"On" and "Off" for Lights, Electrical Switches, etc
Ngram finds "turn on the water" and "turn off the water" going back to 1735 at least. So clearly "turn on" and "turn off" were well-established idioms long before electricity came into common use, and they were simply adapted their electrical use as relatively mundane metaphors.
Two different (but related) guesses as to the origin:
- "Turn on" and "turn off" refer to diverting a stream with a small dam or gate.
- "Turn on" and "turn off" refer to using a petcock as might be used to control the flow of water from a cask
"Turn off" was much more common early on than "turn on", if that gives any hints as to the origin. (Perhaps "open" was used instead of "turn on"?)
Update: Looking some more, Ngram shows that "open the valve" was fairly popular since the 1700s, while "close the valve" did not perk up until about 1880, around the same time that "turn on" became as popular as "turn off". One suspects that the terminology was trying to settle, but kept being disrupted by technology.
I would have preferred to leave this as a comment because it's merely a conjecture, but I can't write a comment this long.
I think off and on may have to do with the intermittence of the light. Sometimes it's "on", sometimes it's "off". This use of off and on is documented in the OED back to the 16th century. It's even in Shakespeare's Tempest "I swam 'ere I could recover the shore, five and thirty leagues off and on." The earliest uses of it are nautical, things like "Shyppes lying off and on at Sea while under Sayle."