- "Yes! Doctor Emmett Brown, brilliant but sometimes misunderstood scientist, had actually invented something that worked! / He needed to record all of this while it was still fresh in his mind. He jumped up and walked briskly over to the reel-to-reel tape recorder on the other side of the room. He picked up the microphone as he flipped the 'record' switch. A quick check of his watch, and he began to dictate."
- —From Back to the Future Part III by Craig Shaw Gardner (quote, page 6)
A reel-to-reel tape recorder was a machine used for recording sound on reels of magnetic tape.
Emmett Brown had a reel-to-reel tape recorder, which he used on the morning of November 13, 1955 to record the events of the night before, in which he mentioned that he didn't remember coming home, but did remember seeing Marty McFly after returning him to 1985 in the DeLorean time machine. Doc theorized that this amnesia was due to a disruption of his own brainwaves, caused by the jigowatt discharge coupled with the temporal displacement field created by the DeLorean.
Presumably, along with his notebook and camera, the tape recorder was the medium Doc used to document his scientific experiments prior to the invention — and his purchase — of the JVC camcorder he owned in 1985.