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I was wondering why does something goes off, when it in fact does the opposite

  • bomb goes off - it blows up
  • alarm goes off - it turns on

Why not goes on?

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To "go off" in both contexts simply means to "activate/perform its intended function". Why does this need a bounty to unearth what's going on? –  FumbleFingers Feb 19 '12 at 3:43
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@FumbeFingers first, this bounty was to recompense Jon Purdy's answer. Secondly, in my mind, and perhaps hmemcpy's mind, "on" is associated with the start of a process (online, on air), rather than the end of a process (offline, off air). –  Theta30 Feb 20 '12 at 18:33
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6 Answers

up vote 22 down vote accepted
+50

To go off in this sense is related to the expression to set off, meaning to start or to be started. It implies that the subject was in a state of rest, then moved off from that state into action. The phrasal verb to go on already has the meaning of to continue.

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Very nice explanation. –  Manoochehr Jan 30 '11 at 20:44
    
Touché ! Just touché . –  Argot Jan 12 at 16:35
    
@Jon: I believe the connotation is much stronger than you paint. Something "going off" has a certain explosive nature to it. Something literally exploding, or something going from silence to loud alarm, etc. –  Wayne Jan 12 at 18:38
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@Jon: Your last sentence is only one side of "go on". It can imply continuation, but a light can "go on". So in the original poster's question, an indicator light may have gone on indicating that an alarm had gone off. –  Wayne Jan 12 at 18:39
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An alarm goes off when the alarm is "released" or "raised". An alarm isn't just a thing that is on/off, it's a state of alert.

Similarly when a bomb goes off, it has moved from a gentle, resting state to an excited, explosive state.

Goes off is often used for something that suddenly and explosively changes from a resting state to one of vigorous action.

  • "The runners have lined up, the starter raises his pistol... and they're off!"

  • "The gun goes off and everything changes... the world changes... and nothing else really matters." - PattiSue Plumer, runner

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Still, I think you are bound by usage. A logical explanation of why "off" was used rather than any other preposition would have satisfied me more. –  Theta30 Feb 18 '12 at 18:55
    
@Theta30: it's an idiom, so don't expect too much. –  Wayne Jan 12 at 18:35
    
@Hugo: And in colloquial American English, the phrase has become independent: "She complained about his cooking, and he went off on her." Meaning he exploded in anger and yelled at her. –  Wayne Jan 12 at 18:36
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While I'm no expert on idiom origin, I'd venture to guess that the phrase is "alarm goes off" because the first alarms were likely those that needed to be wound up. As one winds them up, the mechanical processes inside coil up, around and on top of each other, creating a tension. So when it unwinds...the coils literally come off of one another, causing to tension to be be released as well until finally the coils are all off, and the tension is gone, which allows the other parts of the alarm to vibrate and make noise.

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As an alternative to the other two answers, I think of "going off" as a trap or reaction getting triggered. "The bomb goes off" means that the reaction was triggered. Rest and motion are not really relevant. Same with a "trap going off" — the trap was waiting for a particular event and then *bam* it went off. "Went off" is a phrase for someone exploding in emotion against another and also brings to mind a triggering event.

An alarm fits in this category of words: "The alarm went off." The trigger for an alarm is a particular time and, when the time arrives, *bam* it goes off.

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There are two different meanings in play here:

  • The distinction between working/active ("on") and not-working/inactive ("off").

  • The description of a dangerous, sudden, loud, or explosive act occurring: "setting off" an explosion, "sounding off" at roll call (i.e. shouting out "present", "here", or something like that in response to your name being called).

So you could say "the alarm went on" and would be understood, but it's not idiomatic. The picture is that the alarm is loud (a siren, bell, etc) and indicates danger of some sort. In a computer system, an "alarm" might simply be a light or a sentence printed, but the mental picture still applies.

A light "goes on", so you might even see an indicator light "go on" indicating that an alarm has "gone off".

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It dates from medieval England when households revolved around a fire upon which a pot would be continuously simmering. This was linked to some sort of bell which would chime continuously. If the fire went off, the energy (heat) from the fire would stop the chiming thus causing the alarm. Hence we use the phrase when the alarm goes off!

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Please add your research to this answer. –  MετάEd Mar 14 '13 at 14:52
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