According to a journal article Fahrenheit wrote in 1724,[3] he based his scale on three reference points of temperature. The zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt. This is a type of frigorific mixture. The mixture automatically stabilizes its temperature at 0 °F. He then put a thermometer into the mixture and let the liquid in the thermometer descend to its lowest point. The second point is the 32 degree found by putting the thermometer in still water as ice is just forming on the surface.[4] The third point, the 96 degree, was the level of the liquid in the thermometer when held in the mouth or under the armpit. Fahrenheit noted that, using this scale, mercury boils at around 600 degrees.
Later, work by other scientists observed that water boils about 180 degrees higher than the freezing point and decided to redefine the degree slightly to make it exactly 180 degrees higher.[3] It is for this reason that normal body temperature is 98 on the revised scale (whereas it was 96 on Fahrenheit's original scale).[5]
According to a letter Fahrenheit wrote to his friend Herman Boerhaave,[6] his scale built on the work of Ole Rømer, whom he had met earlier. In Rømer?s scale, the two fixed reference points are that brine also freezes at 0 degrees and water boils at 60 degrees. He observed that, on this scale, water freezes at 7.5 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale (resulting in 30 and 240 degrees, respectively). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he observed to be 96 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power).[7]
