Fahrenheit

Lore & Background

In his initial scale, the zero point was determined by placing the thermometer in a mixture of ice, water, and ammonium chloride or sea salt, forming a eutectic system that stabilizes its temperature automatically. A second point, 32 degrees, was marked as the temperature of ice and water without salts. A third point, 96 degrees, was approximately the human body's temperature. According to a letter to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. Fahrenheit multiplied each Rømer value by 4 to eliminate fractions and then re-calibrated using the melting point of ice and normal human body temperature, setting them at 32 and 96 degrees so that 64 intervals separated the two, allowing him to mark degree lines by bisecting the interval six times.

Reader's Guide

The Fahrenheit scale was the primary temperature standard for climatic, industrial, and medical purposes in Anglophone countries until the 1960s. In the late 1960s and 1970s, the Celsius scale replaced Fahrenheit in almost all of those countries, with the notable exception of the United States. Fahrenheit is used in the United States, its territories and associated states, as well as the Cayman Islands for everyday applications, including weather forecasts, cooking, and food freezing temperatures; however, for scientific research, the scale is Celsius and Kelvin. A temperature interval of 1 °F equals 5/9 °C. The Fahrenheit and Celsius scales intersect at −40. The Rankine temperature scale uses degree intervals of the same size as Fahrenheit, with absolute zero at 0 °R. Early 20th-century commentators suggested that resistance to Celsius in the U.S. included the larger size of each Celsius degree and the lower zero point, and claimed the Fahrenheit scale is more intuitive for describing outdoor temperatures in temperate regions.

Did You Know?

• The original lower defining point of 0 °F was established as the freezing temperature of a solution of brine made from water, ice, and ammonium chloride.
• Fahrenheit's initial scale set the average human body temperature at 90 °F, then 96 °F, about 2.6 °F less than the modern value due to a later redefinition.
• The Fahrenheit and Celsius scales intersect numerically at −40 degrees.
• The Rankine temperature scale uses degree intervals of the same size as the Fahrenheit scale, with absolute zero at 0 °R.