The Fahrenheit Temperature Scale
The Fahrenheit temperature scale is the most common in the United States. This scale was proposed by Gabriel Daniel Fahrenheit in a paper published in 1724.
The Fahrenheit temperature scale sets the freezing point of water at 32 degrees Fahrenheit. Water boils at 212 degrees in this scale. Hence a Fahrenheit degree is 1/180 the temperature difference between the boiling and freezing points of water.
The Centigrade or Celsius Temperature Scale
The Centigrade and Celsius temperature scales are two different names for the same temperature scale. This scale was developed by Anders Celsius in 1742.
In the Celsius temperature scale the freezing point of water is defined as 0 degrees Celsius. Water boils at 100 degrees Celsius. Hence a Celsius degree is 1/100 the difference between the boiling and freezing point of water.
A Centigrade degree is 100/180 or 5/9 times the size of a Fahrenheit degree. That is why the factors 9/5 or 5/9 appear in the formulas to convert Celsius to Fahrenheit or Fahrenheit to Celsius.
The Kelvin Temperature Scale
The Kelvin temperature scale is sometimes called the absolute temperature scale, especially in older books. It was developed by William Thomson, also known as Lord Kelvin, in 1848. A Kelvin degree is the same size as a Centigrade degree. This temperature scale however uses absolute zero, rather than the freezing point of water, as the zero point. In this temperature scale water freezes at 273.15 Kelvins and boils at 373.15 Kelvins. The Kelvin temperature scale should be used in thermodynamic calculations.
Absolute Zero
When an object's temperature increases its internal heat energy causes the random motions of individual atoms and molecules to increase. Random molecular motions are faster at higher temperatures. As the temperature decreases, the random atomic and molecular motions decrease.
The lowest possible temperature is when these random atomic and molecular motions are at the minimum possible energy. This temperature is absolute zero. It is zero in the Kelvin temperature scale. Absolute zero is -273.15 degrees Celsius and -459.67 degrees Fahrenheit.
The Rankine Temperature Scale
The Rankine temperature scale was developed by William John Macquorn Rankine in 1859. It is seldom used today. In the Rankine scale the zero point is absolute zero, but the Rankine degree is the same size as the Fahrenheit degree.
Temperature Conversions
To convert Fahrenheit to Celsius or Centigrade use the following formula:
- Centigrade degrees = (5/9) X (Fahrenheit degrees - 32)
To convert Celsius or Centigrade to Fahrenheit use the following formula:
- Fahrenheit degrees = (9/5) X (Centigrade degrees) + 32
To convert between Celsius or Centigrade and Kelvin use the following formula:
- Kelvin degrees = Centigrade degrees + 273
Further Reading
Knight, R.D., Physics for Scientists and Engineers, Pearson, 2004, p 490-492.
Fahrenheit, G.D., "Experimenta et Observationes de Congelatione Aqulin Vacuo Factae", Philisophical Transactions v 33, p 78-89, 1724.
Fahrenheit, G.D., "Experimenta Circa Gradum Caloris Liquorum Nonnallorum Efullientium Instituta", Philisophical Transactions v 33, p 1, 1724.
Thomson, W., Philisophical Magazine, October, 1848.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
