FAHRENHEIT

For other uses, see Fahrenheit (disambiguation).

Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724.

In this scale, the freezing point of water is 32 degrees Fahrenheit (written "32 °F"), and the boiling point is 212 degrees, placing the boiling and melting points of water 180 degrees apart. Thus the unit of this scale, a degree Fahrenheit, is 5/9ths of a kelvin (which is a degree Celsius), and negative 40 degrees Fahrenheit is equal to negative 40 degrees Celsius.

History

There are several competing versions of the story of how Fahrenheit came to devise his temperature scale. One states that 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 home town of Gdansk (Danzig) (−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 which Ole Rømer's scale had produced in everyday use. Fahrenheit fixed his own body temperature as 100 °F (normal body temperature is closer to 98.2 °F, suggesting that Fahrenheit was suffering a fever when he conducted his experiments or that his thermometer was inaccurate), and divided his original scale into twelve divisions; later dividing 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 an equal mixture of ice and salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and ninety-six degrees as the temperature of blood (he initially used horse blood to calibrate his scale). Initially, his scale only contained 12 equal subdivisions, but later he subdivided each division into 8 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, holds that Fahrenheit simply adopted Rømer's scale, at which water freezes at 7.5 degrees, and multiplied each value by 4 in order to eliminate the fractions and increase the granularity of the scale (giving 30 and 240 degrees). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he took to be 96 degrees); the freezing point of water 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 2 to the sixth power).

His measurements were not entirely accurate, though; by his original scale, the actual freezing 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 freezing 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.

A fourth, not so well-known version of the origin of the Fahrenheit scale depends on Fahrenheit himself being a Freemason (of which there is no definitive evidence). In Freemasonry, there are 32 degrees of enlightenment, 32 being the highest. The use of the 'degree' as well is said to have been derived from the degrees of masonry. This may well be coincidence, but there is no conclusive evidence to the contrary, so the thought persists.

A fifth version maintains that Fahrenheit based 0 degrees on an estimate of the temperature someone would freeze to death, and 100 degrees on the temperature someone would die of heat exhaustion from, therefore making 0 to 100 the livable range for human beings.

A sixth version maintains that Fahrenheit marked the freezing point of water, normal human body temperature, and the boiling point of water. He then divided the span from freezing to boiling into 180 degrees. Setting the normal human body temperature as 100 resulted in the FP and BP being 32 and 212, respectively.

Usage

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in most English-speaking countries until the 1960s. In the late 1960s and 1970s the Celsius (formerly centigrade) scale was phased-in by governments as part of the standardizing process of metrication.

Fahrenheit supporters claim its previous popularity was due to Fahrenheit's user-friendliness. The unit of measure, being only 5/9 the size of the Celsius degree, permits more precise communication of measurements without resorting to fractional degrees. Also, the ambient air temperature in most inhabited regions of the world tends not to go far beyond the range of 0 °F to 100 °F: therefore, the Fahrenheit scale would reflect the perceived ambient temperatures, following 10-degree bands that emerge in the Fahrenheit system:

• 10s Deep Frost.
• 20s Light Frost.
• 30s Cold. Close to freezing.
• 40s Cold. Heavy clothing needed.
• 50s Very cool. Moderate Clothing required.
• 60s Cool. Light clothing.
• 70s Tepid. Summer clothing.
• 80s Warm. Minimal clothing.
• 90s Hot.
• 100s Very hot. Take precautions against overheating.

However, such a correlation is largely the result of habit: in the same way, Celsius supporters might indicate that 0–10 °C indicates cold, 10–20 °C mild, 20–30 °C warm and 30–40 °C hot.

Celsius is, on the other hand, certainly more appropriate for estimation of body temperature by measuring pulse rate. It is well known fact that pulse rate increase of 10 beats per minute corresponds to body temperature increase of 1° Celsius (provided that other influences from physiological and psychological activity are absent).^{[citation needed]}

In Jamaica and the United States the Fahrenheit system continues to be very widely used. In Australia, Canada, New Zealand and Europe, Celsius has been adopted and is widely used. Fahrenheit is sometimes used by older generations however for measurement of higher temperatures while lower temperatures are always measured in degrees Celsius. Most other countries have adopted Celsius as the primary scale in use.

Trivia

The fire point, or kindling point, of paper is claimed by some to be 451 °F (233 °C) (the actual kindling point depends on the type of pulp used in the paper's manufacture, chemical content, paper thickness etc.). For this reason it is the title of the book by American author Ray Bradbury, Fahrenheit 451.

Comparison of temperature scales