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How °F = 9/5(°C) + 32

  1. Dec 18, 2016 #1
    How we got this formula i know that the ratio of scale difference is 180/100 =9/5 but why to add 32, is it because °F at 0°c is + 32
  2. jcsd
  3. Dec 18, 2016 #2

    Vanadium 50

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    Yes. Plug in a few numbers and you'll see.
  4. Dec 19, 2016 #3
    The 0 was moved to be the freezing point of water. Here is some info:


    (interestingly, originally the freezing point of water was 100°C, and the boiling point was 0°C, but Carl Linnaeus made that modification- Good read)
  5. Dec 19, 2016 #4


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    I'm not sure how clear it is in the references, but for seafaring nations like Britain and Denmark, the freezing temperature of salt water is very significant. So setting the 0 of the Fahrenheit scale at that point makes sense. Maritime issues drove the development of clocks, thermometers, etc.
  6. Dec 20, 2016 #5
  7. Dec 23, 2016 #6

    James Pelezo

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  8. Dec 23, 2016 #7
    The other well-known conversion method, based on the -40° coincidence of the two scales is, I think, easier to remember. The fact that I no longer remember it is utterly irrelevant.
  9. Dec 24, 2016 #8
  10. Dec 24, 2016 #9
    I fortified myself to argue with you with the aid of Google, which asserts that seawater freezes at 28.4°F. I seem to remember that Fahrenheit produced the lowest temperature possible to him in the day in his laboratory to get to his zero.
  11. Dec 24, 2016 #10


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    I am certainly not an expert in the history of this. I have seen it said a few times that 0°F is the freezing point of brine. (This reference says that 0°F was the lowest temperature he could get reliably by freezing brine. https://en.wikipedia.org/wiki/Brine . That makes sense to me.) I might have seen it explained as important for sailing, I don't remember. Maybe I just erroneously jumped to that conclusion. Certainly he would have wanted to include the temperature of freezing sea water in his temperature range.
    Last edited: Dec 24, 2016
  12. Jan 1, 2017 #11


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    Is there an explanation as to why the two scales match up so nicely? The defined points are rather arbitrarily chosen.

  13. Jan 4, 2017 #12
    It is because the original definition of Fahrenheit was replaced in 1776 with one where 32 °F is 0 °C and 212 °F is 100 °C.
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