string begins to vibrate, causing the feedback. The dominant A would be the logical note to use since it  signals the upcoming riff that is centered around a D chord. 

This marked the first time on a pop recording that the sound of feedback was employed as described later by Paul, George, and Producer George Martin. [Interview] 
Here is an unedited clip from the original studio take, note the longer length of the feedback, which was shortened for the final release.

I Feel Fine was released on Friday November 27, 1964 and was an immediate million seller in the US and Britain. It marked the first of an unbroken string of 6 number one hits in a row by the Beatles in the US. 

I Feel Fine
From the promo film
The guitar intro
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Stream the original recording on Apple Music