There are several competing versions of the story of how Fahrenheit came to devise his temperature scale.
[3] According to one version, Fahrenheit established the zero (0 °F) and 100 °F points on his scale by recording the lowest outdoor temperatures he could measure, and his own body temperature. He took as his zero point the lowest temperature he measured in the harsh winter of 1708 through 1709 in his hometown of
Danzig (now Gdańsk,
Poland) (−17.8 °C). (He was later able to reach this temperature under laboratory conditions using a mixture of ice, ammonium chloride, and water.) Fahrenheit wanted to avoid the negative temperatures that
Ole Rømer's scale had produced in everyday use. He fixed his own body temperature as 100 °F.
[4] He then divided his original scale into twelve parts, and later divided each of these into 8 equal subdivisions, produced a scale of 96 degrees. Fahrenheit noted that his scale placed the freezing point of water at 32 °F and the boiling point at 212 °F, a neat 180 degrees apart.
Another story holds that Fahrenheit established the zero of his scale (0 °F) as the temperature at which a mixture of equal parts of
ice and
salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and 96 degrees as the temperature of blood (he initially used horse blood to calibrate his scale). Initially, his scale contained only 12 equal divisions, but he later subdivided each division into eight equal degrees, ending up with 96.
A third, well-known version of the story, as described in the popular physics television series The Mechanical Universe, maintains that Fahrenheit simply adopted
Rømer's scale (in which water freezes at 7.5 degrees) and multiplied each value by four to eliminate the fractions and increase the granularity of the scale (giving 30 and 240 degrees). He then re-calibrated his scale between the melting point of water and normal human body temperature (which he took to be 96 degrees); the melting point of ice was adjusted to 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 two to the sixth power).
His measurements were not entirely accurate, though. By his original scale, the actual melting and boiling points would have been noticeably different from 32 °F and 212 °F. Some time after his death, it was decided to recalibrate the scale with 32 °F and 212 °F as the exact melting and boiling points of plain water. That change was made to easily convert from Celsius to Fahrenheit and vice versa, with a simple formula. This change also explains why the body temperature once taken as 96 or 100 °F by Fahrenheit is today taken by many as 98.6 °F (it is a direct conversion of 37 °C), although giving the value as 98 °F would be more accurate.
